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Angle of Attack telemetry

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Angle of Attack telemetry

Old 06-04-2013, 07:58 AM
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Chris_Rayner
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Default Angle of Attack telemetry

I'm not sure if this is the right place, but here goes. I have spent a lifetime trying to improve my model flying, and I think I have reached a rather modest level of skill which is about as far as I can get in this life. I am now looking at telemetry as an accessory branch of the hobby.

Most of the ready made telemetry concentrates on monitoring electronics, height, GPS position, climb/sink, and airspeed. From a practical point of view I think that angle of attack is at least as interesting as any of these, yet there does not seem to be a practical implementation for models. I have searched this and other forums, and although the subject is mentioned from time to time, nobody appears to have published a working system. There was one attempt at doing this for small gliders which involved a balsa wind vane attached to a potentiometer, but it struck me as rather delicate and so not suitable for larger i/c powered planes. Unfortunately I have mislaid the link to this effort.

It strikes me that in any case AoA is only of interest in so far as it is a proxy for stall. Stall being the sudden loss of lift from a wing. This loss of lift should be preceded by disruption of pressure differential between the upper and lower surfaces of the wing. Initially I thought that simple monitoring of airspeed could be sufficient, but the EagleTree pitot system seems to be insufficiently sensitive at the airspeeds mostly of interest; in any case, stalls can occur at varying airspeeds. The pitot system of airspeed detection depends on sensing differential air pressure, so I wondered if the angle of attack and hence stall warning could be implemented by using pressure sensors connected to apertures in the wing at, say 1/3 chord on upper and lower surfaces. Using arduino or similar PIC the signals from these sensors could be compared and angle of attack, or possibly percentage likelihood of stall computed and transmitted by telemetry.

I confess that my technical knowledge and expertise are fairly rudimentary, but I would be interested to hear if any more knowledgeable folk have comments and advice.
Old 06-04-2013, 08:12 AM
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bogbeagle
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Default RE: Angle of Attack telemetry

Many light aircraft employ simple systems which are located in the wing' leading edges.

These systems operate only near to the wing's critical angle ... they don't provide A of A information throughout the wing's operating envelope. The warning information is presented to the pilot either audibly, through the headset ... or visually via a light on the control panel.


After-market systems are available for purchase by home-builders.




I'd suggest that this would be a good place to begin researching.



Old 06-04-2013, 01:29 PM
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otrcman
 
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Default RE: Angle of Attack telemetry

As fundamental as it sounds, measuring AOA is surprisingly difficult. Local flow in an about an airplane must be carefully studied as it relates to actual free stream AOA, and once defined, is accurate for only that aerodynamic shape. Even something as mundane as lowering flaps on an airplane can alter local airflow such that prior measurements become inaccurate. This means that a vendor cannot simply produce a "one size fits all" device for measuring AOA on several different airplanes.

Most of the available AOA indicators sold for full scale airplanes don't actually measure AOA in degrees. They are simply on-off devices that turn on when the airplane approaches the stall angle (described in prior post) or they are analog indicators that provide the pilot with some relative idea of where he is in the AOA range of the airplane's envelope. Such indicators typically have ranges marked in green, yellow and red, but don't actually read out in degrees. In all cases, they have to be calibrated to that particular airplane due to the specifics of local flow peculiar to that aerodynamic shape.

The only somewhat accurate means of measuring AOA without detailed knowledge of the local flow is with a probe that extends well forward into undisturbed air. Rule of thumb placement of such probes is that they should be at least 1 1/2 local chord lengths ahead of the wing leading edge (if wing mounted) or at least one body diameter ahead if mounted on the nose of the fuselage. The probe can consist of a vane/potentiometer affair or a blunt cylinder with an array of static ports, each monitored for local air pressure. Needless to say, any such probe extending out from the nose or wing leading edge is highly subject to handling damage.

Some simple yet accurate means of measuring AOA is still off in the future somewhere, but it certainly would be welcome in R/C flying as well as in full scale. If such a device had been feasible years ago we might not be relying on airspeed indicators so much today.

