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'[PIC]: Autopilot landing system for model aircraft'
2002\06\18@221026 by Tan Chun Chiek

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Hi,
   This idea of an autopilot landing system just popped up in my mind, and
other details came almost immediately. I thought why hasn't anyone done it
before? The runway is something like 10m X 50m. GPS is used for
longitudinal/latitudinal positioning and preferably velocity measurement,
the aircraft would have a GPS receiver onboard, and 2 other GPS receivers
would sit on the both long ends at center of the runway, it's something akin
to differential GPS to provide better accuracy. Height measurement is done
by a ultrasonic transceiver or maybe GPS, are they accurate enough? And
pitch and roll is sensed by a pizeo gyroscope.

How feasible is this concept?

Regards,
Tan CC

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2002\06\18@222731 by Jim

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It's in the planning stage with the big boys ...

Otherwise MLS (Microwave Landing System) was
going to be the standard 'landing aid' to
replace ILS ...

Disclaimer: I did some field work and testing with
TI on an MLS development system in the mid 70's.

Jim

{Original Message removed}

2002\06\18@224048 by Pic Dude

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Few thoughts....

(1) For small model airplanes, you'd need to make really,
really quick corrections to pull this off, and I'm not
sure if GPS (even 3) would be precise enough for that.
But I may be wrong here.

(2) Ever heard of WAAS (Wide Area Augmentation System)?
Originally conceived & started development when SA was
still active.  Not sure what ever happened with it though.
A quick google search for 'WAAS' should net you a lot of
good info though.  Sounds like that's what you want
implement.

(3) Sounds like a full autopilot system developing here,
so stay tuned for the paranoid responses, which I
probably have to agree with more here than for cruise
control.

(4) A lot of electronics in one small plane.  First up,
figure out roughly what electronics will be needed, then
pick a model plane that can handle that extra cargo.
Now figure out the price of that puppy, and if you can
afford it.

(5) Speaking of affording it, you'll need to buy the
planes wholesale for the testing phase. :-)

Cheers,
-Neil.



{Original Message removed}

2002\06\19@032916 by Andrei B.

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--- Tan Chun Chiek <spam_OUTcodeTakeThisOuTspamMXHUB.NET> wrote:
> Hi,
>     This idea of an autopilot landing system just popped up in my
> mind, and other details came almost immediately.
You're aiming for the big league here! I know manufacturers are working
to create safe landing capabilities for airliners, (don't know the
details), but I doubt they have a system that can 100% replace the
human factor in all conditions...

> before? The runway is something like 10m X 50m. GPS is used for
> longitudinal/latitudinal positioning and preferably velocity
> measurement,

I don't want to discourage you, but there are loads of problems...

First of all, I am not sure GPS, (unless military weapons grade, which
I doubt anyone can get their hands on) would be of any real use, other
then to have navigation and get the model in the vicinity of the
airport.
With model aircraft, aside from scale reduction, aerodynamic rules
work the same, but many parameters change dramatically due to the small
size.

Absolute speed measurement, the one that GPS can provide is
practically irrelevant for a model, espacially at landing. It may be
usefull for general navigation, remember, you need to have mapping and
stuff for this.

What you need is airspeed measurement, which GPS cannot provide. This
is the key.
Let's say your model flies well at 30kph and begins to stall below
18kph. If you come in for a landing, with 22kph, with headwind of 10
kph, GPS will say you have 12kph, and by this, will assume that you are
way below stall speed and will accelerate, landing you with excess
speed, and then you wander why the autopilot shoved your plane into the
ground...
Such a small scale air speed sensor is a challenge itself (I think).

Another thing, an autopilot can be built and used ONLY, I repeat, ONLY
when you know very well all the handling characteristics of the plane
in all conditions. Research and flight testing of real airplanes takes
years to gather most of the data for the flight manual and for the
autopilot. Without this, an autopilot is next to useless.
The autopilot needs to know exactly in the given conditions, and the
given orders how to control the plane. Changing the propeller, adding
200grams of weight somewhere, another type of engine, etc, would throw
off completely the autopilot and the planes characteristics will have
to be retested and the autopilot refitted.
A human pilot learns very easily and adapts very quickly to a
situation, but a computer-driven pilot who has a limited knowledge of
the plane and no learning capability (you're not putting an array of
multiprocessor systems with complex neural nets in your model
aircraft...), it won't be able to cope.

