AoA sensors

[Jack Aubrey]

Hello all, I wonder if someone can help me.

I am building a UAV (model aircraft) for a University research programme and I want to install an AoA sensor to integrate into the flight control and nav system.

My instinct is to do it the way it is done on most of the manned fighters I have flown in - an aerofoil shaped vane.

My supervisor insists that a simple bob weight will do and can be kept within the fuselage. In either case, the sensor would be attached to a potentiometer to produce a calibrated DC voltage.

I am not anticipating much negative G or incidence but may need to unload to zero G to recover from stall. Max CL / stall will be ca. 12 degrees.

Any thoughts or suggestions would be welcome.

[Shawn Coyle]

A bob weight within the fuselage to measure AOA????
Not sure what school of aerodynamics the supervisor came from.
A bob weight in the fuselage would be subject to errors when longitudinal accelerations / decelerations take place, unless the sensor is perfectly aligned with the flight path; it would be subject to errors in turns, and turbulence might also affect it in unusual ways.
Stick with something that measures the angle of the airflow. No more complex than the bobweight- it just has to be put outside the fuselage.

[Jack Aubrey]

Very many thanks SC.

I have figured out some diagrams to illustrate the errors that would be obvious in a steady steep climb or dive.

Sometimes tact and corroboration are as important in academia as they are in real life!

Jno Aubrey

[John Farley]

Jack

Presumably he thinks AOA is related to aircraft attitude. You really will have to sort him out.

You could ask him to explain to you how the vertical is related to AOA as you are a bit confused...

John

[Jack Aubrey]

John

Very many thanks for your response.

I think the origin of the problem is that model aircraft/UAV pilots may only be interested in AoA at the touchdown point. I shall point out that Mr Chopper Test (SC) and Mr Hover Jet know better!

Since you have shown interest... manned aircraft tend to use an horizontal cylindrical or conical probe with parallel slots along but slightly either side of rotational axis. Is this just a simple drag balance device or do they measure pressure and servo drive to balance?

Obviously, it would be best to KISS for a small aircraft. On the principle of parsimony, we only really need to relay/record AoA at T/O, stall recovery and landing. If possible, I would like to go for a symmetrical aerofoil section and had thought of a section of model helicopter rotor blade. Any ideas on where to pivot it? Max thickness point, or max pressure point?

[Genghis the Engineer]

Since you have shown interest... manned aircraft tend to use an horizontal cylindrical or conical probe with parallel slots along but slightly either side of rotational axis. Is this just a simple drag balance device or do they measure pressure and servo drive to balance?

I think that you are referring to a combined pressure head. This is anular in section, with the tube through the centre containing pitot (dynamic) pressure, and the outer tube, vented by the slots you describe, containing static pressure. The pitot and static pressures feed the main aircraft pressure instruments, to whit...

- The Airspeed Indicator (ASI) is a comparative pressure gauge, comparing the two.

- The Altimeter is an absolute pressure gauge, measuring Static pressure.

- The Vertical Speed Indicator (VSI) measures the rate of change in static pressure.


Apart from that the errors in this tend to be a function of AoA, these aren't AoA devices in any way.


Having said that, there is a device called a yawmeter, which typically has a cone or cylinder pointed into the airflow, with holes or slots something like 45° above and below the (nominal) horizontal plane. Comparing the two pressures with static can give a very good measurement of AoA (or if mounted in the other plane, sideslip angle). I can't recall ever seeing such a device on an aeroplane but it is quite commonly used in wind tunnels - a standard reference such as Pope and Goyne may offer some more detailed information.

Try this link for some further information too. (http://www-g.eng.cam.ac.uk/whittle/current-research/hph/pressure-probes/pressure-probes.html)

G

[fobotcso]

Genghis, you said:

"...these aren't AoA devices in any way."

Beg to differ a little on this. Isn't JA describing the ADD sensor? Part of the pre-flight used to be a gentle twist to ensure that it was free to be servo'ed in flight.

