air speed sensor for rc plane

[Chris Hibner]

Originally Posted by Gdeaver

A Freescale pressure sensor with the proper ADC can easily give altitude with in 5 ft. To measure air speed use a tube perpendicular to the airflow and measure the pressure drop within the tube. I forget the name for this device it begins with a p. The MPX4115a is the most used sensor for these apps.
There are GPS units about the size of a quarter that interface with a PIC by serial. There are allot of sources on the web on how to decode the packet. A 3 axis digital compass would also be a nice addition but they are expensive. You should be looking on the RC airplane forums. You'll find allot more specific how to's.

The perpendicular tube you mention is called a pitot tube. You determine airspeed by measuring the difference in pressure between the impact air pressure (at the pitot tube) and the ambient pressure (same source you use for your altimeter). Any fluid mechanics book has good explainations on how pitot tubes are used to measure fluid speeds. You will need two pressure transducers.

Someone earlier mentioned that a drawback to the fan (and pitot tube) is that it only measures speed relative to the air and not the ground. This is NOT a drawback. An airplane flies relative to its surrounding air - NOT the ground. The airplane does not care whatsoever what is happening with the ground unless the plane is within ~1 wingspan above the ground (i.e. only takeoffs and landings).

I think you should get rid of the accelerometers and replace them with angular rate sensors (i.e. solid state gyros). Acceleration is not important in an aircraft and can lead to false information. It is the gyroscope that allows pilots to fly in the blind. I guess one accelerometer can be useful as a slip/skid indicator, but that's not important for an autopilot.

You'll have to let me know how it goes. I've long been wanting to develop an RC autopilot. My big goal is that I wanted to do an auto-land feature.

[Josh Joseph]

hey guys I'm actually the friend greg posted the question for - I decided to just register so I can give some feedback on this...

The main problem with an anemometer is that it will be for a micro air vehicle (like the picture greg posted) so the added drag of a device like that is too much.

Also I've considered a pitot tube but I'm afraid because the craft is so small the airflow will be not be nearly uniform enough to get an accurate reading. I could definitely be wrong here seeing as how I've never tested this.

It's all electric powered and is driven by a brushless motor. I've talked to a couple of EE friends of mine and they didn't seem very convinced I could relate current draw to velocity very accurately.

I'd be interested if anyone has tried a pitot tube even on anything as small as a typical RC airplane...or had some luck relating current draw to velocity..or anything else...

I'm currently trying to relate forward/backward acceleration to velocity but seeing as how it's on one of these college project budgets the sensors are anything but high end and at first glance it's seeming like I'm not going to get the accuracy I'd need.

thanks guys

[Tristan Lall]

Originally Posted by Chris Hibner

Someone earlier mentioned that a drawback to the fan (and pitot tube) is that it only measures speed relative to the air and not the ground. This is NOT a drawback. An airplane flies relative to its surrounding air - NOT the ground. The airplane does not care whatsoever what is happening with the ground unless the plane is within ~1 wingspan above the ground (i.e. only takeoffs and landings).

When I mentioned groundspeed vs. airspeed, that's exactly the situation I had in mind; operating near obstacles. Before he'd more fully explained the concept, I was envisioning something like the IARC, where small autonomous aircraft are operated near buildings, even (in the case of helicopters) inside of buildings. In that sort of situation, you have to know where you are in absolute terms to follow a clear path, or otherwise, know the position of all obstacles at all times, and know how to take evasive action when you do see one, while still heading for your destination.

If it's just going to fly around, airspeed is quite sufficient; it's just that you need to also know wind speed and direction at all points, in order to follow a ground-based course using velocity as a reference.