Also take into account that for such a small scale, even small,
apparently insignificant changes can dramatically affect the plane's
handling. As a human pilot, one can adapt quickly, sometimes
instinctively, maybe barely noticing it...
Example: your plane, fully equiped, at 20kph stalls at 14deg. Let's
say that you put a new battery pack with more capacity, but it also
weighs 100 grams more. This will shift the center of gravity of the
plane and now it will stall at 12 deg at 20kph. The difference may
seems small, but it is actually huge!
The autopilot WILL stall this plane, because it assumes that at 20kph,
it is safe to fly at 13deg inclination! Unless ofcourse, you go through
the whole process of getting the new flight characteristics.

Don't even think of putting an autopilot in another type of plane!!!

Well, it can be built, to work in a very limited range of situations
and flight conditions, with less research, but will have to be at least
supervised and override options should be available at all times.

Actually, making such an autopilot would be much easier for model
helicopter then for a plane. Due to the nature of the helicopter, and
the way it flies, (excluding acrobatics, ofcourse), many things are
simpler to handle.
The advantage of model helicopters is the fact that they can reach a
dynamic equilibrium in which they appear static and can hold this. Then
it's a simple matter of slightly lowering the collective...
(simplified)

The plane, although by it's nature tends to fly, while the helicopter
does the opposite, dynamicly balancing the parameters on a plane could
be more challenging then for a helicopter.

I hope this will help.

P.S. keep us posted

=====
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Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\06\19@054908 by Dave King

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At 12:27 AM 19/06/02 -0700, you wrote:
>--- Tan Chun Chiek <codespamKILLspamMXHUB.NET> wrote:
> > Hi,
> >     This idea of an autopilot landing system just popped up in my
> > mind, and other details came almost immediately.
>
>but I doubt they have a system that can 100% replace the
>human factor in all conditions...
>
>ing. Andrei Boros

A parachute? ;-]

Dave

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2002\06\19@075018 by Walter Banks

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"Andrei B." wrote:
>
>  Such a small scale air speed sensor is a challenge itself (I think).
>
This may be one of the easier parts to build. Airspeed measurement is
a function of ratio between ram air pressure and static pressure.
There are lots of pressure sensors these days.

w..

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2002\06\19@095436 by edrupp

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This guy at http://members.shaw.ca/sonde/index.htm has built a really
cool computer controlled glider.  Its not meant to do the final
touchdown, it deploys a parachute.  However the web site gives a idea of
the level of detail required.  Real cool, wish I knew more about flight
dynamics, really fun looking!

Engineering just for fun

Edward



Tan Chun Chiek wrote:
{Quote hidden}

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2002\06\19@095647 by M. Adam Davis

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Commercial GPS updates once per second, and without real DGPS or WAAS is
only accurate to about 10m horizontally and 100m vertically.  Most OEM
GPS receivers accept real dgps data, but a dgps sender is big bucks, and
is not trivial to build.  Using several of the same receivers on the
same location and a computer to computer the difference would be less
accurate than DGPS and perhaps WAAS - you'd still likely have a meter
error.  In any case you should probably throw out the vertical
information from the GPS completely.

Since it only updates once a second you are going to have to use
accelerometers or another positioning system to fill in the gaps - think
about how far an airplane goes in one second, and how far it can go in a
wind gust.  Straight interpolation simply won't work here (unless you
can eliminate all the variables, but even large buildings big enough for
your model are going to have odd air currents)

I suspect you'd spend less time in development if you built your own
positioning system.  Radio would work, using lights on the runway and a
camera in the plane would work, you could even have a transmitter on the
plane, tracked on the ground using three radio receivers with all the
heavy computations done by the computer on the ground.  You may want to
go this route first only to make it easier to program the algorithms on
the computer.

The hardest part, I suspect, is not the actual mechanisms for tracking
and autopiloting the plane but the algorithms that control it:

Example:
Up until recently two legged walking robots have all been built by trial
and error and/or by keeping the center of gravity above points touching
the ground.  The problem with doing it this way is that the algorithms
are specific to the hardware/model they are using and are not
generalized for other models.  When they do get the algorithm working on
another piece of hardware they have to change hundreds or thousands of
variables and re-tweak the algorithm.  Researchers have finally made a
simple model, including equations to describe the model, that can walk
and run in a dynamic fashion - with its center of gravity somewhere
beyond its feet.  This model can be described in just a few variables,
and adapting it to a different piece of hardware is a matter of changing
only those few varaibles, not the algorithm itself.

I imagine you are going to go with the trial and error method, and being
able to program a computer on the ground in the situation is going to
make this a lot easier.  I wouldn't try to do it with a PIC just yet.
Pick some fast high level language, or even create your own program
with a scripting language so you can go through the trials more easily.