ADD = Airstream Direction Detector.

Not AoA to be sure and full of local errors, but once calibrated for an airframe gave (gives) an indication of the angle between the airframe and the relative airflow. The indicator doesn't read degrees but "Units" and can be used for performance flying; typically approach and landing.

[John Farley]

Jack

To amplify fobotcso's post

As you probably realise the advantages of the cylindrical or conical probe as the sensor for the Airstream Direction Detection instrument is that it is reasonably robust compared to the sort of flightest wind vane one sees out on booms ahead of test aircraft. It is also easy to heat and keep from icing up.

It works because the pressure sensed by the two rows of slots (of whichever shape of probe) is fed to the opposite sides of a paddle type vane inside a cylinder that is free to rotate up to perhaps 50 deg and also connected to the probe. Thus as you surmised the probe settles at an angle giving equal pressure either side of the paddle and this angle varies with the direction of the airflow approaching the probe. If you like the probe aligns itself so that the stagnation point of the approaching air is between the two rows of slots.

My kingdom for a picture!

NB If your library has AP3456 you will find a nice picture of the ADD system workings in Vol 3 Part 2 Section 3 Ch 4 Page 2 (AL6)

My recommendation is that you use a wind vane on your model.

Regards

John

[Genghis the Engineer]

To clarify, I thought that Jack was referring to a pitot-static, which is not an AoA device - as you gents rightly say, an ADD is.

I think I agree with John about a vane being preferable - although with something based upon a yawmeter and using some compact electronics you could certainly produce something small and light - if not necessarily cheaply or quickly.

G

[fobotcso]

Jack, it would be interesting to know more about the scale of the project; how big is the model and what flight envelope are you aiming at?

I assume the data are to be recorded or telemetered; quite a challenge for a home-brew project given the small size, weight and dynamic pressures involved. Yes, a lightweight vane is the way to go. Assuming the model is prop-powered, it really ought to be on a boom to reduce errors.

[Jack Aubrey]

Gentlemen

Very many thanks for the comments and suggestions. We will go ahead with the vane idea.

The next step in our programme is to win the British Model Flying Association University Challenge.

www.bmfa.org/events/unichall2003.html

After that is a larger aircraft to fly long distances under semi-autonomous navigation, You may be aware that some American gentlemen flew a radio control model across the Atlantic recently.

The overall project is to develop structural and systems design techniques for extremely light weight aerostructures and mechanisms, as well as other onboard systems to allow missions that have been impossible before.

Basic problem for any robotic machine is to be able to carry enough energy to carry out a meaningful mission. So we aim to build things lighter, find more energy dense power sources, new ways of enhancing efficiency and so on.

fobotsco, we have a relatively cheap and effective way of relaying data omnidirectionally out to a few miles within line of sight - more effective and consistent than some of the systems from commercial sources.

Next thing for us to look at is a cheap and cheerful inertial platform! I am thinking of a fibre optic ring gyro (3 axis) built into the airframe structure, 3 axis accelerometers and an IR based optical sensor to give ideas about what is up and down (read high tech captive cat!!). All fused together (filtered?) by software.

[Tim Inder]

You may already be aware fo the following:
CSM manufacture piezo gyros for use in model aircraft and helicopters - there are two versions -a simple 'rate' gyro and a "HLG" (heading lock) which combines yaw rate sensing with the heading 'lock' facility which is mainly intended to stabilise the yaw of r/c helicopters. Presumably they will have the same effect on the other axes if mounted appropriately.

I have seen an i.r. 'trainer' system which can be used to recover a model to an upright attitude (not sure of the accuracy etc)

Lastly, there is a range of telemetry equipment available for r/c aircraft which can record such parameters as airspeed, altitude, vertical acceleration etc. the manufacturer's name escapes me at the moment, but there have been a series of articles in 'Radio Control Models and Electronics' or RCM&E within the last 6 months or so.

Btw, which uni are you at?

Tim