redcommander

I am curious to see the interest level in a sensor that can could measure true airspeed. The current technology (Pitot static) requires accurate measurement of static pressure (which can be extremely difficult at high altitudes). It also requires accurate air temperature measurements. Finally, a Pitot-static system is inaccurate below about 30 knots.

Does anyone know of any current projects that could benefit from a sensor (1.5" diam x 8"length) that could measure true airspeed (not pressure) and was accurate below 10 knots?

rmh

there are some excellent wind speed meters -use by glider guys- which operate in that speed envelope .
mounted on a plane they would show airspeed -if you flew under 30 MPH---
as for engine rpm -- on my gas engines - I have ignition systems which read and record highest rpm
These ar standard setups on most ZDZs now and some MVVS (I have both types )
these ignitions allow one to tailor prop rpm in flight
If I was into speed aircraft ( which I am not) I would match speeds with prop readings and plot a "best " hp/ prop combo.
on glow stuf you can buy some in air recording devices whch read rpm etc..

redcommander

That is pretty neat about the engine RPM recorders on stock motors. I wasn't aware.

As for the airspeed sensor, I am refering to airspeed sensors which actually measure TRUE AIRSPEED. The glider guys' sensors are almost certainly dependent upon air density, which complicates things at high altitude. Do you know how the "glider airspeed sensors" work?

I've also seen the airspeed sensors that send a sound pulse from an emitter to a detecter, and backout the airspeed by recording the phase shift or time delay to back out the airspeed. But, as we know, the speed of sound is variant with density as well.

I am speaking of a new sensor that actually measures airspeed, invariant with air density (ie. pressure or temp).

B.L.E.

A small windmill style air speed measurement device will give true airspeed although you still won't know how fast your plane could have gone without this extra drag on your plane. It would also have to be out of the propwash and away from any surface that would tend to accelerate the airflow around it, like the top of an airfoil.
Full scale pilots rely on pitot tube indicated air speed readings mostly because uncorrected IAS is a more useful bit of information than corrected air speed is. A plane will stall at approximately the same IAS regardless of temperature or altitude or humidity.

HalH

My Feedback: (12)

I personally can see no apparent reason to have TAS since the airplane flies according to IAS. GPS will give an accurate ground speed and there are systems presently in that use GPS to send GS.

mesae

As I understand pitot-static systems, the main inaccuracy in sensing dynamic pressure difference has to do with the AOA at the inlet (cos(alpha)*freestream vector). At low speed, the AOA becomes large enough to cause a significant reduction in ram pressure (small angle approximation doesn't work anymore). One way to do it would be to use a traditional pitot-static system with AOA, pressure altitude and temperature sensors and process the input before displaying it. It's done this way using various methods on some full-scale airplanes. Of course all that equiment would add weight. Wouldn't want to use a windmill: far too draggy, and that's still "indicated" airspeed anyway.

Why would you want to measure the true airspeed of a model anyway? Are you thinking about a UAV or something high dollar like that? True airspeed is used to calculate ground speed and wind correction angle, but indicated (or calibrated) airspeed is what operating limitations and climb performance are based on.

redcommander

Well, airplanes do NOT fly according to IAS (this is just an "indication", uncorrected for temp and alt). All of our V's in the equations we use for lift, drag, pitching moment, dimensional derivatives.......assume TRUE AIRSPEED. I don't really want to debate this, as it is like trying to debate F=ma. I was just curious to see if anyone had application ideas for a true airspeed sensor in the hobby world. In the UAV world, it obviously has relevance in very low speed aircraft such as VTOLS, helicopters (which also have problems sensing because of all the downwash screwing up static ports), low speed MAVs (micros) and blimps and high altitude blimps (which are notorious for not being able to sense their airspeed).

This is not true. For example, a good rule of thumb is to increase the IAS by 2% for every 1000ft of altitude. If a certain airplane stalls @35 kts @ sea level, 2% of 35=0.7. so at 10,000 ft, this airplane will stall at an IAS of 42 kts, and at 20,000ft, it will stall at an IAS of 49 kts. 49 is not approximately 35.

Yes, this is a problem with low cost Pitot tubes, but most modern probes have a particular shape on the front end that actually make them insensitive to AOA or Beta up to 15 or 20 degrees. The measurement of static pressure is at the root of the airdata measurement problem. Both altitude and airspeed measurements are impaired by the limited accuracy of pressure sensors as the altitude increases and the airspeed decreases. Precise small, light, low cost absolute pressure transducers having a range from zero to sea level pressures for altitude measurements and very low differential pressures for airspeed measurements do not exist. Using .001 “Hg. as a practical threshold for accuracy and resolution of airborne pressure measurements, degraded accuracy of speeds occur below 25 kts and above altitudes of 60,000 feet. More precise measurement of pressure is very difficult and expensive. This defines the lowest speed and highest altitude where good airdata measurements can be made.

what's GS

mesae

Indicated best angle and rate of climb speed change with a change in altitude, converging at the absolute ceiling.