-Adam

Tan Chun Chiek wrote:

{Quote hidden}

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2002\06\19@131537 by Jeremy Furtek

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----- Original Message -----
From: "Walter Banks" <KILLspamwalterKILLspamspamBYTECRAFT.COM>
Sent: Wednesday, June 19, 2002 4:41 AM
Subject: Re: [PIC]: Autopilot landing system for model aircraft


> "Andrei B." wrote:
> >
> >  Such a small scale air speed sensor is a challenge itself (I think).
> >
> This may be one of the easier parts to build. Airspeed measurement is
> a function of ratio between ram air pressure and static pressure.
> There are lots of pressure sensors these days.
>
> w..

I think that the subtle and challenging part of doing this (accurately) is
measuring static pressure. The presence of the aircraft influences the flow
field. For full size aircraft I think that they expend some effort to find a
position on the aircraft that will give an accurate reading for static
pressure under a representative set of operating conditions.

A wind tunnel test ($$$) of an aircraft may include placing pressure taps
along the model along the centerline. If you plot the pressure coefficient
(comparing measured pressure to the static pressure measured well upstream
or far away from the aircraft) along the axis of the aircraft, good
locations for the static pressure tap would be places where this value is
zero.

For a flying model, to determine the static pressure, I think that they may
sometimes drag an instrumented object far behind the aircraft, and compare
that to the taps on the aircraft.

For an R/C, you could do something similar. Another alternative would be to
determine the best location computationally, a la CFD.

Just my $.02.

Jeremy Furtek
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2002\06\19@132611 by Pic Dude

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Missed a bunch of this thread, but just looking at this...
Why can't you just use the pressure *inside* the aircraft
for the static pressure (assuming it's not airtight)?  If
so, then it would be really easy to do using any of the
low-cost gauge or diff pressure sensors out there.

Cheers,
-Neil.



{Original Message removed}

2002\06\19@133114 by Bob Japundza

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Because pressure is usually positive inside the fuselage.

The best place for a static port is about 1/3 the way between the trailing edge of the wing and the horizontal stabilizer.

Bob


> {Original Message removed}

2002\06\19@134656 by Pic Dude

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"positive"???  You mean positive relative to ambient,
rather than positive absolute, right?

If so, then why?  I would expect that it should just be
ambient, unless the gaps are in such a location that it
has a ram-air effect.  In either case, should it be
easy to calibrate/compensate for this in code?




{Original Message removed}

2002\06\19@140543 by Bob Japundza
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Yes, positive, in relation to ambient.  Since the fuselage has a shape, bernoulli's (sp?) principle still applies.  Since there are curves on the fuselage, the air takes longer to travel from the nose of the aircraft to the tail around the curved surface rather than a straight line, thus, that air will be at a lower pressure, no different than the top surface of the wing. There is positive pressure inside the fuselage since air going past the fuselage is at negative pressure.

In my airplane the canopy skirts bow out slightly in flight, due to the positive pressure inside the cockpit, even with the vents closed (I experimented with plugging any holes in the cockpit where air could seep through, to keep the canopy from bowing out, nothing worked; air always manages to get in).  The cockpit of an aircraft is not a good place for static pressure due to the reason above, having positive pressure inside.  Most IFR-certified single-engine aircraft such as the Cessna 172 etc. have an alernatic-static selector valve, that switches the static source to the cockpit if the one on the aft fuselage becomes iced-up or blocked otherwise.  Not the best solution, but it is better than nothing.  Or, you could break the glass of one of your pitot-static instruments, such as the vertical-speed indicator, and by breaking the glass you in effect have a static source inside the cockpit.

You can compensate for static errors by experimenting with different locations along the aft fuselage, or by raising or lowering the static port from the surface, or by putting an air dam in front of the static port.

Bob



> {Original Message removed}

2002\06\19@142039 by Drew Vassallo

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>Yes, positive, in relation to ambient.  There is positive pressure inside
>the fuselage since air going past the fuselage is at negative pressure.

I don't think this is true.  Or maybe you're just explaining it in the wrong
terms.  In fact, the pressure inside the fuselage is BELOW the ambient
static pressure.  The fuselage cannot "pressurize itself" just because
there's an airflow over its surface.

