Rear Prop for a 337G 1975

[Jack Dick]

Hello All,

A couple of Months ago I purchased N20EC a 1975 Skymaster from Dave in Mn.

I have had the worst luck with my rear prop. After getting the plane back to it's new home at D38 Canandaigua NY the first time I flew it (for about an hour) I noticed a fair sized chunk out of the rear prop. The damage was caused by a bolt which had hit it and left the impression of the bolt head on the back edge of the prop. had the prop repaired and O/H at the same time. I had thought that I had picked up up some junk from a taxi or runway. I again flew the plane and after the flight saw another small but detinct chip min the bade. This time an A&P was able to correct the minor problem. I had the rear panels pulled and found a broken exhaust clamp bolt had been the cause of both incidents. After the repair and a good inspection of the rear engine compartment went flying again. Well as luck or bad luck would have it after landing there was an even bigger chunk out of the prop than both the other times put together. The blade can not be repaired. I have 3 questions for you folks who have owned 336's or 337's for years.

1/ does anyone have a prop for sale.

2/ Have I had just plain bad luck in the last 3 flights.

3/ Is there an STC for anything to stop the crap comming out of the back if it breaks, such as mesh covers.

The Skymaster has been a dream for many years and I must confess that if this a recurrent problem, my love for the plane could disipate.

Thanks folks.

Jack

[Mark Hislop]

Jack:

Sorry about your bad luck.

I've had my Skymaster since 1996. I've only had one small nick in the rear prop in all that time. I've had much more wear and tear on the front prop. The rear prop still has all of it's original paint from the last overhaul on it. I have to touch up the leading edges and rear surfaces of the front prop every year at annual. And I only operate from paved runways.

I don't know of any mechanism to keep objects from exiting the rear engine and hitting the prop. Frankly, it's something that never occurred to me. After you problems, I think I'll keep a closer eye on any possible loose objects in the engine compartment.

Mark

[skymstr02]

My O-2A has 1/4 inch stainless steel mesh on the rear cowling rear opening to keep loose hardware from exiting the cowl enclosure. The O-2's are normally aspirated, you didn't mention if your airplane was boosted. This may be a difference.

Check your illustrated parts manual to see if you should have this on your aircraft.

[Jack Dick]

Thanks for the response folks, I feel better knowing that it is not a inherant problem.

For Skymsr02, it is a normally aspirated engine, but your mentioning that the 0-2A has mmesh I fing interesting.

Mark, thank you too.

hharney, How did the the exhaust bolt break in your incident? I am still at a loss. Other engineering buddies have looked at the nut and remaining threaded portion (intact, just broken) and have had every concievable solution, over torque at installation, heat crystlization coupled with vibration and expansion shear. Unfortunatley they all leave me cold as I keep coming back to the base physical properties of it would or maybe I might say should take almost 13,000 pounds of stress to shear in a normal configuration and while there may be some reduction because of heat fatigue, do not believe it to be a large enough factor.

Incidently, the bolt in question. On the top of the exhaust at the rear, is a support bracket that is connected via tubular arms to the top of the two rear engine mounts. The bolt in question passes through all three with the exhaust support bracket being in the middle.

Thanks again

Jack

[Jim Rainer]

Jack, I have a '76 normal that I've put 670 hours on in the last 8 years. I've had only one nick and it was serious enough to require an A&P to file it out.

I religiously start any roll - taxi or takeoff - with the rear engine and bring the front engine in after starting to roll. I taxi mostly with the rear engine (unless it's running hot) unless I know the surface very well I am taxiing over.

This is my 3rd 337 and I believe 95% of rear prop damage results from items blown rearward by the front prop. Maybe this will help you.

[kmack3211]

Jack,
Just thought you would like to know that 20EC and 30EC, now 34EC owned by Jerry DeSantis, were originally bought new by the same company in the Indianapolis IN area. Both were sold by the estate of the company owner without the log books that went missing. I bid on 20EC but was too late, it sold for around $21k, so I wound up with 30EC which was the pressurized model.
Just a little bit of trivia, good luck with your new ship.
Ken

[hharney]

I have had my C model since 1976 and have had one incident like yours. I too had a bolt come loose from the muffler bracket (might be something to check more oftern or during preflight) that struck the blade. It too made a perfect impression of the bolt head in the blade.

Other incidents of interest may be the several issues (4 - 6) of FOD from unimproved runways mainly. I fly a lot of back country strips in Idaho (before I moved to Michigan) and it's part of owning a Skymaster. In 2002 I installed Wood Composite MT three blade props and since that time I have had two incidents with the rear blade. With the MT's you just mix a little 5 minute epoxy, apply, let it cure, sand it off and paint. Really pretty simple and inexpensive. Even if I had to replace a complete blade the cost is 25% of the McCauley's.

Below is the trailing edge of the first FOD contact. The FOD contacted the front of the rear blade but exploded the rear of the trailing edge of the blade.

This damage took the better part of 1 day to repair.

[skymstr02]

Here's a scan from the parts book on the O-2A's showing the screen installation.

[hharney]

The damage that occured in my case with the muffler attachment hardware, (when I had original McCauley blades) came through the exhaust pipe holes not the prop hub area. I have removed the screen on my ship years ago. It is one of those items that continued to vibrate loose and got to the point where I was tired of re-riviting so I finally just removed it. I have seen several other 337's without this screen also.

[skymstr02]

By removing the screens, you've altered a type designed part. What did you use for approval on the 337 form?

[hharney]

If I can remember back then, an A&P removed them but I am not sure about the legality. I assume the cooling aspect might be better though. Do others have these removed?

[walter holloway]

Sir
Have the rear engine completely gone over by an A&P for loose things....then run the rear engine on the ground and have it rechecked. Simply no substitute for close inspection.
I have several rear props, rebuilt and ready to go If your interested.
Good luck
Walter

[Jack Dick]

Thank you all for some good information.

My choices came down to, buy one new blade and have it shaved to match the existing blade, or buy two new blades and have a spare which cpild be used if a mishap occured in the future.

I decided on the two new blades and as I had just had the hub O/H had them shipped to Sensenic in PA. They are really fine people and did a great job, as they had done on the frint and rear prop a couple months ago.

I also had every nut and bolt replaced on the rear engine. The nut and bolt in the rear exhaust support braket was included in the replacement (that was the one which caused the big chunk aka #3 prop ding) and after installation had the ends sealed (the nut and bolt are recessed). My smart A & P also made it so it could be removed, during the annual.

After about 4 hours I had the panels pulled and everything inspected. All looked good, but I think I will have the panels pulled every 20 hours or so for inspection.

My annual was due in May but I had it done early while the plane was down because I am planning on attending the great fly in. It all depends on my safety pilot being able to go with me.

Again Thanks for the help and I really enjoy logging on and seeing what is happening.

Jack

[Jack Dick]

Walter,

Thank you for the offer of rear blades, I probably would have done it, but had already ordered the new blades.

Thank you.

Jack

[rrice]

Are you still looking for a rear prop?