Replace Cowling Questions??

[GrayHawk]

I fly an older (1991) RV-6A, originally built with all piano hinge attachment of the original gel coated cowling where the ram air scoop came as a separate piece. Engine is O320-D1A with usual FAB, vertical induction through carb. Plane originally has a Warnke wood prop, but now has a Sensenich metal prop with a 4" extension. Total time is about 1700 hours, which has resulted in:

1. Cowling is cracked in places and getting worse.
2. Cowling was too close to alternator pulley resulting in wearing a arc shaped hole through cowling.
3. Prop change & extension resulted in wide spinner to cowling gap.
4. Rear attachment hinge on upper cowling worn out & replaced with screws with rivnut attachment tabs.
5. Some other issues also

So as part of my improvement work on this plane I'd like to give it a new cowling. I've gathered a couple of orphans towards this goal: an original gel coated cowling (separate air scoop) & a newer 'green' cowling where the scoop is integral. I thought the newer one was for an O320 but when it arrived I see it is marked O360.

Questions are:
1. Is there any reason(s) I can't use the newer cowling?
2. Attachment at the firewall end will need to be something other than piano hinge. What is the best retrofit attachment methods to use, short of taking the whole firewall apart.

Thanks,

 

[Mike S]

There is a 6 here that had a later cowling retrofitted after the original was damaged.

One thing that I remember pretty well is that the newer unit is a bit different in shape, and size------end result was cooling issues, took a long time to figure out it was due to the baffling fit.

Moral-----be sure your new cowl is compatible with your old baffle setup. You may have to do a bit or re-work in that area.

Good luck.

 

[az_gila]

Cowl length

I fly an older (1991) RV-6A, originally built with all piano hinge attachment of the original gel coated cowling where the ram air scoop came as a separate piece. Engine is O320-D1A with usual FAB, vertical induction through carb. Plane originally has a Warnke wood prop, but now has a Sensenich metal prop with a 4" extension. Total time is about 1700 hours, which has resulted in:

1. Cowling is cracked in places and getting worse.
2. Cowling was too close to alternator pulley resulting in wearing a arc shaped hole through cowling.
3. Prop change & extension resulted in wide spinner to cowling gap.
4. Rear attachment hinge on upper cowling worn out & replaced with screws with rivnut attachment tabs.
5. Some other issues also

So as part of my improvement work on this plane I'd like to give it a new cowling. I've gathered a couple of orphans towards this goal: an original gel coated cowling (separate air scoop) & a newer 'green' cowling where the scoop is integral. I thought the newer one was for an O320 but when it arrived I see it is marked O360.

Questions are:
1. Is there any reason(s) I can't use the newer cowling?
2. Attachment at the firewall end will need to be something other than piano hinge. What is the best retrofit attachment methods to use, short of taking the whole firewall apart.

Thanks,

Gray Hawk... one issue you might find is cowl length.

You now have a "long" cowl if you are using a 4 inch prop extension.

You need to check that both of your "new" cowls are "long"... but IIRC, the green one only comes in a short version.

The other option is to buy a shorter spacer and shorter prop bolts from Sensenich... which would involve many $100's of dollars...... but would allow the use of a newer "short" cowl.

gil A

 

[GrayHawk]

Keep in mind that the current prop & extension are not the original ones, but the cowling is the original.

Two of the problems to be 'fixed' are that the current old cowl is too short relative to the spinner (spinner to cowling gap) and the alternator pulley location (rubbed causing hole); so the first thing I did was measure the replacement cowls to make sure they were long enough to fit.

I would stick with the 4" prop extension as cutting the new cowling to that length would provide more clearance between alternator pulley and inside of cowling. I expect to redo the baffling and seal as part of all this. I would also make other cooling improvements as part of the process (sealing the sides of the ramps at the inlet openings).

 

[rvholic]

Also check the height of the FAB, the carb of o320 is smaller than 0360 carb. then the scoop hole does not coincide.

I had that trouble when I installed my cowl because they send me a 320 cowl and I am using a 360 engine.

 

[dav1111]

Cowl change

First suggestion would be keep the existing cowl and learn to lay up fiberglass to correct all the problems with the esisting cowl and order a Skybolt Cam Locks RV-4 kit and optional backer strips to change the hinge pins to cam locks. If there is no problem with the esisting lower cowl hinge pin attachment to the firewall you could keep the hinge pin setup there and just do the cam locks across the top of the firewill and down each side after drilling out the hinge pins. I just did a change over to cam locks from hinge pins and that part of the process wasn't realy hard.

If you don't have much expierence with fiberglass layup stop by Lubbock and I will give you some help on your cowl. I now have lots of fiberglass experence and my buddy Bob in Las Vegas keeps trying to draft me for at least a weeks work on his project. Unlike my buddy Rick he hasn't yet offered me all the perks that Rick offered recently on Matronics for the rental of a child for small places.