The reason that your canopy is bowing outward is that there is an acting
pressure LOWER than ambient static on the outer surface due to the airflow,
not because of a pressure HIGHER than ambient static inside the fuselage.
The pressure inside the fuselage starts out AT the total, or static,
pressure prior to takeoff.  Once in motion, the pressure fuselage gradually
starts DROPPING due to the negative pressure (from the airflow) acting
outside the fuselage.

Try this example of the same effect:  go down the highway in your car.  Open
the window halfway.  The pressure inside the car DROPS because the trapped
static (total) pressure inside the car tries to escape to equalize the
negative pressure flow going past the window.  So, the actual pressure
inside the car (or fuselage) will actually be LOWER than the static ambient
pressure.

--Andrew



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2002\06\19@142056 by Drew Vassallo

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By the way, this thread is getting EXTREMELY off-topic.  Of course, it's not
for me to judge, but I just thought I'd bring it up before Dale silences us
all :)

--Andrew

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2002\06\19@144122 by Pic Dude

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Hmmm.... I know that if I have a port on the side of
a car, it will usually have lower pressure on that
side than ambient.

I'm thinking of a way to create a system with ram-air
and suction-air (patent pending :-).  Pretty much a
ram-air tube as on a typical pitot-static system, and
a similar tube in reverse (facing the back of the
aircraft).  With similar sized tubes, and one butted
right behind the other, static should be the midpoint
of the 2 readings, or more importantly, airspeed would
be half of the new value.

Something like that.

> "Or, you could break the glass ...".
Exactly why I thought of using the pressure inside.

Cheers,
-Neil.



{Original Message removed}

2002\06\19@164404 by Dave King

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> > "Or, you could break the glass ...".
>Exactly why I thought of using the pressure inside.
>
>Cheers,
>-Neil.

The problem with internal pressure is that it will lag behind the
actual quite a bit. The "in case of emergency break glass" is
done on the bigger machines but only as a last resort as it
produces a lot of errors. Mind you if you have wondered into
a cloud/storm and had your static ports ice over anything at
this stage is better than nothing.

Dave

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2002\06\19@165212 by Walter Banks

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Pic Dude wrote:
> Why can't you just use the pressure *inside* the aircraft
> for the static pressure (assuming it's not airtight)?  If
> so, then it would be really easy to do using any of the
> low-cost gauge or diff pressure sensors out there.

Cabin pressure may be positive or negative. A simple way to
get static pressure is the way many full size light A/C do.

Ram air pressure is a tube pointing straight forward outside
of the propeller wash. From a wing for example)  Static is
a tube in parallel to the RAM air tube with the end capped
and 3 or 4 small holes drilled in its sides. Both the tubes
and the hole sizes are non critical.

w..

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2002\06\19@175310 by Olin Lathrop

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> The reason that your canopy is bowing outward is that there is an acting
> pressure LOWER than ambient static on the outer surface due to the
airflow,
> not because of a pressure HIGHER than ambient static inside the fuselage.
> The pressure inside the fuselage starts out AT the total, or static,
> pressure prior to takeoff.  Once in motion, the pressure fuselage
gradually
> starts DROPPING due to the negative pressure (from the airflow) acting
> outside the fuselage.
>
> Try this example of the same effect:  go down the highway in your car.
Open
> the window halfway.  The pressure inside the car DROPS because the trapped
> static (total) pressure inside the car tries to escape to equalize the
> negative pressure flow going past the window.  So, the actual pressure
> inside the car (or fuselage) will actually be LOWER than the static
ambient
> pressure.

This depends on where the holes are in the fuselage are and what their
orientation is.  If you could open the windshield as a window, air would
rush in and inside pressure would be higher than outside.  A different
location and orientation can just as easily create a suction causing lower
than ambient cabin pressure.


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2002\06\20@021907 by Andrei B.

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> For an R/C, you could do something similar. Another alternative would
> be to
> determine the best location computationally, a la CFD.
>

Unfortunately, in aviation, especially in such pioneering areas, there
is no purely theoretical solution. Experimental data is crucial.


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Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\06\20@022528 by Andrei B.

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--- Walter Banks <RemoveMEwalterTakeThisOuTspamspamBYTECRAFT.COM> wrote:
> "Andrei B." wrote:
> >
> >  Such a small scale air speed sensor is a challenge itself (I
> think).
> >
> This may be one of the easier parts to build. Airspeed measurement is
> a function of ratio between ram air pressure and static pressure.
> There are lots of pressure sensors these days.
>
No doubt, but there are a few tiny details:

- small size, small weight
- accuracy for low air speeds. (I think we're not talking about jet
models with miniature turboreactor that flies at over 100kph...)
- related to the above : high resolution.


