warren52nz
Nov 08, 2005, 04:37 PM
I've worked out a way to measure your headspeed without a tachometer using the note that comes off the blade tips.
First of all you need a calibrated tone generator which you can get from:
http://mda-vst.com
I't's called "Wavetool" and you run "Sig-Gen.exe" for this exercise. It's Freeware.
Set the Sig-Gen to "Sine", -6dB and 85 Hz and play it out your PC speakers. If your speakers can't reproduce that note then you could try headphones. Run up your heli on the bench or in your hand with zero pitch and compare the note that comes off the tip of the blades (watch your ears!). Adjust the Sig-Gen by typing in new values in the "Hz" field until the note from the heli and the PC are the same. Adjust the PC speaker level until the two notes are about the same volume.
When you're close, you'll hear a "beat" or a wavering in the volume of the note as the two notes interfere with each other. When the beat frequency drops off to zero (no changes in volume), you're right on the note.
Read the "Hz" value off the screen and multiply it by 30 (for a two blade helicopter). What we're doing is changing "Hz" (cycles/second) into rpm (cycles per minute). To do this we multiply by 60 (seconds in a minute) and divide by two because there are two blades contributing to the note.
So 85 Hz equates to 2,550 rpm. If you know your headspeed, approximately, you can divide that by 30 to get into the ballpark on the Sig-Gen. I've confirmed the method by comparing my results to a digital tach although it's pretty trivial physics and doesn't really need proving.
Hope someone else finds this useful. Thanks to whoever wrote the Sig-Gen program.
First of all you need a calibrated tone generator which you can get from:
http://mda-vst.com
I't's called "Wavetool" and you run "Sig-Gen.exe" for this exercise. It's Freeware.
Set the Sig-Gen to "Sine", -6dB and 85 Hz and play it out your PC speakers. If your speakers can't reproduce that note then you could try headphones. Run up your heli on the bench or in your hand with zero pitch and compare the note that comes off the tip of the blades (watch your ears!). Adjust the Sig-Gen by typing in new values in the "Hz" field until the note from the heli and the PC are the same. Adjust the PC speaker level until the two notes are about the same volume.
When you're close, you'll hear a "beat" or a wavering in the volume of the note as the two notes interfere with each other. When the beat frequency drops off to zero (no changes in volume), you're right on the note.
Read the "Hz" value off the screen and multiply it by 30 (for a two blade helicopter). What we're doing is changing "Hz" (cycles/second) into rpm (cycles per minute). To do this we multiply by 60 (seconds in a minute) and divide by two because there are two blades contributing to the note.
So 85 Hz equates to 2,550 rpm. If you know your headspeed, approximately, you can divide that by 30 to get into the ballpark on the Sig-Gen. I've confirmed the method by comparing my results to a digital tach although it's pretty trivial physics and doesn't really need proving.
Hope someone else finds this useful. Thanks to whoever wrote the Sig-Gen program.