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FliteTest P-38 Build - Prop Question

#1
Finishing up my P-38 and it appears to be awesome. First twin engine. One Newbie question - The motors are counter rotating. From the front, the Left rotates clockwise, the R rotates counterclockwise. The power pack comes with 2 sets of props, one is a "multirotor prop" and the other is a "multirotor pusher prop" with an "R" designation (I assume for "reverse"). My assumption is that the Right engine, rotating counterclockwise gets the "normal" prop and the one rotating clockwise gets the "pusher" or "reverse" prop. Is this correct. (all directions are viewing from the plane from the front).
 

Grifflyer

WWII fanatic
#2
Yes that is correct, the normal prop should spin CCW when viewed from the front and the R prop should spin CW when viewed from the front. Always make sure the numbers on the props are facing in the direction of travel.

Welcome to the forums!!!
 

Monte.C

Well-known member
#4
Which side should get which direction? I imagine you'd be pushing air either up or down on the horizontal stabilizer. Is that right? Which is preferred?
 

Monte.C

Well-known member
#12
I don't think it really matters, as long as the props are counter rotating I don't think you'll be able to tell a difference.
Agreed.
I found this complete explanation on another forum, https://www.rcuniverse.com/forum/el...-motor-aircraft-counter-rotating-props-3.html , scroll down to post #66.
At a high angle of attack, @ takeoff or in high alpha, the blade spinning downward experiences a greater airspeed, "biting more air", thus creating more thrust. This shifts the thrust axis a little towards that side off of center. Spinning the props downward at the fuselage makes it so the thrust axis is closer to the fuselage when at a high angle of attack. In full scale aircraft, if you lose an engine, then it makes you just a bit more recoverable/controllable under those conditions.
There you go.
And I agree with Grifflyer, for us it doesn't really matter.

Just the one other thing - I still suspect on some designs it'll affect your trim, either pushing up or down on the horiz. stabilizer. We'll see; I'm building a WWII twin engine medium bomber right now. :)
 
#13
I don't think it really matters, as long as the props are counter rotating I don't think you'll be able to tell a difference.
Well - there's a different prop (a reverse prop) that goes on the clockwise motor. As long as you get that right, it might not matter. BUT, to be clear, the recommended orientation as directed when I built my p-38 with differential thrust: is as follows:
LOOKING FROM IN FRONT OF THE PLANE: the right prop (which is actually the left engine of the plane) turns clockwise and has a reverse prop
the left prop (which is actually the right engine of the plane) turns CCW and has a standard prop
Check out the build video for the flitetest P-38 starting at 2 hours and 51 minutes into the video.
However, wikipedia notes the opposite (keep track of orientation from behind or from in front when reading) so I remain a bit confused tho my p38 flies great set up per the flitetest video. Wikepedia:


And from Wikipedia:
Counter-rotating propellers, also referred to as CRP, are propellers which spin in opposite directions to each other.[1] They are used on some twin- and multi-engine propeller-driven aircraft.
The propellers on most conventional twin-engined aircraft spin clockwise (as viewed from behind the engine). Counter-rotating propellers generally spin clockwise on the left engine and counter-clockwise on the right. The advantage of such designs is that counter-rotating propellers balance the effects of torque and P-factor, meaning that such aircraft do not have a critical engine in the case of engine failure.
Drawbacks of counter-rotating propellers come from the fact that, in order to reverse sense of rotation of one propeller, either the engines must be adapted to turn in opposite directions or one propeller must have an additional reversing gearbox.
 
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The Hangar

Well-known member
#14
Well - there's a different prop (a reverse prop) that goes on the clockwise motor. As long as you get that right, it might not matter. BUT, to be clear, the recommended orientation as directed when I built my p-38 with differential thrust: is as follows:
LOOKING FROM IN FRONT OF THE PLANE: the right prop (which is actually the left engine of the plane) turns clockwise and has a reverse prop
the left prop (which is actually the right engine of the plane) turns CCW and has a standard prop
Check out the build video for the flitetest P-38 starting at 2 hours and 51 minutes into the video.
Funny John does it that way cause Bixler said in the sea duck build video he thinks they fly better with the (looking from the front of the plane) the right motor spinning CCW and the left motor spinning CW. That’s why I always set mine up like to at, but like @Monte.C and @Grifflyer said, for our little models, it doesn’t really matter much. I need to replace a prop on my A-10 so l might just set it up they way you’re doing it and see if I can tell a difference at all.
 

Rhaps

Well-known member
#15
Interesting tidbit from Twin Mustang https://en.wikipedia.org/wiki/North_American_F-82_Twin_Mustang

"... In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control. This proved not to be the case when the aircraft refused to become airborne during its first flight attempt. After a month of work North American engineers finally discovered that rotating the propellers to meet in the center on their upward turn created sufficient drag to cancel out all lift from the center wing section, one quarter of the aircraft's total wing surface area. The engines and propellers were then exchanged, with their rotation meeting on the downward turn, and the problem was fully solved.
 

sprzout

Knower of useless information
Mentor
#16
If you want, check out the Sea Duck build. That was one of the first twin engine builds, and Josh went over the setup in a well explained video for setup. He even talks about the counter-rotation and why you want it. :)