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Oct 04, 2006, 06:07 PM
Speed Demon
GregG's Avatar
Thread OP
Discussion

Measuring 30,000+++ RPM


With the current crop of BL motors turning upwards of claimed 60K RPM, it gets interesting to try and get an accurate RPM reading on a direct drive prop. Looking through the ads on a few of the hobby supplier web pages I've seen tachometers that measure 29K-32K RPM.

I know that the measurement can be taken with one stripe on the spinner and the reading doubled. However that just isn't the easiest way to do it. What do you use to measure higher RPM's?

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Oct 04, 2006, 06:43 PM
I don't have an answer, but another question on your subject. Would you actually run a prop at 60,000 RPM direct drive? I understand some of the faster pylon planes are running 44,000 RPM with something like the APC carbon reinforced. By the way, where do you buy these special strong props?
Oct 04, 2006, 07:49 PM
hot air rises...
jfinch's Avatar
Record the spinning prop and use spectrogram: https://www.rcgroups.com/forums/show...85&postcount=8
Oct 04, 2006, 09:35 PM
Registered User
davidfee's Avatar
I use an old Futaba FP-DT1 digital tach, which goes to 60k RPM (+/- 200).

http://www.futaba-rc.com/manuals/dt1-manual.pdf
Oct 04, 2006, 09:46 PM
Castle Tech
Joe Ford's Avatar
Quote:
Originally Posted by davidfee
I use an old Futaba FP-DT1 digital tach, which goes to 60k RPM (+/- 200).

http://www.futaba-rc.com/manuals/dt1-manual.pdf
Ditto! One of the guys at the flying field gave it to me when I first started R/C.
Oct 04, 2006, 11:13 PM
Caution:Makes sharp left turns
Troy's Avatar
My supposedly cheapo Magnum tach has given me accurate numbers up to almost 40,000RPM. The only time I ever have issues getting a good reading is in low light but a simple flashlight solves that quick....that works for indoor fluorescent lighting, too.

dessertracer, nobody is using anything exotic for props. The guys who are running up to 40K RPM are running very small diameter and so the rotating inertia is lower like a ducted fan. What scared me was spinning a 7.5" prop at 37K...
Oct 04, 2006, 11:42 PM
Castle Tech
Joe Ford's Avatar
Quote:
Originally Posted by Troy
What scared me was spinning a 7.5" prop at 37K...
You sir are NUTS!!! Glad to know I'm in the right place.
Oct 05, 2006, 02:16 AM
Registered User
Code:
What scared me was spinning a 7.5" prop at 37K..
I am pleased to hear that you have achieved a Mach 1.1 tip speed.
Oct 05, 2006, 02:20 AM
Registered User
ezmo's Avatar
I assume 60000rpm is the max witouth prop, then stuff begins to rip apart in motor???
Oct 05, 2006, 06:20 AM
Speed Demon
GregG's Avatar
Thread OP
Quote:
Originally Posted by ezmo
I assume 60000rpm is the max witouth prop, then stuff begins to rip apart in motor???
The 60K+ RPM reference is using other peoples claims in this forum. I have no idea if it's with a prop or without. I just wondered how they measured the RPM.
Oct 05, 2006, 09:31 AM
And You're Not
The TNC Tach will read 60K RPM without any modifications.
Oct 05, 2006, 09:56 AM
Caution:Makes sharp left turns
Troy's Avatar
Quote:
Originally Posted by David H
I am pleased to hear that you have achieved a Mach 1.1 tip speed.
It hurts your ears.... it's also terribly inefficient.

60K RPMs are most likely referring to motor RPM before the gearbox; F5B motors for example.
Oct 05, 2006, 11:23 AM
I plan to take over world
sayno2glo's Avatar
It would be better to change to lower kv motor and bigger prop. Better to stay under 0.85M as the drag will grow about tenfolds when going to 1.1Mach
Oct 05, 2006, 11:55 AM
Flying motor mount master
fly_boy99's Avatar
Man I thought I was nuts to run up props in the 30k range withing
a few feet of myself.

If I went above 35k then I would place the entire setup in a back room
with a sliding glass door, grab the TX, hook up the Micrologger and
find out the data after I pull pieces of prop out of the wall.

B

p.s. I have had a blade thrown from a folder recently and it's very
humbling.
Oct 05, 2006, 12:42 PM
I plan to take over world
sayno2glo's Avatar
I just did a school practice which is very interesting conserning this subject.
First pic is with Mach 0.65 and you see it's right at the limit of braking the speed of sound at low pressure point.
Second pic is at Mach 0.9 and the drag coefficient has risen from 0.009 to 0.08.
The profile is at 3 deg alpha and it's 10% thick. That alpha is about the same as props should have.


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