If you order the cam locks be sue and get the optional backer stirpes, and buy a cheap nibler from Harbor Freight to do the V cuts on the backer stirpes.


In the alternative you might consider talking to Sam & Will James at James Aircraft about their combination plenum/cowl for the RV-4 assuming nothing has been done to the new cowl and you can return it.

This would potentially solve the issue of the baffle fit using the new cowl.

 

[GrayHawk]

Also check the height of the FAB, the carb of o320 is smaller than 0360 carb. then the scoop hole does not coincide.

I had that trouble when I installed my cowl because they send me a 320 cowl and I am using a 360 engine.

This one is probably a real problem & will cause me to use the spare gel-coated cowling with some extra work to attach the air scoop.

 

[gvgoff99]

Old Cowl

If you decide to repair the old cowl, look out for oil saturation. The new fiberglass will not "stick" to anything that has even a hint of oil on it. Fiberglass, in the cowling area is susceptible to having the oil weep into the fiberglass from even the most minor of leaks. Capillary action and all that stuff, you know.

 

[GrayHawk]

If you decide to repair the old cowl, look out for oil saturation. The new fiberglass will not "stick" to anything that has even a hint of oil on it. Fiberglass, in the cowling area is susceptible to having the oil weep into the fiberglass from even the most minor of leaks. Capillary action and all that stuff, you know.

I will NOT repair the old cowling. It is too far gone. I have my choice of new gel coated or new green cowlings as the replacement.

 

[fl-mike]

I had to do a lot of repair/mods to the white cowl on my project, and if I knew then what I know now, I would have started fresh with a new green/pink. The easiest part was changing the hinge over the top to the 1/4 turn fasteners. I kept the hinge along the sides and the bottom.

Know up front that this change will take a loooong time. Much longer than you think (isn't that true for everything).

If you change over, I would plan on:

• New cowl
• new baffles
• new prop extension
• new FAB

 

[GrayHawk]

I had to do a lot of repair/mods to the white cowl on my project, and if I knew then what I know now, I would have started fresh with a new green/pink. The easiest part was changing the hinge over the top to the 1/4 turn fasteners. I kept the hinge along the sides and the bottom.

Know up front that this change will take a loooong time. Much longer than you think (isn't that true for everything).

If you change over, I would plan on:

• New cowl
• new baffles
• new prop extension
• new FAB

My hinges over the top and along the bottom are long gone (wear & replacement before I got the plane).

What kind of quarter turn fasteners did you use? Any photos of the fastener strip or mounting?

Everyone mentions changing the prop extension. That is not going to happen. I consider it one of the 'givens'. Besides it helps with the aft CG you get on RV-6s sometimes under certain loading & fuel conditions.

I'll double check everything, including engine alignment and will shim if needed; but quick measurements show both of the new cowlings are long enough for the 4" prop extension. Baffles & FAB will be adjusted as necessary.

Measuring along the top from back edge of cowl to front edge of cowl and back edge of spinner:
-Current old cowl - 35"
-Current spinner - 35 7/8"
(yes, a 7/8" gap)
-New white cowling - 36 3/8"
-New green cowling - 35 3/4"

 

[rvbuilder2002]

Everyone mentions changing the prop extension. That is not going to happen. I consider it one of the 'givens'. Besides it helps with the aft CG you get on RV-6s sometimes under certain loading & fuel conditions.

I'll double check everything, including engine alignment and will shim if needed; but quick measurements show both of the new cowlings are long enough for the 4" prop extension. Baffles & FAB will be adjusted as necessary.

The prepreg (green) cowl was specifically made for only a compact hub constant speed prop. or a fixed pitch with a 2 1/4" prop extension (which is why everyone keeps mentioning replacing it). I am quite positive it will not work with your 4" extension.
You may be measuring that the cowl is long enough but I am pretty sure you are probably measuring extra flash area on the cowl that is intended to be trimmed off (usually between 1.5 - 2 inches). If you try to use it without trimming it off, it will be too big and will not fit the perimeter size of the firewall.
As a side note about your alt. pulley interference problem...it is not because of the cowl, it is because either too long of a belt was installed (alt. is rotated to far away from the engine to get it tight or a bracket/alternator combination was used that just doesn't work well on RV's.
The belt that used to be supplied with the B&C alternator kit (and as far as I know still is) was way too long and always caused this problem.

 

[Bob Axsom]

Alternators and Exhaust pipe clearance

Scott's post is interesting. I have a B&C alternator and I had a cowl wear problem even though there was clearance in the static condition. I cut off a thick stainless steel butter knife blade and bonded it to the inside of the cowl as a strike plate and that has worked well for several hundred hours. My wife has accepted the fact that desparate problems require desparate measures.

I bought one of the original honeycomb cowls when they were first developed and offered as an option, as my original cowl. The alternator pulley contact problem was experienced on that cowl.

