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  #1  
Old 10-17-2019, 03:43 PM
cduster cduster is offline
 
Join Date: Aug 2019
Location: Hertford, NC
Posts: 21
Default Engine Angle

Some reading, including a bit aimed at Reno types, got me questioning engine mount offset. I guess the conventional wisdom is that straightening the engine angle out and perhaps setting it with the slightest down angle has the potential to increase speed and handling qualities with only a slight increase in rudder requirements on takeoff.

Is there a prevailing opinion on the optimum left/right offset? How about on the vertical plane?

Every picture that I can find of a Rocket engine mount looks completely stock but mine came to me with a 1/8" washer between the mount and the Lord vibration isolators on the lower mount points. I assume this might be to center the spinner and I can't help but wonder if it is having any detrimental effect.
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  #2  
Old 10-17-2019, 03:47 PM
Tom Martin Tom Martin is offline
 
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Location: Ontario, Canada
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Default

With time the lower isolators compress, which drops the motor, the washers bring it back to the original setting.
It may be time to replace the isolators. Have a aircraft mechanic take a look at them.
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  #3  
Old 10-17-2019, 07:08 PM
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F1Boss F1Boss is offline
 
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Location: Taylor Texas
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Default Yes

Setting the engine straight with about .5 deg down is a good TLAR setting. You'll get more speed in cruise, and use less left rudder on the descent. Your right leg will get a workout on the take off - but that is only 20 seconds or so. You will need more right leg pressure in the climb too.

A cockpit adjustable rudder trim would be most helpful if this mod is done.

One of the past Reno pilots claimed 5kt around the course with the engine straightened out. Also, Cirrus did that same thing to get the SR22 to reach the speed goal the company had in mind when installing the 550 over the 360.
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  #4  
Old 11-04-2019, 09:17 AM
cduster cduster is offline
 
Join Date: Aug 2019
Location: Hertford, NC
Posts: 21
Default

I moved my bottom left 1/8" washer to the top right which let the engine sag a tiny bit and straightened it out just a hair. One completely subjective test flight later and I think that I like it. Just from that tiny little correction, I can already tell the plane is a little faster and does indeed require a bit more rudder on takeoff. Cruise rudder trim seems pretty good as is.

I'd love to really straighten it all the way out but that looks like kit will involve extensive cowling modifications, or maybe even making a completely new one. I wish I wasn't such a fiberglass weenie.

For those of you in the know, does the stock engine mount offset the rear of the engine to the left, or is it centered up and the crankshaft exits a bit to the right of the centerline?
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  #5  
Old 11-04-2019, 02:10 PM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
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Default

Per RV-4 plans the rear end (engine attach points) are offset 1/2" to the left and 1/4" fore-aft on the conical mount.
Dynafocal mount specifies 1.25* offset.
DWG 33.

Finn
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  #6  
Old 11-04-2019, 04:31 PM
cduster cduster is offline
 
Join Date: Aug 2019
Location: Hertford, NC
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Quote:
Originally Posted by FinnFlyer View Post
Per RV-4 plans the rear end (engine attach points) are offset 1/2" to the left and 1/4" fore-aft on the conical mount.
Dynafocal mount specifies 1.25* offset.
DWG 33.

Finn
Thanks Finn.

If I understand that correctly, 1/4" forward on the LH side means that my 1/8" shim took half of the offset out of the rigging. One more washer would require a little fiberglass rework, but it shouldn't be anything drastic.
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  #7  
Old 11-04-2019, 07:01 PM
FinnFlyer FinnFlyer is offline
 
Join Date: Aug 2018
Location: Bell, FL
Posts: 193
Default Not quite ...

On the conical mount ...
Upper right engine attach point is 11.5 - 5.25" = 6.25" from upper right firewall bolt. Distance between bolts is 23". So a 1/8" washer under upper right firewall bolt would only be moving the upper right attach bolt forward .125 x 6.25 / 23 ~ 1/32". Well, you subtracted 1/8 and added 1/8 so perhaps 0.25 x 6.25 / 23 ~ 1/16". But is it a conical mount?
Seems you'll get more out of putting washers under the engine bolts. And probably more fiberglass work.

Finn
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Last edited by FinnFlyer : 11-04-2019 at 07:05 PM.
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  #8  
Old 11-07-2019, 12:22 PM
fixnflyguy fixnflyguy is offline
 
Join Date: Jan 2006
Location: Winston-Salem, N.C.
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Default Offset is on all RV's..

On the -4, as well as all the RV series I know of the offset is built into the mount. It requires no shims, and the cowling is noticeably designed to set the spinner gap at the correct angle. For the average person, a look at the cowlings standing on the bench, one would think they are horribly bad out of square. All works out in the end, and its a pretty good "cruise" alignment...slightly off the topic, I flew backseat in my friends P-51, and he dialed in a BUNCH of rudder trim for take-off (checklist item)..but 1500 HP.
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