Performance data and speed limitations in most airplane manuals are specified in IAS or CAS. I'm not sure I know what you mean by "airplanes do not fly according to IAS". All airplanes really care about is pressure; they don't "sense" true airspeed at all. Maneuvering speed IAS (Va), for example does not vary with altitude or temperature, since the ability to generate lift is reduced at the same rate and for the same reason that indicated airspeed decreases with altitude, keeping Va (IAS) the same for all reasonable altitudes. Va (TAS) does vary with density. This means the entire V-n diagram is based on IAS.

Please note that I am only taking exception with your apparently unconditional statement, "...airplanes do NOT fly according to IAS...". Whether IAS or TAS is important depends on what speed(s) we are considering, and whether we are designing an airplane, creating performance tables for it, or flying it.

GS means ground speed.

I suppose someone might be able to help more if we know more about your requirements. If you're talking about something large and expensive, I offered a suggestion. If you're talking about something with a 10" wingspan that will fly at 5 mph, I'm afraid I'm no help.

<edit> Changed "unqualified statement" to "unconditional statement".

HalH

My Feedback: (12)

Airplanes most certainlly do fly in relation to IAS. The wing has no idea what the TAS is nor does it care.

redcommander

Is the velocity term in dynamic pressure (q=1/2 (rho) V^2) TAS? I thought it was. What I meant was that lift and drag are usually computed with respect to dynamic pressure, and in its most common form, related to true airspeed. Is this correct?

redcommander

I see what you are saying,

q=1/2*Density@sealevel*IAS^2 This assumes IAS doesn't have compressibility effects
or
q=1/2 *actual density*TAS^2

So pilots probably like IAS because they always know the SSL density.

Tall Paul

It's the "rho" in the dynamic pressure equation that eliminates TAS from considieration.
Dynamic pressure flies airplanes, at sea level, or 70,000 feet.
The TAS required for flight will change, but the plane's flight characteristics are related solely to "q", not TAS.
A U-2 that stalls at 55 knots (CAS) at sea level stalls at the same speed at 70,000 feet. At sea level it's going 55 knots, TAS. At 70,000 feet, it's going 400 knots, TAS. but the loads on the plane are the same as at sea level.

mesae

From q=(rho*u^2)/2, it can be seen that if density is decreased, speed (TAS) must be increased to maintain the same q. Since IAS is (ideally) derived from q minus free-stream or ambient pressure, this equation indirectly shows the relationship between TAS and IAS.

So, you were not wrong when you wrote that TAS is used in stability calculations. TAS is implied, and so is IAS.

Pilots like IAS because it's what's in front of them most of the time. And it is a reliable indirect measure of q, which is something that can break airplanes. TAS by itself doesn't measure q, so it's not terribly useful to a pilot trying to out-climb a tree, or keep the wings on his mount during aerobatics or turbulence.

redcommander

OK, got it. I think we were all thinking the same things(TallPaul must be a LM guy? did I see you give a presentation to SAE 2 years ago at Palmdale on th SR-71?) Anyway, this is a great conversation.

The prospect for accurately measuring TAS allows a new approach to computing air data parameters that avoids the use of static pressure measurements and its associated measurement problems. Adding true speed to the measurement mix of total pressure (Pt), and total temperature (Tt), allows all other air data parameters to be calculated as follows:
Knowing the specific heat, Cp , for air and using the measured true speed and total temperature allows the calculation of ambient temperature, Ta :

CpTt = Cp Ta + ½ Vt2

from which the speed of sound, a , and Mach number, M, is evaluated as a=(gR Ta)1/2 and Mach number, M = Vt /a
Using the measured total pressure and the true speed or the calculated Mach number allows the calculation of atmospheric pressure and pressure altitude:

Pt/ Pa = (Tt/Ta)3.5 or = (1 + 0.2 M2 )3.5

Having the total pressure and temperature, ambient pressure and temperature and the Mach number all other air data parameters (Vc, Ve, et cetera.) can be calculated from standard air data equations.
If the true speed is corrected for flow inclination, which can be readily measured, the computed pressure is the free stream ambient pressure and is not subject to position errors that plague Pitot static based air data system measurements. This can be done through a simple cosine correction to the measured true speed, Vt

Vt = Vt cos è

where è is the angle between the measured true speed vector and the flight path velocity vector. Combining the true airspeed and GPS velocities allows the explicit computation of the instantaneous wind velocity and direction.


