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  #1  
Old 06-19-2019, 10:39 PM
fbrewer fbrewer is offline
 
Join Date: Sep 2017
Location: Leander
Posts: 77
Default Cowl Fit Problem -- fixed pitch RV 6 to constant speed

So we finally got to mounting the new hartzell CS prop.

We then held our breath while fitting the upper and lower cowling.

Dang, not enough clearance from the spinner backing plate to the cowl. Actually, there was no clearance.



Our expert EAA advisor is saying in a perfect world, we need 3/16' spacing between the cowl and the spinner backing plate. Agree.

Here are the three possible solutions:

1. Cut off the front of the cowl about 1/4 back. Trim 1/4" from the upper and lower cowl front, and then re-layup the front of the cowl.

2. Cut off 1/4" from the back of the cowl( essential the piano hinge and other dsuz fasteners.

3. Remove the 3/16" spacer separating the prop hub from the spinner backer plate. The would move the backer plate forward 3/16". We would then also move the mid spinner attachement plate also forward 3/16" by moveing the 3 x 1/16" washers to between the prop hub and the mid spinner attachment plate. We will also have to remove some more material from the backer plate, so the plate will not rub against the prop hub as it gets 3/16" closer to the hub.

You can see the spacer in this picture:



This last possibility is likely how we will proceed.

I appreciate any and all comments, especially if there is a better way.

Last edited by fbrewer : 06-20-2019 at 06:23 AM.
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  #2  
Old 06-20-2019, 06:36 AM
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agirard7a agirard7a is offline
 
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Location: Newport, RI
Posts: 655
Default Spacer

I had a similar problem with my cowl when I converted from a nose wheel to a tail wheel. The engine Mount was shorter which created a tighter space between the spinner and cowl. Vanís allowed me to put spacers between the mount and fire wall. (Aprox 3/16Ē). I also but an 1/8Ē spacer between the prop spacer and the aft spinner plate. Hope this helps. Check in with Vanís to make sure.
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  #3  
Old 06-20-2019, 10:47 AM
rvbuilder2002 rvbuilder2002 is offline
 
Join Date: Jul 2005
Location: Hubbard Oregon
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Do you plan to do any aerobatics?
If the spacer (in fact if even the recommended washer) is removed, the blades will contact the spinner back plate when the prop is in full course pitch.

This may not ever happen if you don't do aero, but the correct way to solve it is to modify the cowling.
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  #4  
Old 06-21-2019, 06:48 AM
fbrewer fbrewer is offline
 
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Scott,

Can you explain why aerobatics will cause the prop to touch the spinner back plate if the 3/16" spacer is removed from between the prop hub and the spinner back plate (when in full course pitch)?

We have rotated the prop an cannot see that range of motion.

TIA
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  #5  
Old 06-21-2019, 07:12 AM
rv7charlie rv7charlie is offline
 
Join Date: May 2006
Location: Pocahontas MS
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If you end up having to modify the cowl, have you considered laying up an extra 3/16"-1/4" of fiberglass on the *inside* of the nose area in the cowl, and then sanding the nose back 3/16" ? A belt sander with aggressive grit would take off that much in short order. You *might* get away with not having to paint the whole cowl, and you wouldn't disturb the cowl fit.

Don't know about your cowl, but mine has a slight taper toward the nose, so it would be a roll of the dice on whether removing 3/16" on the back would shrink the circumference enough to cause issues with re-fitting it to the fuselage.

Charlie
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  #6  
Old 06-21-2019, 08:13 AM
fbrewer fbrewer is offline
 
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Location: Leander
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Charlie,

Yes, we also discussed the taper issue from front to back and possibly/probably causing a hinge alignment issue.

Shoot, with the current fit, the hinge is difficult to get back together :-)
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  #7  
Old 06-21-2019, 11:37 PM
rvbuilder2002 rvbuilder2002 is offline
 
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Quote:
Originally Posted by fbrewer View Post
Scott,

Can you explain why aerobatics will cause the prop to touch the spinner back plate if the 3/16" spacer is removed from between the prop hub and the spinner back plate (when in full course pitch)?

We have rotated the prop an cannot see that range of motion.

TIA
During aerobatics it is common for the prop to position against the high pitch stops.
Typically the trailing edge of the blades just barely clear the spinner bulkhead with the spaced and one washer installed when at high pitch. I am aware of numerous cases where the washer wasn't installed and there was evidence of wear/fretting from contact between the blades and the bulkhead.

When you say "rotated the prop" I assume you mean rotated the blades? If so, and you are sure they were fully turned against the high pitch stops then I guess you are ok, but not what I would expect. Especially if you remove the spacer. The other down side to removing the spacer is that the amount of material you will have to remove from the bulkhead to provide clearance for the body of the prop hub, will leave very little material left for attachment. I would be concerned about the longevity of the installation.
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  #8  
Old 06-22-2019, 02:07 AM
jliltd jliltd is offline
 
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Location: Rancho San Lorenzo
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Could you get prop bolts 3/16" longer and add additional 3/16" spacers or a 3/16" thick machined shim plate aft of the bulkhead? I realize this would add weight and could have the potential for increased pin bending loads in the bolts if they ever loosen. And maybe more crankshaft dynamic bending. That class in college was at 7am and I was always hung over. ��
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Last edited by jliltd : 06-22-2019 at 02:10 AM.
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  #9  
Old 06-22-2019, 10:42 AM
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Hartstoc Hartstoc is offline
 
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Quote:
Originally Posted by fbrewer View Post
Scott,

Can you explain why aerobatics will cause the prop to touch the spinner back plate if the 3/16" spacer is removed from between the prop hub and the spinner back plate (when in full course pitch)?

We have rotated the prop an cannot see that range of motion.

TIA
If the prop blade is rotated to the full-coarse pitch stop and does not contact the backing plate, then it wonít in flight either, but keep in mind that high G-loading and gyroscopic loads during abrupt maneuvers will flex the motor mounts so be sure you have adequate clearance of spinner and cowling and between blade trailing edges and cowling to allow for this movement. Inspect the cowl after the first several flights at progressively higher G and maneuvering loads to be sure the spinner is not making contact with the cowl.

You can try compressed air for rotating the blades, but be sure they go to the stops. In flight, the governorís internal pump boosts engine oil pressure into the 300PSI range but it should not require that much sitting stationary. - Otis
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  #10  
Old 06-24-2019, 04:33 PM
fbrewer fbrewer is offline
 
Join Date: Sep 2017
Location: Leander
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Members,

I had another suggestion that I would like your thoughts on.

Add 3/16" to 1/4" spacing between the engine mount and the dynafocal mount.

What are the engineering issues with this, other than a longer moment arm of the motor (nominally 1/4" more forward)?
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