Am much easier and cheaper version would be a small propeller working
as a windmill. Then have a PIC measure the rpm optically. This might
work better for low airspeed then differential pressure sensors.
It would need however accurate calibration.


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Societatea Romana de Radiodifuziune

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2002\06\20@022748 by Andrei B.

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--- Pic Dude <RemoveMEpicdudeKILLspamspamAVN-TECH.COM> wrote:
> Missed a bunch of this thread, but just looking at this...
> Why can't you just use the pressure *inside* the aircraft
> for the static pressure (assuming it's not airtight)?

If you have a frontal opening to your aircraft, your inside pressure is
already influenced by speed and is no longer useful in such
measurement.


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2002\06\20@030509 by Andrei B.

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Here are some more thought about the landing system, and! on-topic.

I don't know how today's ILS works, but I do know how the Lorentz low
visibility radio guided landing helper worked during WWII.
Generate 2 adiacent radio beams that overlap a little. Beams could be
several km wide, but the overlap is quite narrow.

The beam on the left is modulated with audio pulses with let's say 25%
fill ratio and the beam on the right is modulated with audio pulses
(same freq) with fill ratio 75%, in sync with the other beam.
When you are in the overlap area, you hear a continuos sound. If you
hear short pulses and long pauses, you are too far to the left, so go
right. And the reverse.

Now assume the particularities of models and an airport for them.

Assume that your model has some navigation built into it and it can
position itself mapwise in an pretty good corridor to come in for a
landing. (assume GPS here, also see below for more).

Have an ultrasonic altimeter onboard that CAN measure heights down to
let's say 10cm and up to several tens of meters. Now you have a good
height indication (GPS is totally useless for this due to unacceptable
tolerances 10m error, 100m error, accuracy of the map you reference
to).

Build a Lorentz like beam system, but instead of radio waves, use
ultrasound. Have the pair of transmitters at the far side of the
airfield and a receiver onboard (you can definitely build it with a
PIC!!!) that can control the plane's direction left/rigth, after it
gets adequately aligned to the airfield by other means, so that it
stays on course guided by the Lorentz beam.

Navigation :

Assume that you are flying at an airfield usually used by you and
other model aircraft. You could build on the ground the necessary
support systems for a navigation system that will work around that
airport.

Although I have absolutely no technical knowledge about how GPS
actually works, I guess it could be an extension of the Oboe system
developed and used for aircraft navigation by the british, also during
WWII. This system was considered to be the most accurate radio
navigation system developed during the conflict.

The Oboe system : you have several stations on the ground at different
location. Each transmits a pulse modulated radio wave, and all are in
sync. Each station is identified uniquely.
A receiver will pick up one station and use it as a reference, then it
picks up another station and computes the time difference between the 2
signals. By some math (which I do not know), one can compute a
hyperbola which is the geometric locus where the receiver can be, with
one of the stations (i think the  reference one) in it's focal point.
Now pick up a third station and compute the time difference between
the signals and you get a second hyperbola. Where the two hyperbolas
intersect, there you are. If you are in doubt and you may have 2
intersections, measure the difference between signals 2 and 3 and get a
third one. all three will have a common intersection point.

(I assume GPS and similar systems use a similar method, using atomic
clocks to have the signals in sync and expanding the concept to 3D, to
include height and to consider the fact that you cannot simplify to 2D
when your source stations are orbiting above...)

According to the accuracy of the measurements and the floating point
precision used, one can get a pretty accurate navigation system.

Now take this concept and try to apply it to sound instead of radio.
This is because sound travels much slower and on such short distances,
computing picosecond differences between signals is not a practical
thing to do.

I leave it to you to extrapolate.

One more thing: please, if anyone develops such a system, let us know
about it, and hopefully, will be under GPL.

P.S.
And let's not forget that WE ARE TALKING ABOUTMODEL AIRCRAFT, not a
military jet, not a huge airliner. I think, in general, a model
aircraft is not flown in hurricane winds, and it doesn't go up in a
thunderstorm so it get's lost in the clouds, etc,etc,etc.



