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Why the EagleTree Airspeed Sensor is Superior to Doppler or Radar or GPS.

Old 11-02-2007, 01:10 AM
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Moonwalker
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Default Why the EagleTree Airspeed Sensor is Superior to Doppler or Radar or GPS.

Most max. speed results posted on the internet for a given power system on a given airframe have used Doppler sound anaylsis or Radar. Now, I believe a revolution has taken place with the tiny little Eagletree micrologger with airspeed sensor.

Below I consider factors which I consider make the airspeed sensor superior in terms of comparing power sytems for given airframes when compared to the traditional RC speed measurement of radar or doppler and even GPS. Feel free to criticize my analysis or add to it as you see fit. NB - This analysis assumes straight and level measurements - not dives.


1) Many Doppler analysts fail to consider actual air temp at the time of video or sound recording to make it more accurate. Air temp effects the speed of sound which is the source of doppler measurements (since Waveoscope allows for compensation for temp, one must merely be vigilent when using it but doppler can also be fooled by throttle variations and one must have a "clean" sound file);

2) With Doppler, as with Radar, one must consider the "Cosine Effect"; that is, as the angle of approach increases relative to the measurement source - recording source or radar - speed calculated is lower than actual speed relative to the ground directly below. When you get a speeding ticket, the benefit of the cosine effect goes to the motorist because the cop is rarely in the middle of the highway with his radar gun. With Doppler it is no different. Unless the approach is right at the source at 0 degrees and right away from it at 180 degrees, you will have the cosine effect. Not a problem for Eagletree Airspeed sensor or GPS.

3) Airspeed is the true indicator of what a given power system will push an airframe to. Consider an aircraft flying at an airspeed of 100 mph at 180 degrees to a 90 mph hurricane. Doppler and radar will give you 10 mph speed measurement (straight and level) but the plane is actually cutting through the fluid (air) at 100 mph. The power to overcome aerodynamic drag is airspeed, not ground speed. Same limitation with GPS, it will register only 10 mph straight and level in the above situation.

4) Both Doppler and Radar measure the speed of the aircraft approaching the recording device or radar gun. The cameraman in videos or radar gun operator is usually mid-distance between the start of the speed run and the point at which the pilot can no longer hold throttle because the aircraft is a visual dot and about to leave his visual perception. A plane moving at at high speeds is often covering over 200 feet per second and, thus, the pilot is limited to holding full throttle for about 5-6 seconds with 20/20 vision. The graphs of Eagletree airspeed sensors often show the plane still accerating at a high rate when throttle is cut and speed peaks because the pilot can no longer see the aircraft. GPS is not limited in this respect but radar and doppler are unless the operator of them is at the end of the field to record the last portion of the speed run, which would, in effect, make it difficult for the pilot to judge whether he is flying directly at and over the operator.

My conclusion is thus that Airspeed sensors will eventually supplant Doppler, Radar and GPS (all of which measure ground speed) as a preferred form of measurement for comparing power systems on a given airframe.

Last edited by Moonwalker; 11-02-2007 at 07:42 AM.
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Old 11-02-2007, 04:49 PM
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Ryan CSRC
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You make very good points. Ground speed really is a pretty poor way to guage airplane performance. How do you think low pressure zones around the airplane effect the airspeed indicator? How different will the readings be if it is placed on a wingtip, versus, say directly behind the prop? I do not know the answer to these, I am truly curious.

Ryan Lefevre
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Old 11-02-2007, 08:52 PM
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Moonwalker
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Originally Posted by Ryan CSRC View Post
You make very good points. Ground speed really is a pretty poor way to guage airplane performance. How do you think low pressure zones around the airplane effect the airspeed indicator? How different will the readings be if it is placed on a wingtip, versus, say directly behind the prop? I do not know the answer to these, I am truly curious.

Ryan Lefevre
www.CommonSenseRC.com
The Go To Guys in Electric Power
Hi Ryan:

I think the accuracy would be adversely effected by the prop wash and any of the various air currents that swirl around and over the airframe. The speed measured is the differential between the air pressure at the pitot tube and the static port.

The manufacturer of this device advises on this and says to put the pitot tube on the wing infront of the leading edge or any other area where it will be exposed to new and undisturbed air. The static port should be placed such that it is not getting blasted by air. Personally, I mounted the pitot tube right out of the tip of the nose of a Funjet by about 1 inch so that it is the first portion of the plane that cuts through the air - no air disturbance by even the first mm of the nose. Initially, I had the static port in the fuse behind the ram air intake for cooling. It was getting partially blasted by air coming into the fuse during flight. The manufacture said to create a static tube by sealing the sensor and running the static tube out the side of the fuse so that it was being exposed to the oncoming air at 90 degrees. I found this too cumbersome. Therefore, I cut a "room" for the sensor in the bottom of the fuse and then taped over the sensor in the room such that the "door" was closed to the "room" where the static port (inside the loose shrink wrap of the sensor) was housed. It was not airtight but good enough such that the air rushing into the fuse simply bypasses the static port. This resulted in an increase of 20 mph on average compared with the earlier set-up where the static port was getting blasted with air. Others, have encapulsated the sensor with cotton balls and tape so that it does not get directly blasted with air but can still "breath".

Others with the same prop, batt. and motor combos on this airframe have replicated my results within a few MPH with the same Micrologger and airpspeed sensor. Regardless of the absolute accuracy of this set-up in MPH it provides a good baseline with which to test various combos. Interestingly, the Eagletree results of power versus speed accord well with the exponential power versus speed equation for a given airframe. For example, I hit 109 mph with 444 watts in 18 Celcius and 135 mph with 922 watts in -3 celsius (only 800 watts in my room which was 20 celcius).

With 444 watts at 109 mph the power versus speed equation predicted the power to get to 135 mph pretty close. That is, 135/109 = 1.2385 which cubed is 1.8997. 1.8997 x 444 watts is 843 watts.
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