Author Topic: Tale of Two Planes (Read 1550 times)

Motorman · « **on:** May 01, 2015, 06:22:02 PM »

I've been playing with props, rpm and lap times and came across something. My Banshee and my Tanager both have the same system on them with the exception the Tanager has an Ice Lite 50 and the Banshee has an Edge lite 50 also, one has an external start button on the Hubin timer and the other doesn't. Everything else is the same prop, lines, motors and flew them same day. The Banshee was set for 10,320 RPM and the Tanager for 9,570. The larger Tanager was faster 5.00 sec vs 5.46 sec for the Banshee.

After flying I checked the rpm with a tach and found the Tanager was set for 9570 but turned 9990. The Banshee was set for 10,320 but turned 9390. I bumped the Banshee up to 10,960 and it tached 9960 but didn't have enough battery power to fly it again.

Do you find this kind of discrepancy from one timer to the next normal or is it some kind of defect?

Thanks,
MM

John Cralley · « **Reply #1 on:** May 01, 2015, 09:49:42 PM »

You are little short on detail! What batteries are you using? Are they 4 cell? What is the motor KV? Are those Rpms in/near the sweet spot for that KV and battery combination? Are you sure that the ESCs are programmed the same? For that matter what mode are you using in your ESC program? Are the timers FM-9s and if so what program are you using to set the Rpm?

If you can check the pulse width put out by the timer, then program each timer to the same Rpm and see if you get the same pulse width. If they are identical pulse widths, then the difference is in the ESC or the motors.

Some motors have been reported to have KVs that are quite a different from what the manufacturer/vendor report in their specs, so the tachometer is the proof of what you are actually getting from your setup.

If you swap the motors you can determine if the difference is in fact related to the motors being different (i.e.different KVs).

The remote start button is irrelevant.

Robert-Jan · « **Reply #2 on:** May 02, 2015, 07:52:48 AM »

KV has nothing to do with the RPM.
KV gives you an indication of the maximum RPM with the used battery.
The number of poles is relevant to the RPM.
If you run in govenor. The ESC will set the electrical RPM fixed. If you have more poles the actual RPM will be lower.

Greetings Robert-Jan

John Cralley · « **Reply #3 on:** May 02, 2015, 09:08:15 AM »

Right Robert-Jan. Don't know what I was thinking.

Don't think that the pole count would be different in this case.

Bob Hudak · « **Reply #4 on:** May 02, 2015, 09:13:30 AM »

Demon drag and airplane weight are also to be held accountable for some of the discrepancy you notice.
Bob

Motorman · « **Reply #5 on:** May 02, 2015, 09:11:49 PM »

Flew the Banshee today. Now that I matched the rpm with a tach they are very close in speed. Still strange how the rpm setting is much higher.

John, My question was kind of a general one. Do governor RPM setting usually match actual RPM numbers, How much variation is normal?

Thanks,
MM

Keith Renecle · « **Reply #6 on:** May 04, 2015, 11:38:54 PM »

Hi MM, your timer system and esc are setting the governor rpm by means of the throttle pulse width, and that obviously (from what you say) does not relate directly to the rpm. I would guess that you are using the FM-9 system plus a CC esc? The Jeti Spin esc's used with their Jeti box programmer, uses a reference number in the esc program to get the rpm to within 50 revs of the set reference. That works very well, but is not easy to do with a throttle pulse width system. You have to then compensate by using a tach to get the rpm spot-on and once that is set then you can use your programmer to add or subtract rpm from that point.

Keith R

Author Topic: Tale of Two Planes (Read 1550 times)

Motorman

Tale of Two Planes

John Cralley

Re: Tale of Two Planes

Robert-Jan

Re: Tale of Two Planes

John Cralley

Re: Tale of Two Planes

Bob Hudak

Re: Tale of Two Planes

Motorman

Re: Tale of Two Planes

Keith Renecle

Re: Tale of Two Planes