Dick
Old 06-04-2013, 06:54 PM
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WoodCutter
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Default RE: Angle of Attack telemetry

On the Pilatus PC-12 I fly at work, the A.O.A indicator is on a short probe out ahead of the wing. The problem I could foresee on a model outside of the obvious electronic and mechanical complexity and the inherent weight of such a system is the need to be able to calibrate the system. You would have to do systematic testing, preferably in a wind tunnel with your exact wing section in order to establish your working range.This is just my opinion but an observant pilot can already develop a "feel" for the aircrafts angle of attack by carefully noting the direction the A/C centerline is pointing versus the direction of travel that the airplane is moving. It's been many years since I've inadvertently stalled or snapped in with a model if you develop this skill enough. Another useful technique is to take careful note of the stick position required to stall the aircraft. This is surprisingly uniform regardless of the speed of the model assuming you don't mess around with the rate switches on the transmitter too much.Just my 2 cents....
Old 06-05-2013, 09:04 AM
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Chris_Rayner
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Default RE: Angle of Attack telemetry

Thanks to all for your observations. I have found details of a full size system similar to what I was proposing here [link=http://www.advanced-flight-systems.com/Products/AOA/aoa.html]here[/link].

Reviewing their literature it is clear that each aircraft needs to be more or less completely assessed to get the system working properly. With a model this is likely to be most easily done by simply flying the model in various states of trim up to the stall (and with adequate height for certain recovery) and recording the measurements generated. There appear to be readily available pressure sensors of small enough size and cost to make this feasible. I think a couple of these could be installed in the tips of build up wings with a small hole in the upper and lower surfaces connected to a small plenum chamber containing the sensor. With the sensors connected to a small PIC with memory it should be possible to record the traces of pressure changes and see if the data can be made useful as stall warning.

I agree with Woodcutter, this is not necessary if you concentrate and develop a feel for your 'planes. But then the whole business of flying model aircraft is optional, I just choose to do this because I fancy doing it. A 'Lone impulse of delight' if you like.
Old 12-16-2019, 05:32 PM
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TheNonConformist
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Try google searching TC18-7.pdf there's a picture on page 18 of what I'm pretty sure you're looking for...this website seems to not like my screenshot ot me posting links cause I'm a noob
Old 12-16-2019, 08:42 PM
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I think what you will find as you investigate more is that the wings on most of our model aircraft fly at a very shallow AOA in their normal flight mode. Obviously this is somewhat airfoil and wing loading dependent. An example would be with a 20oz wing loading and a symmetrical airfoil the wing will support the aircraft weight at around .75 to 1 degree of AOA. At normal flying speed any additional AOA simply results in a climb. The airflow in relation to airplane travel attitude remains fairly constant. landing speeds may see the AOA at as little as 5 degrees.
Old 12-19-2019, 05:47 AM
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ahicks
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How would an "angle of attack" sensor compare to a stall sensor used on full scale aircraft?

I get how the stall sensor in light planes (full scale) are built into the leading edges and do their thing, but the pictures of the sensor on the failed 737 Max look like they are just hanging a small flag on the outside of the fuse and measure the angle it's running at compared to the fuse. Air flowing over the "flag" will keep the flag parallel to air flow no matter the angle of the fuse. It's up front to keep it in fairly clean uninterrupted air.

The flag is going to stay parallel with the airflow going over it, so they simply have to compare the difference between the angles of the flag and the fuse to get their info - which would really be angle of attack, no? They can then use the angle of attack and airspeed to safely predict stall conditions - assuming the angle sensor(s) aren't frozen up, and the software is working correctly.

So, to bring this into the RC world, angle of attack sensor could be a small flag mounted on a sensor that would measure the angle the flag was running at as compared to the fuse. Could work something like a gas gauge where the float has been replaced by a flag running on the outside of the fuse? -Al

Last edited by ahicks; 12-19-2019 at 05:56 AM.

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