=====
ing. Andrei Boros
Centrul pt. Tehnologia Informatiei
Societatea Romana de Radiodifuziune

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2002\06\26@110607 by Brandon Fosdick

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Jeremy Furtek wrote:
> I think that the subtle and challenging part of doing this (accurately) is
> measuring static pressure. The presence of the aircraft influences the flow
> field. For full size aircraft I think that they expend some effort to find a
> position on the aircraft that will give an accurate reading for static
> pressure under a representative set of operating conditions.
>
> A wind tunnel test ($$$) of an aircraft may include placing pressure taps
> along the model along the centerline. If you plot the pressure coefficient
> (comparing measured pressure to the static pressure measured well upstream
> or far away from the aircraft) along the axis of the aircraft, good
> locations for the static pressure tap would be places where this value is
> zero.
>
> For a flying model, to determine the static pressure, I think that they may
> sometimes drag an instrumented object far behind the aircraft, and compare
> that to the taps on the aircraft.
>
> For an R/C, you could do something similar. Another alternative would be to
> determine the best location computationally, a la CFD.

The easiest way to get a static port on a model airplane is to not use a port.
Most model airplanes aren't anywhere close to airtight, but aren't open to the
wind either. In general the air pressure inside the airframe is close enough to
the ideal static pressure. Of course, this assumption breaks down if the
pressure sensor is too close to a "crack" (wing attach point, gear doors,
firewall...). And most importantly, you need to calibrate the sensor. Some
syringes (w/o needles) and fuel tubing can be very handy for that.

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2002\06\26@110617 by Brandon Fosdick

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"Andrei B." wrote:
> I don't want to discourage you, but there are loads of problems...

Not really, any standard aircraft dynamics textbook has most everything you need
to do this sort of this. Mine even has a section(s) devoted to the topic. As for
GPS...autoland systems existed long before GPS.

{Quote hidden}

Human pilots learn through trial and error and so can the computer. All of the
systems I've worked with for model airplanes ignore the modeling problem and
just use PID or PIDD controllers. The gains for which are determined through
trial and error.

{Quote hidden}

Stall is not a function of pitch angle, its a function of angle of attack and
airspeed, which are related. As long as the autopilot has a control law wrapped
around the airspeed and knows not to go below the stall speed, it should be
fine. Adding an angle of attack sensor will help to.

{Quote hidden}

Achieving steady state flight with a stable airplane is easier, since it will
tend to stay at equilbibrium (hence "stable"). The difference is that for an
airplane the equilibrium state involves forward velocity. Helicopters are
inherently unstable and require constant attention to stay at the steady state
point (excluding autogyro). There's a reason why papers airplanes fly better
than paper helicopters.

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2002\06\26@110620 by Brandon Fosdick

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Tan Chun Chiek wrote:
>     This idea of an autopilot landing system just popped up in my mind, and
> other details came almost immediately. I thought why hasn't anyone done it
> before? The runway is something like 10m X 50m. GPS is used for
> longitudinal/latitudinal positioning and preferably velocity measurement,
> the aircraft would have a GPS receiver onboard, and 2 other GPS receivers
> would sit on the both long ends at center of the runway, it's something akin
> to differential GPS to provide better accuracy. Height measurement is done
> by a ultrasonic transceiver or maybe GPS, are they accurate enough? And
> pitch and roll is sensed by a pizeo gyroscope.
>
> How feasible is this concept?

Have you looked at commercially available systems? The MP2000
(http://www.micropilot.com) is capable of autoland. Its expensive and not much
fun to work with, but it gets the job done.

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2002\06\26@110622 by Brandon Fosdick

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"M. Adam Davis" wrote:
> I suspect you'd spend less time in development if you built your own
> positioning system.  Radio would work, using lights on the runway and a
> camera in the plane would work, you could even have a transmitter on the
> plane, tracked on the ground using three radio receivers with all the
> heavy computations done by the computer on the ground.  You may want to
> go this route first only to make it easier to program the algorithms on
> the computer.

An inertial navigation system isn't too hard to do.

> The hardest part, I suspect, is not the actual mechanisms for tracking
> and autopiloting the plane but the algorithms that control it:

Actually, tuning the algorithms (w/o crashing) is the hardest part. The
"algorithms" can be simple uninformed control laws.

{Quote hidden}

Can yo provide any more info on this? I've been interested in walking stuff for
a while, but haven't been able to find any good papers on it. The best thing
I've seen so far is the trodon at MIT's leg lab, but they don't appear to be
talking about it.

> I imagine you are going to go with the trial and error method, and being
> able to program a computer on the ground in the situation is going to
> make this a lot easier.  I wouldn't try to do it with a PIC just yet.
>  Pick some fast high level language, or even create your own program
> with a scripting language so you can go through the trials more easily.

Actually, the MP1000 used two PIC's (I forget which ones) and could keep a
vehicle in the air. Although its handling qualities left much to be desired and
it certainly couldn't land.

