What problems may I have with Over Revving RPM -

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What problems may I have with Over Revving RPM -

Postby NRE » Wed Jul 25, 2007 10:54 pm

On todays flight the plug that shows the rotor RPM came loose in flight. During autorotation the pilot over revved the Franklin 235 up to 4000 RPM. We were trying to land in a difficult patch and the RPM slipped up during autorotation.It remained at 4000 for 20 - 30 sec. We set the Hiller B/C model down for inspection. After everything checked out, we flew for 35 min back to base. What issues could I be looking at due to the excessive RPM?
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Postby administrator » Thu Jul 26, 2007 3:41 pm

With an overspeed like that, and that long you will have to have basically everything inspected. If the engine overrevvd like that the mainrotor and tailrotor are also overrevvd. You will probably don't see anything with the bare eye but this is really serious. I would have the main rotor blades overhauled at least replace the retention straps and all the nuts and bolts. Rotorhead needs overhaul as well. The engine should be inspected, basically an overhaul.

The main problem with an overspeed like this are the extreme centrifugal forces on the main / tail rotor, with the accompanying vibrations.

A note on the whole thing; if a gauge fails, you don't have to enter an autorotation. Even if the engine RPM gage stops, there's no panic as long as you hear the engine.

I'm really sorry for you this happened, but don't take this lightly.
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Postby Pandaz3 » Sat Jul 28, 2007 3:33 am

Overreving the engine is even worse unloaded, wrist pins and bearings really take a beating. You really endangered yourselves by flying it back. I would be Leary of everything that rotates. Serious isn't a strong enough word for this, thank your lucky stars.
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Ref: Hiller Inspection guide Rev 1 Jan 1997

Postby ted.stokes » Tue Jul 31, 2007 1:41 am

pg. 30
a. Power Plant (refer to manufacturer's recommendations regarding rpm limitations)
1. Inspect the engine for cracked or broken cylinder heads or barrels; remove and inspect the oil screens, oil filter and sump plug for metal particles.
2. remove and inspect the sump plug and oil screen at the 10th hour of engine operation following an overspeed condition.

8. AFTER ROTOR OVERSPEED (in excess of 415 rpm)
1. Whenever the main rotor rpm has exceeded 415 rpm, remove the main rotor blades and inspect the main rotor head assembly and components for the following conditions: bending or shear deformation of inboard and outboard main rotor retention pins; elongation of the retention pin holes in the hub and forks; deformation of drag link attachment pins and holes.
2. Replace any questionable parts.

Whenever the main rotor head is removed for overspeed, perform the main rotor lock nut inspection listed under Special inspection column, Table III."

That's it according the the factory documentation. I would pay close attention to all attachment parts in the main head. You will need to see what Frankln has to say about the rpm limits.

I hope this helps.
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Postby Hiller360 » Wed Aug 01, 2007 3:38 am

According to Franklin Engine Co. Service News FSN 39 regarding overspeeding,anything over 3720 rpm is a code D condition which states" Over 120% rated rpm.Engine will be removed from service for overhaul,inspection and repair".

Sorry to be the bearer of bad news ,but its better to find something cracked on the bench than climbing out of a hole over some trees.
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Over Reving Engine

Postby administrator » Thu Nov 29, 2007 8:47 am

I had submitted a few months ago a problem I had when my instructor over
spun my Franklin 235 on my Hiller B/C model after the Rotor RPM wire
came loose in flight. After the over spin, I unknowingly flew the Hiller
back to base ( about a 45 min flight )The Hiller flew perfect. After
reading the comments from you and other Hiller Pilots, I took my
Aircraft 5 hours away to a Franklin dealer. He called today and said he
looked down one of the cylinders and said that the piston was pitted
from overheating. He suggested a complete overhaul and said the
approximate estimate would be $ 20,000 ±.
Being a new Hiller owner, and not being a Franklin mechanic, does this
sound in line? And if the engine got so hot and the pistons were so
badley pitted, Why would it run so great on the way back. I plan on
going out to take a look at the cylinder next week. I'm just trying to
get as much feed back as I can prior to going back out. I am sure he is
correct on his analysis, I'm just interested in your thoughts and

P.S. What other problems may I encounter?


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Postby Rod Baker » Thu Nov 29, 2007 8:12 pm


This may be correct due to the fact that 235 cly's are hard to find from what I know. I will ask a few friends and find out for you. The question is how bad did you overrev the engine that might be a mandatory teardown due the the fact the lower end may be ready to come undone. Also over speed may require trans tear down and rotor head. look back on the previous entrys on overspeeds good knowledge base. Much better that curse the bench that pick out a wreck, ......did the instructor have insurance??? being that most times now everyone has to including the instructors.
Rod Baker
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Over Rev

Postby ted.stokes » Sat Dec 01, 2007 11:57 pm


I would shop around. Last summer, I checked with Gulf Coast Helo and they priced rebuilding my extra engine at $14-15K. That prive included all the accessories on the engine. I would have someone perform the Hiller inspections on the M/R Head and Transmission per the Hiller manuals.

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Re: Over Reving Engine

Postby NRE » Wed Jan 30, 2008 3:28 am

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Over revving a Franklin

Postby 6V350 » Fri Feb 08, 2008 3:04 pm

It seems no one can answer your questions about overspeeding with any experience, only quotes from manuals and such. You are the one with
the real experience now.
Back in the early eighties when I was new to Hillers and Franklins, I was talking to Charlie Hart of Syracuse NY(a Franklin overhaul shop supervisor) on the phone and he told me the factory ran a 210hp engine at 4900rpm in a test cell for a work shift(about 8 hrs, I presume) with no problems. They were going for 5000rpm but didn't quite make it.
Spray pilots routinely used 33-3400 rpm for takeoff until translation.

I would say that if you have piston pitted it was not due to over speed.
The factory timing setting is 28 degrees BTC but I find that this leads to pre-ignition when using high power settings such as hovering for extended periods when run on 100LL. Feels like an intermittant miss or kick or quiver. I initially thought it was spark plugs, wires or magneto problems. Drove me nuts. Nothing I did helped. It wouldn't show up till after 2 or 3 minutes of hovering. So it seemed a heat related problem. So one day the light went on in my head and I got the idea to back off the timing a bit so I figured out the correct direction and turned the mags to the end of the slots.
The problem vanished and I saw no reduction in power either. Later I checked the timing and it was about 25 degrees. So this is what I use now.
Not quite enough lead in 100LL for 10.5 to 1 compression.
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