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Author Topic: Does "downthrust" become "upthrust" when using a pusher prop?  (Read 1896 times)

Offline Peter Germann

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Does "downthrust" become "upthrust" when using a pusher prop?
« on: January 31, 2011, 04:39:08 AM »
Having used 0.5° motor downthrust (and + 0.5° stab incidence) on a couple of PA.75 airplanes during the last couple of years, I've built my new electric airplane the same. (see "Signorina Elettra" in "List your setup")
Up comes the trend to use clockwise running pusher props.
Do I now have to adjust my motor thrustline to 0.5° up? It would be fairly easy to do as I have left open an air inlet around the spinner.
Thanks for comments, Peter Germann
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Peter Germann

Offline Dean Pappas

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #1 on: January 31, 2011, 10:29:24 AM »
Hi Peter,
"Miss Electric" ... I like it.
The reasonable conjecture is that you will have to reverse these adjustments.
We eagerly await the results of your test: possibly the first of its kind to be published!

Regards,
   Dean P.
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Offline Igor Burger

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #2 on: January 31, 2011, 01:23:26 PM »
Simple answer is YES ... and more detailed answer will be .. as well as everything else  VD~

Seriously ... when I tried pusher, I needed to change so many things, that I better went back to tractor. I think pusher needs redesign of whole fuselage and there will be still some features which we cannot change, for example landing gears. But there are still some advantageges of pushers. Question what is really better ...we will see after a time :-)
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Online Dennis Toth

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #3 on: January 31, 2011, 03:04:50 PM »
Peter,
My ships are set for zero-zero so going to the pusher was a little easier.The first thing first thing I noticed with the pusher was that the ship would hug the ground a little longer on takeoff. I am told that this has to do with the prop swirl being reversed. Some of the free flight guys use to use the swirl to circle the ship one direction during power launch and then transition to the other direction for the glide. The next thing was the very secure feel in the outside maneuvers particularly the outside portion of the square 8 and hourglass. For me I like that the precession move the nose out on the outside turns, it may be all psychological but its just been easier for me to handle these maneuvers.

Best,           DennisT
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Offline Peter Germann

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #4 on: February 01, 2011, 03:40:12 AM »
Quote from: Igor Burger on January 31, 2011, 01:23:26 PM
Simple answer is YES ... and more detailed answer will be .. as well as everything else  VD~

Seriously ... when I tried pusher, I needed to change so many things, that I better went back to tractor. I think pusher needs redesign of whole fuselage and there will be still some features which we cannot change, for example landing gears. But there are still some advantageges of pushers. Question what is really better ...we will see after a time :-)

Thanks, friends.

Igor, does "everything else" include stab incidence? If so, then I am afraid that such a change might be to complicated to do and would have to wait for the time when I build the next "electric lady".
Meanwhile, I will put +0.5° upthrust in and see what it does to me. Wheather permitting, its 0° C outside and smells snow, I gladly will report asap here.
regards, Peter
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Peter Germann

Offline Igor Burger

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #5 on: February 01, 2011, 03:48:28 AM »
No, it started hunting, but I cannot change stab incidence, and I also did not want to change elevator to flap position in neutral, so I solved it only on handle, it worked in calm, but any small wind caused hunting again, so I think it will need also another incidence, but you can substitue stab incidence by engine thrust offset to some extent. Effect of downthrust and incidence is exchangable to some extent. You will see difference only in wind.
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Offline Peter Germann

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #6 on: February 07, 2011, 09:55:40 AM »
Coming back from the field, here is the report announced last week:

Data:
"Signorina Elettra"  (63oz 666 in/sq)
C.G.: 20% MAC or 6.5in from flaps hingeline fwd.
55°F at 1'150 ft/msl
Lines 64ft x 0.015in eye-eye
5.5 sec/lap
Duration, end of spool-up until shutdown: 5:30
Motor: AXI 2826/12
ESC: CC 75 ICE in high gov mode
Timer: FM-9 in New Phoenix High Mode
Batterie: 2'600 mA/h  5S
Prop: APC 13 x 4.5 E P
RPM: 9'500
Min. Power: 290W    max Power: 540W
Initial voltage: 20.9 V    End of flight voltage:18.1V (3.62V/cell)
Consumption: 1'840 mA/h (70%)
Recharge time at 1.5 C: 35 Min.

Findings re "Upthrust"and reverse running prop

I did 6 flights today with 0.5° motor upthrust and (approx.) 0.2° positive stab incidence. Very light wind and some turbulence caused by nearby shrub. What I, at this point in time, have found is:

The airplane feels stable w/o any hunting tendency and still turns very (perhaps to-) sharp. (Had to reduce handle spacing)

Line tension goes back in insides and gets stronger in outsides. The effect is well noticable in the upper loop of the vertical eight and in turns 2 and 3 of the hourglass. I suspect that the benefit of effect, outweighs the getting a bit lighter in the insides and will carry on flying the pusher for the time being.

Next I plan to bring fwd. the C.G. a bit in order to allow me to go back to my normal 4in handle spacing

rgds. Peter Germann


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Peter Germann

Offline Keith Renecle

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #7 on: February 08, 2011, 12:31:57 AM »
Hi Peter,

Thanks for posting your very interesting test results. I also experienced extra hunting with my Electron using the pusher prop (12 x 6 APC), but I have not yet had the time to investigate more. My Electron is an in-line design similar to the Yatsenko models. I did enjoy the extra line tension in the hourglass, where my model tends to lose it in bad weather, so I will try to trim the problem out.

Just a point about the sharp turn rate that you mention. With electric power, the main weight is no longer the engine up front, it is the battery and this is closer to the CG. Therefore the barbell effect is not there and the turn rate is much faster with less overshoot as well. To me, this is one of really good benefits of e-power. Just by the way, your model is a most interesting design as usual. Well done!

Keith R
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Offline John Sunderland

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #8 on: February 08, 2011, 01:23:36 AM »
I posed this question awhile back and they all said " I dunno, let us know what you find out!" :o...My next thought was OK this may require some design reversal and consideration!
Then I thought, forget it...it is easier to change back to tractor. :P
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Offline Peter Germann

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Re: Does "downthrust" become "upthrust" when using a pusher prop?
« Reply #9 on: February 08, 2011, 03:06:54 AM »
Quote from: Keith Renecle on February 08, 2011, 12:31:57 AM

Just a point about the sharp turn rate that you mention. With electric power, the main weight is no longer the engine up front, it is the battery and this is closer to the CG. Therefore the barbell effect is not there and the turn rate is much faster with less overshoot as well. To me, this is one of really good benefits of e-power. Just by the way, your model is a most interesting design as usual. Well done!

Keith R

Thank you for mentioning the mass moment of inertia, Keith, I have not thought of what you call the "barbell effect".
With the nose being 265 mm (10.4") long (spinner-backplate), the 330 Gr battery sits approx. at the same place as would a 220 cc tank (216 Gr when 3/4 full). For the AXI being approx. 160 Gr. lighter than a PA .75, while being equidistant to the C.G., the difference must be significant. While this explains the airplane's readiness to turn sharp without, as you say, a great tendency to overshoot, I still wish to make the controls feel a touch more solid.
As the motor temperature after landing is way below 50°C (120°F) I feel safe to replace the current open tip spinner by a heavier metal type which will bring the C.G. forward by approx. 5 mm while at the same type bringing back a bit of the mass moment of inertia "lost". Cranking in some crosstrim (flaps up + elev.down), just like in accurate formation flying, might help, too.
rgds Peter


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Peter Germann

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