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2002\06\26@110624 by Brandon Fosdick

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Walter Banks wrote:
>
> Pic Dude wrote:
> > Why can't you just use the pressure *inside* the aircraft
> > for the static pressure (assuming it's not airtight)?  If
> > so, then it would be really easy to do using any of the
> > low-cost gauge or diff pressure sensors out there.
>
> Cabin pressure may be positive or negative. A simple way to
> get static pressure is the way many full size light A/C do.
>
> Ram air pressure is a tube pointing straight forward outside
> of the propeller wash. From a wing for example)  Static is
> a tube in parallel to the RAM air tube with the end capped
> and 3 or 4 small holes drilled in its sides. Both the tubes
> and the hole sizes are non critical.

This is definately the better way to do it, although I've found that as long as
you calibrate the sensors well enough and the cabin doesn't have too many holes,
the cabin pressure is "close enough". But then, I've only tried this in a pusher
configuration, a puller might be different.

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2002\06\26@110627 by Brandon Fosdick

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Pic Dude wrote:
>
> Hmmm.... I know that if I have a port on the side of
> a car, it will usually have lower pressure on that
> side than ambient.
>
> I'm thinking of a way to create a system with ram-air
> and suction-air (patent pending :-).  Pretty much a
> ram-air tube as on a typical pitot-static system, and
> a similar tube in reverse (facing the back of the
> aircraft).  With similar sized tubes, and one butted
> right behind the other, static should be the midpoint
> of the 2 readings, or more importantly, airspeed would
> be half of the new value.
>
> Something like that.

The really really old airplanes used a venturi to get airpseed. Thats
essentially what you're talking about. Some wind tunnels still do it that way.

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2002\06\27@005410 by M. Adam Davis

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Unfortunately no, I attended a tech luncheon at the University of
Michigan.  The  presentation was done by a professor and graduate
student who were working on exactly this with a french university.  I
don't even recall walking away with any documentation on it...

Ah nevermind.  I spent a few minutes at the engineering site and found it:
http://www.eecs.umich.edu/~ewesterv/biped/

-Adam

Brandon Fosdick wrote:

{Quote hidden}

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2002\06\27@135136 by Peter L. Peres

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>Can yo provide any more info on this? I've been interested in walking stuff for
>a while, but haven't been able to find any good papers on it. The best thing
>I've seen so far is the trodon at MIT's leg lab, but they don't appear to be
>talking about it.

There's a book called 'walking legged robots' or such. I do not remember
the proper title.

Peter

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2002\06\27@135152 by Peter L. Peres

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On Wed, 26 Jun 2002, Brandon Fosdick wrote:

{Quote hidden}

Stall depends on the efficiency of the aerofoils at the given parameters,
which include density (altitude), airspeed, aoa, and wing load, among
other things (which include wing surface quality and any defects). I am
not an expert on this but these things are *complicated*.

{Quote hidden}

Steady state does not exist in flight. Stable airplanes do not exist. What
appears to be temporarily stable is a quirk in a balance equation that
predicts a crash either by stall or by pitching into the ground sooner or
later.  Either that or you are spending lots of energy in wasted
compensation moments that represent a significant percentage of the real
forces required for 'stable' (say, efficient) flight. A (paper) helicopter
with a serious coning angle and/or large mass/momentum at the rim
(frisbee) is very stable for the duration of its flight, same for a model
airplane or paper airplane.

It follows that you can have stability two ways:

1. Waste a lot of energy (engine power) on oversized static aerodynamic
moments that ensure that the plane is 'stable' (large dihedral, large
arrow, large stabilizer, low cog, etc etc). This costs fuel and/or
efficiency (low glide number) and/or maneuvrability.

2. Use active feedback control on an instable or critically stable
airframe to keep it flying 'stable'. This costs lots of time to get the
controls right and yields best results. The feedback can be a live pilot
or a machine. By the time the machine does everything the pilot knows how
to do, it should be about as smart as the pilot.

3. (the third way of two <g>). Use as much of <2> as you dare and keep as
much as you need of <1> in the design just in case <2> is not so good.

definitely not an expert,

Peter

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2002\06\27@201315 by Tony Nixon

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Brandon Fosdick wrote:

> Stall is not a function of pitch angle, its a function of angle of attack and
> airspeed, which are related. As long as the autopilot has a control law wrapped
> around the airspeed and knows not to go below the stall speed, it should be
> fine. Adding an angle of attack sensor will help to.

