Help with designing airspeed indicator with BS2

MRM RCModels

Greeting to all,
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This is my first post here and like my handle implies, I design/sell rc planes.·My aero design skills are pretty good, but my circuit design skills aren't·· The more I stare at the schematics, the more confused I get·· I would like to deploy the·flaps (driven by servo) automatically depending on·the airspeed·of the aircraft.·I'm thinking of using (click on hyperlinks): a dual pressure sensor [noparse][[/noparse]one input in aircraft, the other in the air stream], a differential analog to digital converter [noparse][[/noparse]to compare the two pressure values and convert the signal so the BS2 can read it], and BS2 [noparse][[/noparse]to drive the motor].·How do you wire the pressure sensor to the differential ADC·so the·numerical output·can be read by the BS2? Since it's in an RC Plane, I'd really like it to run of the circuit for the 4.8V/1100mA RX battery.
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Am I·on the right track or is there a better way? Any suggestions are welcome. I'm just tired of banging my head against the wall trying to figure this out.
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Thanks,
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Kelvin

bobledoux

You are using a classic aircraft design of a pitot and static port. If the aircraft angle of attack doesn't vary much when you want to deploy the flaps just use the pitot. Most pressure sensors are voltage output so you could use a comparator chip, like an LM393. A resistor trimmer would set the voltage for flap operation. The comparator output would be high or low. So it could directly drive the Stamp pin.

This simple arrangement means the flaps would drop even when the plane is sitting reading for takeoff. High G forces might also cause the flaps to drop due to poor alignment of pitot tube with airflow.

MRM RCModels

Thanks for the reply, but I would not want the flaps fully deployed at all times. Mainly I wanted to use this system during final approach when the AoA doesn't match the direction of travel. This why I wanted to go with the classic differential pitot/static port configuration, so I could program the STAMP to deploy the correct·degree of flap for a given airspeed (i.e. 45 degrees·at 20 knots, 35 degrees at 40 knots, etc.).

Kelvin

kelvin james

Hmmmm I thought i held the copywrite on the name Kelvin


It may be more than what you want to get into, but i think a gps module would be worth a look. It will provide speed, altitude, direction,etc. It gives the speed info that can be used to control the servo for the flaps. It updates every second, which should be okay at slower speeds when landing.Since you only want the auto positioning of the flaps upon landing, an extra channel from the radio could be used to switch that program function on and off, or it could be done with some well thought-out programming. Parallax sells a gps module on their website that would be ideal. GPS on a model plane opens a lot of doors for monitoring and control.

W9GFO

I'm using the Parallax GPS module on one of my planes. So far I use it to fly a preprogrammed route.

The GPS is not suitable for indicating airspeed. It indicates ground speed only. For deciding when to deploy flaps you really need a pitot. Same goes for altitude, the GPS accuracy for altitude is pretty course. A barometric altimeter is needed if you want to maintain an altitude or make altitude based decisions.

I have a variety of pressure sensors from www.freescale.com/. They have differential pressure sensors where there is only one output - you only need to use one channel of your ADC. That would be ideal for measuring airspeed. Many of their sensors are available as samples, that's how I got them.

How about showing us some of your planes?

MRM RCModels

Thanks for the website link W9GFO. I went to the site but found it not that user friendly. Would you be able to suggest a part number for a differential pressure sensor with a single output? My first design is 0.90cc sized·twin boom pusher called the Modkat. It has retracts, fiberglass fuse with build-up wings. It will go into production when the new workshop is completed late this year.

And since you asked, here's some pics:

kelvin james

Since i do not own and fly a rc plane with GPS, my knowledge on this is somewhat limited, to say the least. BUT, i find the claim that " speed in the air is different than on the ground " to be totally unfounded. Travelling in a straight line at roughly the same altitude ( like a landing approach ) from point A to point B, there is no difference in the air or on the ground. Maybe i have missed some unique phenomena in physics that relates to this discrepancy. I would like to see an explanation of this for curiousity sake.