[Al Skierkiewicz]

Greg and Josh,
Since speed is going to be relatively slow and I am guessing that this competition will take place indoors there is not likely to be much variation in barometric pressure and little in cross or head winds there is an easier way to do this. I would just use a reflective photo detector to count prop revolutions. You can find a number of reflective devices from Digikey. Add a little piece of audio splicing tape, (from Radio Shack, silver adhesive backed tape) to the back of the prop and you will not affect it's balance or drag coeeficient. Attach the device to the side of the motor and calibrate by running the airplane over a fixed distance to determine prop speed vs. disstance traveled. Unless a strom comes through while flying, the ambient pressure would stay relatively constant and regardless of battery voltage, the prop RPM would still give a good indication of air speed.
It's hard to tell from the picture, but you may be able to add something to the motor shaft on the side opposite the prop and count revs that way as well.

Al

[Greg Needel]

this is not really for a competition, it is a spin off project from Josh's employment that will probably be the ground work for a senior design project. The device is going to be used outside. and that picture is just an example not our final product. Although i think that the propeller counting might work outside still because of our low altitudes we are running there still wont be that much pressure difference, hence the reason we don't want to use pressure sensor to do speed.

[Chris Hibner]

Originally Posted by Greg Needel

...because of our low altitudes we are running there still wont be that much pressure difference, hence the reason we don't want to use pressure sensor to do speed.

Airspeed measurement has nothing to do with what altitude you're at. Well, as the air gets thinner, the conversion constant to convert your measured pressure difference into true airspeed is different, but the altitude makes no difference in the ability to measure it. If your airspeed is too low, that might be a problem, but the altitude is independent.

Your low change in altitude will make it easier and cheaper - you should be able to do a sufficient job with only one pressure transucer (the one at the pitot tube). Since your altitude changes will be small, you won't need to know the ambient pressure.

[Al Skierkiewicz]

For outside runs, counting prop rotation will not give you repeatable results. Head winds and cross winds vastly effect air speed vs. ground speed. Without a correlation there you will not be able to run a course in auto. Is there a possibility for triangulation from the ground to be transmitted to the plane for course changes? That would allow you to save weight and computing by performing all the functions on the ground.

[Gdeaver]

One problem your going to have is that with that size airframe and power you have almost no electronics payload budget. If you go up in scale and to gas power, there are some off the self options. What I' m thinking off is a pocket PC with a GPS add on. The position, altitude, velocity, heading are available. Use the pocket PC as the main brain. Visual studio Ide can be used to program the pocket PC. The Intel processor has more power than a microcontroller. With the proper cable a pocket PC can out put serial commands. There are servo controller chips based on pic16's that can take the serial command from the pocket pc and generate the hobby servo PWM signals. A rs232 converted is needed. the next part is to use gyro's designed for helicopters and planes. There are single axis units and 3 axis units. They are what stabilizes the RC helicopters. With control of 3 axis, stable straight level flight is possible. These gyro's are designed to take PWM input from the RC receiver and mix these commands with the stabilizing out of the gyro to give smooth control. Release the joy sticks and the plane will return to straight level flight. The Pocket pc controlled servo chip replaces the RC receiver in auto pilot mode. Don't underestimate the magnitude and cost involved with this project.

[seanwitte]

www.eagletreesystems.com

also check rcgroups.com

[Chris Hibner]

Originally Posted by Tristan Lall

When I mentioned groundspeed vs. airspeed, that's exactly the situation I had in mind; operating near obstacles. Before he'd more fully explained the concept, I was envisioning something like the IARC, where small autonomous aircraft are operated near buildings, even (in the case of helicopters) inside of buildings. In that sort of situation, you have to know where you are in absolute terms to follow a clear path, or otherwise, know the position of all obstacles at all times, and know how to take evasive action when you do see one, while still heading for your destination.

If it's just going to fly around, airspeed is quite sufficient; it's just that you need to also know wind speed and direction at all points, in order to follow a ground-based course using velocity as a reference.

Ah yes. If your purpose is to follow a groud track, you will need to reference the ground. If you purpose is controlling the performance of the airplane, airspeed is what you need.

In either case you will still need an airspeed sensor or else with any wind you either stall the plane or break it apart.