As I understand it, stalling on ONLY a function of angle of attack.

You can stall a plane at and attitude, at high speed, and you can also
fly a plane below the stall speed (however you need altitude to regain
flight energy before loading the wings).

To verify this, if the wing has 0g loading, how can it stall. However,
if you are below the stall speed and the wing loading goes above 0g,
then the lift bank balance is in arrears and it will stall. Same thing
flying at the stall speed, there's nothing left in the lift bank for the
wings. Lowering an aileron in this state will stall that wing
immediately. Disasterous near the ground.

As my areobatics instructor stresses, pull the stick back to position
'X' at any speed and the wings will stall, so you may be better
monitoring the back pressure on the "stick" to detect when a stall is
imminent.

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2002\06\27@203247 by Brendan Moran

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> As my areobatics instructor stresses, pull the stick back to position
> 'X' at any speed and the wings will stall, so you may be better
> monitoring the back pressure on the "stick" to detect when a stall is
> imminent.

I think that there may be two separate discussions in progress here.  When
you mention stalling, Tony, you are speaking specifically of a wing stall,
rather than not having lift at all.  They are two different things.

I know of one Russian prop stunt plane that had so much prop force that it
could literally hover using its prop.  Not to mention that many fighter jets
will not stall going straight up unless the throttle is low (for going
straight up, less than 90% counts as low) but by that point, the wings are
no longer causing any lift, and therefore are stalled, while the aircraft
clearly continues to have lift, and is therefore not stalled.

The other guy (sorry forget the name and it's waaayyyy back in my PIC box)
who was talking about all the different factors was, I think, talking more
about when the aircraft would cease to gain lift at all.

What I'm trying to say is that a wing can stall and the aircraft can
continue on un-stalled (though if the aircraft is stalled, I think you're
almost guaranteed to have all wings stalled).  It depends on all the factors
originally mentioned.  If you've built a model jet fighter, and it can climb
straight up, chances are that if you punch the throttle, you'll never stall
the little beggar, though the wings may only be serving as fins at that
point.

--Brendan

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2002\06\27@211410 by Lee Jones

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>> As my areobatics instructor stresses, pull the stick back to
>> position 'X' at any speed and the wings will stall, so you may
>> be better monitoring the back pressure on the "stick" to detect
>> when a stall is imminent.

> I think that there may be two separate discussions in progress here.
> When you mention stalling, Tony, you are speaking specifically of a
> wing stall, rather than not having lift at all.  They are two
> different things.

Lift has several components:

1) pressure differential lift (classic wing lift),
2) flat plate lift, and
3) vertical component of propulsive force.

> What I'm trying to say is that a wing can stall and the aircraft
> can continue on un-stalled

As long as the total of all lift vectors exceeds the weight, the
aircraft will not descend -- even if the wing is technically (in
the clasic sense) stalled.


> (though if the aircraft is stalled, I think you're almost
> guaranteed to have all wings stalled).

Not true.  You can easily have one surface, the wing, stalled while
another surface, the stabilizer/elevator, is not stalled.  You can
also have part of a single surface, the wing, stalled while another
part of the same surface is not stalled -- tuned by installation of
"stall strips" on the leading edge.  If all surfaces stalled at the
same time, the aircraft would be uncontrollable.

                                               Lee Jones

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2002\06\29@061146 by Peter L. Peres

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On Thu, 27 Jun 2002, Brendan Moran wrote:

>I think that there may be two separate discussions in progress here.  When
>you mention stalling, Tony, you are speaking specifically of a wing stall,
>rather than not having lift at all.  They are two different things.

Stall is defined as the condition where the intended laminar flow around
the aerofoil ceases being laminar to a great extent. It has almost nothing
to do with wings or aircraft except the above mentioned aerodynamic
condition just so happens to lower the wing efficiency (lift/drag ratio
for that aoa) below what is acceptable for an aircraft for flying. Stall
also affects other things, like turbine blades, fast-moving cutting tools
in air or lubricant, fans, structures exposed to high speed air or gas
or liquid flows, etc etc.

As you have said, if the engines are powerfull enough you 'can't' stall.
Also usually aerodynamic stall has a latching behavior, in that the
transition between stalled and unstalled mode is abrupt in both
directions.

Peter

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2002\06\29@135123 by Dale Botkin

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Guys, this is *well past* the [OT]: point, please change the topic tag for
any further posts.  Should have sent this two days ago.  Thanks.

Dale
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On Fri, 28 Jun 2002, Peter L. Peres wrote:

{Quote hidden}

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