Author Topic: Balance for Electrics? (Read 8035 times)

FLOYD CARTER · « **on:** February 22, 2015, 11:21:35 AM »

I can't decide if this should go in "Design" or "Electrics". I have read that the CG on an electric stunter should be more forward than on the identical glo powered model. I don't understand the dynamics of this theory.

My electric stunters get balanced after test flights, based on how it is reacting. Eventually I have found the correct CG and line guide setting. I don't know if this would be different if I took the electric out and installed a glo.

I guess that people who have built a kit intended for glo, and have installed electrics, have found the CG shown on the plans is too far aft for electrics. All my electric stunters are my own design, so I haven't noticed any such change.

Floyd

Motorman · « **Reply #1 on:** February 23, 2015, 11:44:49 AM »

Electric doesn't sag so you get more prop blast over the controls making them more effective so more nose weight balances the effect.

MM

Dennis Toth · « **Reply #2 on:** February 23, 2015, 01:47:16 PM »

Floyd,
I believe the idea is that because there is no shift in CG from beginning to end of flight the controls are more consistent. Also, the electric power comes up a little sooner then with glow. It keeps your speed stays up through the maneuver and the ship can handle the CG in a more forward location. Some flyers feel this can reduce the hop coming off corners. I personally feel that if the ship is stable with the rearward CG you need less control input to turn and that helps keep the speed up even more. Bottom line is balance it where you can fly the smoothest then practice, practice, practice.

Best, DennisT

Phil Krankowski · « **Reply #3 on:** February 24, 2015, 12:33:41 PM »

It would be nice if someone had some near identical designs in electric and glow and could post side by side after trimming.
Phil

Brett Buck · « **Reply #4 on:** February 24, 2015, 05:48:34 PM »

Quote from: Phil Krankowski on February 24, 2015, 12:33:41 PM

It would be nice if someone had some near identical designs in electric and glow and could post side by side after trimming.
Phil

Paul Walker has done this. His CG wound up further forward and the leadouts further back:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=392235&mesg_id=392235&page=#392246

Note also an interesting side topic on precession.

Brett

Phil Krankowski · « **Reply #5 on:** February 25, 2015, 06:26:54 AM »

Quote from: Brett Buck on February 24, 2015, 05:48:34 PM

Paul Walker has done this. His CG wound up further forward and the leadouts further back:

http://www.clstunt.com/htdocs/dc/dcboard.php?az=show_topic&forum=103&topic_id=392235&mesg_id=392235&page=#392246

Note also an interesting side topic on precession.

Brett

Thank you Brett.
Phil

Igor Burger · « **Reply #6 on:** March 05, 2015, 01:22:36 AM »

Quote from: FLOYD CARTER on February 22, 2015, 11:21:35 AM

I can't decide if this should go in "Design" or "Electrics". I have read that the CG on an electric stunter should be more forward than on the identical glo powered model. I don't understand the dynamics of this theory.

The main reason was already mentioned, electric has all the time "full tank" so if you compare with empty tank on IC, then average CG position with 1/2 tank will move CG aproximately to the same place.

BUT ... there is also another reason, ICs of the same size and weight have usually larger props making more stabilizing effect (like flywheel) so electric with smaller prop will need little more forward CG. This effect knows everyne who went to light hollowed carbon props. If I change 27g prop to the same size 16g prop, model turns much much easier to corners :- )) ... so it will need little nose weight for the same flying (actually we solve it by battery position).

FLOYD CARTER · « **Reply #7 on:** March 14, 2015, 12:15:42 PM »

The two electrics I now have use a 12-8 prop (the heavy ones). A glo engine of the same power (ST51) would use the same prop, but maybe lower pitch. The electric stunters I have seen (and copied) are not running any smaller props.

I'm still waiting for a light weight electric prop. The CF props now made are all too big.

Floyd

Tim Wescott · « **Reply #8 on:** March 14, 2015, 01:45:41 PM »

Quote from: FLOYD CARTER on March 14, 2015, 12:15:42 PM

I'm still waiting for a light weight electric prop. The CF props now made are all too big.

I think the expectation is that if you're a big enough boy to buy a CF prop, then you're perfectly capable of whacking it down and repitching it as necessary. If you don't feel up to it you might be able to talk Alan or Chris into selling you one that's pre-whacked, or getting someone else in the PNW stunt community to do the job for you.

Of course, I think that part of the attraction of tweakable props is that you can tweak them again and again to get exactly the right combination.