LilDi

If the plane is flying into a 30 mph wind and the top speed of the plane is 30 mph, then at full throttle, the planes ground speed will be zero mph, while the air speed will be 30 mph, since the air·passing over the wings is at 30 mph. It will·look like a helocopter hovering in place.

kelvin james

GPS tracks the actual movement of the module in the plane. Point "a" longitude/latitude to point "b" longitude/latitude, is a calculation of distance and time, which gives "actual" speed of the aircraft. I have been in real airplanes that use this, so i don't see the problem.

OSOK

Kelvin, like stated before, if a plane is flying at 30mph on a 30mph headwind, the airspeed (speed of air passing over its wings) will be 30mph, but the ground speed (speed of aircraft relative to the ground) will be 0.

If the plane is flying at 15mph with a tailwind of 5mph, it's groundspeed will be 20mph but its airspeed will be 15mph.

The GPS will only give you groundspeed, which won't work for landing speeds, etc.

W9GFO

Kelvin said...
I would like to see an explanation of this for curiousity sake.

Kelvin, IAS (indicated Air Speed) is the important measurement here. It works by measuring the pressure that the moving air exerts on a pitot tube. A pitot tube is simply a tube with an open end that points into the airstream. The faster you go the more pressure in the tube. Rather than reading the pressure in psi the gauge or instrument is read in MPH or Knots. The airspeed indicator will have some very important airspeeds marked on it. An extremely important one is stall speed. That is the slowest the airplane can go and still be controllable. There will also be maneuvering speed - the fastest speed at which you can deflect a control surface fully and VNE, never exceed speed - a speed at which if you exceed it your airplane may fly apart. There are others, those are just some common examples.

The beauty of the airspeed indicator is that no adjustments are needed for different altitudes. Whether you are at sea level or 20,000 ft the stall speed, let's say 30 mph, is still the stall speed. As you go higher in altitude the air gets thinner and an airplane must travel faster to stay aloft. But you needn't worry about increasing your Indicated Air Speed, as the air gets thinner you will automatically go faster to maintain the same indicated airspeed. The air will need to hit the pitot tube faster to create an equivalent amount of pressure. It is important to remember that Indicated Air Speed is not an accurate measurement of how fast the you are traveling through the air. That's why we call it Indicated Air Speed. True airspeed would take altitude into account but we don't really care about that.

For making a decision such as when to deploy flaps the only important speed is the Indicated Air Speed. GPS will tell you how fast you are going from point a to point b but that is quite useless in this application.

An example, The GPS says 20 mph. Well, if there is absolutely no wind then fine, your airspeed will be 20 mph. Ok now lets say that there is a 10 mph breeze. What does the GPS 20 mph mean? It could mean that your airspeed is 30 mph and you are pointed directly into the wind. It could also mean that you are traveling with the wind and your airspeed is actually 10 mph. The one thing that you can count on is that if there is any wind, whatever the GPS says your speed is - it will not be your airspeed.

Post Edited (W9GFO) : 8/5/2007 5:44:50 AM GMT

W9GFO

MRM said...
Thanks for the website link W9GFO. I went to the site but found it not that user friendly. Would you be able to suggest a part number for a differential pressure sensor with a single output?

I intend to use this one for airspeed - www.freescale.com/webapp/sps/site/prod_summary.jsp?code=MPX5010&fsrch=1.

It measures from 0 to 1.45 psi. Which should be good for up to about 300 mph. I have not used it yet in an aircraft, I am trying to make the switch to propeller power so that I can make a more capable autopilot. For what you want to do though the BS2 would work just fine.

p.s. Sharp looking plane!

FerretMK4

You should try to get something with a smaller range for good stall speed control. You're going to be down in the couple % of full scale with this one, and if you want to do progressive flap control with say an 8-bit A/D you're going to be working on only a couple of bits, which is too coarse: it'll be like an on-off switch. If you can use a 16 bit (or more) A/D you'd be fine. I'm just getting started with Stamp myself and at this point I don't think you can handle 16 bit data. Someone correct me if I'm wrong, please.
Remember to use a real pitot tube, not just a tube pointing forwards. The real pitot has side ports to sense the non-dynamic air pressure. You could use a port inside the plane, IF you can confirm by using an absolute pressure sensor that the inside of the plane doesn't adopt a positive or negative pressure. I've done this on racecars, and there isn't anywhere that has neutral, constant pressure.