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Jimmy Young
Joined: 24 Nov 2007
Posts: 182
Location: Missouri City, TX
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 Posted: Mon Apr 13, 2009 5:51 am Post subject: engine offset mount question |
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I have a question regarding offsetting an engine to compensate for prop wash on rudder. Currently it takes a lot of right rudder to roll out straight on take off. If I have a left to right crosswind, it takes full hard right rudder to maintain a straight track. If I have a right to left crosswind, very little rudder is required to keep her straight. My engine is offset slightly to the left right now. I have about 3.75" from the prop to the aileron tube on the pilot's left side, and about 4.75" on the right. Is this set up helping the situation or making it worse? It seems to me since the engine is behind the CG, it would make it worse. My prop turns counter-clockwise looking at it from behind the airplane. I do have a rudder tab on the left side, and the plane flys straight & level with very little stick input once up. Any thoughts on this?
Thanks -
Jimmy Y
FS II, 95 hrs on the Generac V-twin
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Jimmy Young
Missouri City, TX
Kolb FS II/HKS 700 |
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lucien
Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM
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| Posted: Mon Apr 13, 2009 6:28 am Post subject: Re: engine offset mount question |
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| Jimmy Young wrote: | I have a question regarding offsetting an engine to compensate for prop wash on rudder. Currently it takes a lot of right rudder to roll out straight on take off. If I have a left to right crosswind, it takes full hard right rudder to maintain a straight track. If I have a right to left crosswind, very little rudder is required to keep her straight. My engine is offset slightly to the left right now. I have about 3.75" from the prop to the aileron tube on the pilot's left side, and about 4.75" on the right. Is this set up helping the situation or making it worse? It seems to me since the engine is behind the CG, it would make it worse. My prop turns counter-clockwise looking at it from behind the airplane. I do have a rudder tab on the left side, and the plane flys straight & level with very little stick input once up. Any thoughts on this?
Thanks -
Jimmy Y
FS II, 95 hrs on the Generac V-twin |
Sounds familiar. My FSII required a lot of left rudder on the takeoff roll (mine had the 503 with the prop turning right-hand, swinging the big ol 68" WD 3 blade on a 3.47 C box) but didn't have a lot of torque problems in the air.
Mine also didn't have any offset in the mount, but I should think that would help. Perhaps not.
I mitigated the problem by just applying power slower especially if there was a crosswind so the torque wouldn't overcome the rudder. By the time the tail came up I could go to wide open without having to hold as much left rudder pressure.
Soft-field takeoffs where I held the tail down longer with back pressure on the stick, tho, would still require the hefty left rudder until I got to flying speed.....
LS
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LS
Titan II SS |
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John Bickham
Joined: 10 Jan 2006
Posts: 170
Location: St. Francisville, LA
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| Posted: Tue Apr 14, 2009 4:46 am Post subject: Re: engine offset mount question |
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Hey Jimmy,
I'm not an expert on this at all. But my experiences are similar to Lucien's. I'm of the opinion it has more to do with the roll of the prop wash hitting one side of the rudder more than the other.
I can get into a jam when I apply throttle very quickly requiring a lot of right rudder to correct. Rudder wash push, at slow roll speed with little rudder authority. Looks like the 2-strokes push one way and the 4-strokes push the other due to prop rotation.
I guess I'm saying I got the same issues. If it where me and you have the runway length, experiment with slower throttle input and see how that comes out. I start crooked when I'm doing max TO's and get ahead of it with early rudder input. Usually have enough rudder authority when I get back to the centerline.
I would leave it set for best cruise if it where me.
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Thanks too much,
John Bickham
Mark III-C w/ 912UL
St. Francisville, LA
I know many pilots and a few true aviators. There is a distinct difference that I have the greatest respect for. |
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Jimmy Young
Joined: 24 Nov 2007
Posts: 182
Location: Missouri City, TX
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| Posted: Tue Apr 14, 2009 6:55 am Post subject: Re: engine offset mount question |
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Lucien & John-
Thanks for your input. That is pretty much how I handle it now, by easing in to full power. I also keep the tail on the ground a bit longer and it helps pin the rear from going right on me.
My main question though is, on a pusher set up if an engine is offset to the left a bit as mine is now, does that promote the plane to turn more to the right or left, since the engine is behind the center of gravity?
On another topic; John, I wish I could take a couple of days off right now. I would fly over to Nauga to meet the Wandering Wench and her wingman! I know they will have a great time at your strip. I'm going to Sun & Fun next week and hope to meet her there!
Jimmy Y
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Jimmy Young
Missouri City, TX
Kolb FS II/HKS 700 |
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John Hauck
Joined: 09 Jan 2006
Posts: 4639
Location: Titus, Alabama (hauck's holler)
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| Posted: Tue Apr 14, 2009 7:07 am Post subject: engine offset mount question |
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I'm going to Sun & Fun next week and hope to meet her there!
Jimmy:
If you see me at Lakeland, ask me about experimentation with off set engines
on Kolbs. Be glad to share what I learned from that. Also, for the same
price, I'll share with you what I learned from off setting the leading edge
of the upper vertical stabilizer.
john h
mkIII
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John Hauck
MKIII/912ULS
hauck's holler
Titus, Alabama |
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lucien
Joined: 03 Jun 2007
Posts: 721
Location: santa fe, NM
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| Posted: Tue Apr 14, 2009 7:14 am Post subject: Re: engine offset mount question |
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| Jimmy Young wrote: | Lucien & John-
Thanks for your input. That is pretty much how I handle it now, by easing in to full power. I also keep the tail on the ground a bit longer and it helps pin the rear from going right on me.
My main question though is, on a pusher set up if an engine is offset to the left a bit as mine is now, does that promote the plane to turn more to the right or left, since the engine is behind the center of gravity?
On another topic; John, I wish I could take a couple of days off right now. I would fly over to Nauga to meet the Wandering Wench and her wingman! I know they will have a great time at your strip. I'm going to Sun & Fun next week and hope to meet her there!
Jimmy Y |
It should, as we did that all the time on our r/c planes to counteract torque (tho that was in a tractor config).
But I'm going along with the theory that the yawing tendency is due to the rotating downwash acting on the rudder rather than torque on FS II. Mine yawed the opposite way you'd expect if it were torque (right hand prop = left hand torque should = yaw to the left). So I'm not sure that offset is going to be very affective (it doesn't seem to be in your case?).
Instead you want to reduce the angle of the prop wash on the rudder and the easiest way to do that is to be moving through the air faster when the prop is turning the fastest. I.e. slower application of power.
That's what I did on my FS II. And actually, the only time I had to put the left pedal to the floor or close to it was going to WOT right away. Getting some forward speed first always reduced that a lot and made it easily manageable. There wasn't really any flight situation I ever got in where I needed full power immediately anyway so that technique seemed to be the best cure....
LS
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LS
Titan II SS |
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