Even more damaging was the insufficient clearance between the Vetterman crossover exhaust pipes and the cowl. The incorrect timing of my LASAR ignition system by Lycoming caused extreme heat which intensified the problem but even after I worked through that problem there was severe cowl burning and delamination. The cowl immediately adjacent to the exhaust pipe from cylinder #2 to the cowl outlet was turned into charcoal and require extensive repair. I cut off the pipes at the slip joints to raise them away from the cowl as far as possible and made new shorter steel links. I also lined the inside of the lower cowl with some very expensive aluminized thermal blanket material from Aircraft Spruce. Long after I worked through this problem (a year or two) I talked to Larry Vetterman an he told me that thclearance changed in the new cowls (green) and he was not aware of it. Many cowls were damaged and he replaced many incompatible exhaust systems with system that fixed the problem (I had fixed my own). He said the cost almost broke his company. This is not an attack on either company, both of which I consider excellent, but if you have an old exhaust system and a new cowl be prepared for this possible problem. The most severe location is where the pipe turns aft below cylinder #2. You mentioned that your cowl has the scoop already in place. Mine did not so your new cowl is different than my new design cowl in at least this regard.

As far as the hinge pin installation is concerned I had one problem. My bottom set of hinges failed very early and I replaced them with aluminum plates, platenuts, #8 flathead screws and dimple washers. The rest has served well and maintains perfect alignment. I do have one stress relief modification at the front of the cowl. I bent pull eyes in the ends of the side hinge pins and I cut out a small notch on each side of the upper cowl to allow the the pull eyes to drop below the surface at the forward end. I made a 0.016" aluminum (2024T3) cover plate for each side that attaches with one flathead screw and platenut on the inside of the outboard section of the cooling air inlet. The cowl halves are modified slightly with fiberglass and aluminum (to mount the platenut) to provide an overlap of the upper and lower cowl so this screw fastens the upper and lower cowl together in a lap joint. The cover plates then wrap around the front of the cowl on each side covering the split line and the pull eye hole. At the outside termination of the cover plate I have two vertically aligned flathead screws going through the upper and lower parts and through the upper and lower cowl into platenuts. One screw on each side goes into the upper cowl and the other goes into the lower cowl so on the outside there is an aluminum splice across the cowl halves. The holes in the cover plates are dimpled for the screws and the low drag strong joints reduce the load on the hinge pins.

Bob Axsom

 

[GrayHawk]

The prepreg (green) cowl was specifically made for only a compact hub constant speed prop. or a fixed pitch with a 2 1/4" prop extension (which is why everyone keeps mentioning replacing it). I am quite positive it will not work with your 4" extension.
You may be measuring that the cowl is long enough but I am pretty sure you are probably measuring extra flash area on the cowl that is intended to be trimmed off (usually between 1.5 - 2 inches). If you try to use it without trimming it off, it will be too big and will not fit the perimeter size of the firewall.
As a side note about your alt. pulley interference problem...it is not because of the cowl, it is because either too long of a belt was installed (alt. is rotated to far away from the engine to get it tight or a bracket/alternator combination was used that just doesn't work well on RV's.
The belt that used to be supplied with the B&C alternator kit (and as far as I know still is) was way too long and always caused this problem.

Thanks Scott,
That is good useful information. I hadn't realized so much 'flash' would need to be trimmed off. I guess I will need to do more careful measurements and stick with the original type cowling. I'll also consider changing the alternator belt when this all happens.

 

[Bob Axsom]

Has Van's determined a belt replacement?

Scott, has Van's determined a belt replacement for the B&C belt? When I replace the prop blades some day I would like to install a proper belt for the application.

Bob Axsom

 

[Kyle Boatright]

Scott, has Van's determined a belt replacement for the B&C belt? When I replace the prop blades some day I would like to install a proper belt for the application.

Bob Axsom

Take the belt you have to the local auto parts emporium. They have a tool to measure it. Once you know the measurement, buy one that is a couple of centimeters shorter, and another one that is even shorter. That's what I did. In the end, I installed the one that gave me the best clearance and have not had any cowl scuffing since.

 

[Bob Axsom]

Very good tested input

This first hand validated problem fix method for the Alternator pulley intermittent cowl contact problem is valuable - Thanks. Current builders can get immediate benefit.

Bob Axsom

 

[rvbuilder2002]

Take the belt you have to the local auto parts emporium. They have a tool to measure it. Once you know the measurement, buy one that is a couple of centimeters shorter, and another one that is even shorter. That's what I did. In the end, I installed the one that gave me the best clearance and have not had any cowl scuffing since.

Bob,

What Kyle said. The return what ever belt(s) you didn't use.

I use the shortest belt that I can make work. I fit the belt around the alt. pulley and the ring gear pulley and then put the ring gear support on the crankshaft flange. This will give you the most clearance possible.