This method would be beneficial since static pressure wouldn't have to be measured! Would anyone be interested?

redcommander

TallPaul is right. I got screwed up here[&o] This is an example of how to get severely confused with all these speeds.

mesae

You would still need to know IAS, or some measure of dynamic pressure, to know how to avoid exceeding operating limitations. And you write that you might still need to do a cosine correction for large inclinations, to be sure we are measuring free-stream velocity as closely as possible. This is essentially the same type of processing that is currently done, either automatically or manually using CR or E-6B computers, or read from charts, using pitot-static systems, and applying gas formulas and trig to get TAS, and windspeed, when ground velocity is known. I don't see why it wouldn't work, you are just starting at TAS, and computing everything else the opposite direction of what is currently done, if I read your post correctly.

My question, since you still haven't defined a mission, is what would be the benefit to such a system? Would such a system be light and small enough to fill the gap between zero and 30 mph (I guess that is what you are asking)? Pitot-static systems have served well thus far, and are simple and accurate enough when their limitations are understood.

I really see two parts to my question. Could such a system be made light and inexpensive enough to be used on small UAVs? And, why would it be better than what we are using now? I guess I'm mainly referring to UAVs here because I don't know of many recreational RC models that have ANY type of speed sensing.

BTW, I assumed when you first mentioned 30 mph (or knots, whatever) as a lower limit, I assumed you were indirectly relating that to AOA and x-component loss. I think I see now that you were referring to the sensitivity of the diaphragms used in pressure sensing.

redcommander

Yes, that is correct. Start with TAS and work backwords. One benefit is that static presure doesn't have to be measured. This sensor could be very small, and could be used on micro UAVs (potentially even working on nano UAVs). The other advantage is that it should be accurate to very low airspeeds (below 10 kts)

Now, How does one measure True Airspeed. There isn't such a thing as a density sensor! I'm surprised no one has asked this question. Lets hear some thoughts? I have one method, but would like to hear yours first. My method requires near real-time processing , and about $50 in parts.

mesae

Laser? Doppler? Sonar? Ultrasonics? That equipment isn't in my area of expertise. Interesting though.

I imagine some UAV types might be interested. I flew UAVs professionally for several years part-time before starting to fly full-scale full-time for a living. Like everybody else, they are always looking for a better, lighter, cheaper way.

Another thought: If you are thinking about certificated airplanes, I doubt it would catch on any time soon. The FAA requires pitot-static systems for certification, and even if they allowed somebody to add another type of system, they would probably not allow it to eliminate a pitot-static system, since the concequences of a failure without a backup are potentially dire.


correction: "I'm imagine"?? egad.

redcommander

well, I don't know squat about lasers (except i don't think I can get one at radioshack), but I do know that speed of sound depends on altitude (T), so corrections would have to be made when using sonar as well. I believe this is true for ultrasonics as well (I am assuming you are talking about looking at the phase shift with an emitter/detector pair in combo with the dopplar effect) . This implies that the use of sound and ultrasound cannot measure TAS, although I have seen these type of airspeed sensors.

redcommander

Hey TallPaul, I just noticed you hav a picture of KU's SAE 2003 plane on your website! I was on that design team and piloted for KU from 2003-2004 in both east and west coast comps. We won the east competition in 03 with that bipe. got smoked at the west though. Man that high desert does wonders on engine performance.

B.L.E.

The speed of sound is independent of air density due to pressure. Differences in density due to temperature will change the speed of sound.

The speed of sound in air = square root of (1.4 x pressure/density) When you go up to a high altitude, the reduction of air pressure causes a corresponding reduction of air density and so the speed of sound remains the same. Heating the air causes a reduction of air density without reducing the pressure and so the speed of sound increases.

Tall Paul

.
I recall thinking someone paid a tribute to the DH-4, with that plane.
The 2006 west event will be at Sepulveda this year.

redcommander

Yeah, I can't believe we didn't hit a few Joshua trees.

I just had a thought...I suspect it is very difficult to measure very low airspeeds with sonar. For example, if you are trying to measure 5 mph at a temp where the speed of sound is 750 mph, you would have to be able to know the speed of sound very accurately in addition to being able to record the sound pulse time very accuratley since 5 is only about 0.67% of 750. This sounds expensive.