Quote from: FLOYD CARTER on March 14, 2015, 12:15:42 PM

The two electrics I now have use a 12-8 prop (the heavy ones). A glo engine of the same power (ST51) would use the same prop, but maybe lower pitch. The electric stunters I have seen (and copied) are not running any smaller props.

If the in-air RPM is inversely proportional to pitch, then your electrics would still be showing less gyroscopic effect. I can't remember if the gyroscopic effect is proportional to the gyro wheel's rotational rate, or that rate squared, but the faster you spin the wheel (propeller, in this case), the more gyroscopic effect you'll experience.

Brett Buck · « **Reply #9 on:** March 14, 2015, 04:20:52 PM »

Quote from: Tim Wescott on March 14, 2015, 01:45:41 PM

I think the expectation is that if you're a big enough boy to buy a CF prop, then you're perfectly capable of whacking it down and repitching it as necessary. If you don't feel up to it you might be able to talk Alan or Chris into selling you one that's pre-whacked, or getting someone else in the PNW stunt community to do the job for you.

Of course, I think that part of the attraction of tweakable props is that you can tweak them again and again to get exactly the right combination.

If the in-air RPM is inversely proportional to pitch, then your electrics would still be showing less gyroscopic effect. I can't remember if the gyroscopic effect is proportional to the gyro wheel's rotational rate, or that rate squared,

I*omega

The in-flight RPM is not inversely proportional to pitch*, that would only be the case if it was screwing its way through a solid like wood screw, which it is not.

Brett

* true pitch, not measured pitch.

Russell Bond · « **Reply #10 on:** March 14, 2015, 07:29:34 PM »

Floyd,
Check out the three blade props Igor sells. 16 grams!

There is the 12 x 5 and the 11 x 5.
I use the 12 x 5 on my heavier model.

Tim Wescott · « **Reply #11 on:** March 14, 2015, 07:37:57 PM »

Quote from: Brett Buck on March 14, 2015, 04:20:52 PM

The in-flight RPM is not inversely proportional to pitch*, that would only be the case if it was screwing its way through a solid like wood screw, which it is not.

Well, yes, OK, fine. But if you go down in pitch then you'd better go up in RPM to make up for it, all else being equal.

Brett Buck · « **Reply #12 on:** March 17, 2015, 10:04:37 AM »

Quote from: Tim Wescott on March 14, 2015, 07:37:57 PM

Well, yes, OK, fine. But if you go down in pitch then you'd better go up in RPM to make up for it, all else being equal.

Certainly. To the degree we care about it, and over the range we use in practice, you can do some "back-of-the-envelope" calculations assuming a proportional RPM.

For the most part, precession in yaw, torque or spiral slipstream, whatever, is far down in the noise compared to trim variations/errors, and people tend to attribute problems they have with trim to these sorts of issues, particularly if there has been a recent discussion of the topic!

Brett

Dennis Toth · « **Reply #13 on:** March 25, 2015, 08:05:14 PM »

The problem we have in ECL is battery sizing available to us. At times we have the equivalent of having a 5 oz tank and to have the better performance we need 5 1/2 oz. In order to get that extra 1/2 oz of juice we have to put in a jump to 6 oz. The next step up battery can cost you 1 - 2 oz in dead extra battery weight. In order to have the needed reserve and keep the weight in check we run slightly slower lap times and run slightly higher pitch props at lower rpm. The higher rpm option that IC's have cost us a lot in amps drawn. Once batteries get a little better we will be able to get the low pitch/ high rpm setups with the 3 - 4 inch pitch range which will really add to the performance of the electrics.

Best, DennisT

Author Topic: Balance for Electrics? (Read 8035 times)

FLOYD CARTER

Balance for Electrics?

Motorman

Re: Balance for Electrics?

Dennis Toth

Re: Balance for Electrics?

Phil Krankowski

Re: Balance for Electrics?

Brett Buck

Re: Balance for Electrics?

Phil Krankowski

Re: Balance for Electrics?

Igor Burger

Re: Balance for Electrics?

FLOYD CARTER

Re: Balance for Electrics?

Tim Wescott

Re: Balance for Electrics?

Brett Buck

Re: Balance for Electrics?

Russell Bond

Re: Balance for Electrics?

Tim Wescott

Re: Balance for Electrics?

Brett Buck

Re: Balance for Electrics?

Dennis Toth

Re: Balance for Electrics?