Mailing List flyrotary@lancaironline.net Message #12806
From: Charlie England <ceengland@bellsouth.net>
Subject: Re: [FlyRotary] Re: Rusty's Engine Mount (and Ed's too)
Date: Thu, 11 Nov 2004 13:21:42 -0600
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Russell Duffy wrote:

OK, now that's just not fair.  We "turbo" guys still are not catching you as it is...  I guess it's time to consider the C-drive as well.  Rusty, how did your engine mount end up?  Did you put in the shims, or is your mount askew in the wrong directing?  Ed, same question.  I was hoping you meant that you were going to abandon the turbo, but having a C drive AND a turbo would certainly be interesting.    My mount is still offset to the left (wrong direction).  I could shim it to straight, but not to a right offset.  After flying it a while, and talking to Tracy (no offset in his mount), I'm leaning toward leaving mine like it is.  Tracy's description of rudder usage on takeoff is the same as mine, so the best I can do is lessen the problem a little.  In cruise, it really doesn't take much of a trim tab to take out all the yaw, and my feeling is that this isn't any more than many RV's have anyway.  In other words, I'm talking myself into believing that the only time the offset is causing me a problem is during takeoff, and climb.  Keep in mind though that this is a significant issue.  I can't use full throttle until I get a fair amount of airspeed to make the rudder effective.  Even then, it's a handful (or should I say legful).  The real question is how much of the problem is from the wrong offset, and how much is just from the torque on that long prop.  It could be that no amount of correct offset would make up for putting that much power/prop on an RV-3.
snipped
Rusty (spending my day off with State Farm, and the dentist)

My money would be on the prop diameter, rather than mount offset.

FWIW, I've flown Globe Swifts with 200hp Lyc & 210hp Continental. After quite a few hours in the Lyc, I flew a friend's Cont powered & it had a *much* stronger desire to hunt runway lights. 5% hp difference but 200% more rudder needed on takeoff. I attribute that to a bigger prop on the Cont.

I would never give up any diameter available to me if I'm forced to use a fixed pitch prop. Vance Jaqua (sp?) published a chart on the 'other' list a few years ago showing efficiency curves for 72" & much smaller dia props a few years ago. the large prop doesn't lose out to a smaller one until over 200mph but eats the smaller one alive on takeoff.

If RV's have any shortcoming for me, it's that they don't have enough rudder. How about taller vert stab & rudder tips? Jim Ayers did that on his RV-3 & was able to put his comm antenna in the VS tip.

Charlie




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