(29-10-2012, 07:24 PM)Flying fisho Wrote: Great review Jason, sounds like you had loads od fun. Any idea of top speed (my topic of interest at the moment )

G’day Wayne,

I do have a pretty good idea of the speed they were doing, from the fastest pass I videoed, it’s around 154 kph.

My engines are running a bit rich, being still new, I didn’t want to risk them running too lean and overheating. There was no point in pushing them to absolutely flank speed for a bit of practice, I will save that for race day. : D

Because your interested in how fast models go (caught the speed bug ay ?? )

I should mention how I arrived at these numbers.

This is how the speed was measured. It’s the most reliable and accurate method I can find. Even tho’ it gives lower numbers than the GPS method I’m much more inclined to believe the results.

The speed was measured buy analysing the Doppler shift in the sound of the prop blades from a video/audio recording of the plane flying a low, close in, pass.

It should be a very accurate method of measuring the speed as it relies on the frequency difference between the plane approaching and departing. Any recording error that shifts one frequency should also shift the other the same amount and not affect the difference between the two in any way. The difference is what is used to measure the speed.

There is a small error tho’. The error one might expect is a cosine error but it’s very small and will always make the measurement on the lower side of what the speed actually is. How much error there is depends on how close the pass is and the straightness of the path that the model flies during the recording. If the model flies in a reasonably straight line for at least 50m approaching and departing the recording point and passes the recording point no further than 10 yards away then. The error should be less than 2% and that will mean a measured speed of at least 98% of the actual speed.

If you maintain the 50m straight line approach and departure and fly with in 15 meters of the recording point then. The error should be less than 5% and that will mean that the measured speed will be at least 95% of the actual speed.

You can do a little cross check of any measured speed by calculating the pitch-speed of your prop and multiplying it by 0.8 to allow for slippage. For a good prop will grip only about 0.8 of it’s pitch speed and a not so good one about 0.65

These numbers are affected by the drag of the air frame and allow some latitude either way. But for a quick cross check they are not too bad.

If I look at the frequency of the prop noise divide by two (two for two blades) you get the engine speed. Not suprisingly this is the same number as the frequency of the engines exaust note. around 335 Hz.

If we multiply this frequency by 60 to get Hz into RPM (cycles per second) into (Revs per minuet) we get around 20,000 RPM which is about right, considering when the plane is in the air the engine unloads a bit and revs harder than it does static running, on the ground.

A 10X6 prop reving at 20,000 rpm = 120,000 inches per minuet

120,000 X .0254 = 3048 meters per min

3048 X 60 / 1000 = 182 Kph

182 Kph X 0.8 = 146.304 (Pretty close to the measured 154 Kph)