Any problems with inverted mounting of Hirth?

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Peterson

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I am currently considering purchasing a Hirth F20 twin plug 650cc inline twin. I have a set of plans that recommends mounting the engine upside down for clearance and using a 4 belt reduction unit. Being a two stroke, oiling shouldn't be an issue and flipping the carbs right side up is easy enough. Are there any known issues from such a set up? Thanks
 

don january

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Inverted will it affect your belt reduction? as far as prop on or near thrust line
 

Peterson

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Inverted will it affect your belt reduction? as far as prop on or near thrust line
The plans show an inverted engine hung under the cantilever engine mount with he belt redux drive mounted on top to keep the thrust line where it needs to be without the engine sticking up into the forward field of view.
 

don january

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I would guess if you have a set of plan's for the installation of this engine in that foremat it is a proven design. I say sound's good and go for it
 

Peterson

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The plans suggest a Kawasaki 440 for the single seater, I'm building a two seat version and the 65hp Hirth comes in at the right power to weight ratio for an added passenger seat. I plan to save weight by putting flight controls and instruments in the rear seat only, as solo flights must be made from the rear for weight and balance anyways. I just haven't heard about mounting Hirth inline twins inverted and was wondering if this has been known to cause reliability issues or not.
 

don january

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I have never heard or read any negitive information in this application. I personally can not see any problem, maybe exhaust could be an issue but eazy to cure. I think I would put a duel safty cable hooked to the engine and mounted to the mount just in case a prop failure or mounting bolts break.
 

Peterson

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I have never heard or read any negitive information in this application. I personally can not see any problem, maybe exhaust could be an issue but eazy to cure. I think I would put a duel safty cable hooked to the engine and mounted to the mount just in case a prop failure or mounting bolts break.
What would you suggest for the exhaust? 2stroke exhaust has always confused me
 

Peterson

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Cool! Please post build pictures!

Lynn
Will post pics when I start, realistically a few months away before I start with the fuselage and empennage. Have a chance to pick up what looks like a good engine choice for it though and wanted to know what happens to inverted 2 strokes before I go that route.
 

lake_harley

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I don't have personal experience, but I've read discussion threads where some have had to deal with "wet" plugs on a inverted 2-stroke if it's been sitting for a while. I seem to recall some discussion of a "protruding tip" spark plug that apparently lessens the problem, at least for some people. At worst, I'd think you might need to pull the plugs and clean/dry them, or put in a clean set if the engine had been sitting and oil drained past the rings. It probably makes a difference too depending on the shape of the combustion chamber and the spark plug location.

Lynn
 

Aesquire

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Exhaust for a 2 stroke is tuned for maximum power at the desired rpm. Length, expansion chambers, it's a musical instrument.

The ( simplified ) idea is to get the "pop" as the exhaust port opens to travel the length of the pipe, and when it hits the end ( which may be inside the muffler ) it sends the pressure wave back down the pipe where it is supposed to be timed to stuff the gas air mix back into the cylinder, right before the exhaust port closes. It's all tuned for a certain frequency, and works less efficiently as you go faster or slower.

It's where the term "on the pipe" comes from. By all means search the forum. Pipe design is a black art and a math problem rolled into one. ( though some will disagree ) Run the pipe the manufacturer recommends is the best advice.
 

Himat

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Exhaust for a 2 stroke is tuned for maximum power at the desired rpm. Length, expansion chambers, it's a musical instrument.

The ( simplified ) idea is to get the "pop" as the exhaust port opens to travel the length of the pipe, and when it hits the end ( which may be inside the muffler ) it sends the pressure wave back down the pipe where it is supposed to be timed to stuff the gas air mix back into the cylinder, right before the exhaust port closes. It's all tuned for a certain frequency, and works less efficiently as you go faster or slower.

It's where the term "on the pipe" comes from. By all means search the forum. Pipe design is a black art and a math problem rolled into one. ( though some will disagree ) Run the pipe the manufacturer recommends is the best advice.
Two stroke pipe design is not that much black art, rather an example of where it is difficult to apply the math.
A two stroke tuned pipe can as you say be considered a musical instrument, even if not everybody do like the music played.
The important thing to remember is that the exhaust system on a two stroke engine is part of the induction system, a tuned pipe both help extracting the exhaust from the cylinder by inertia forces and as you say stuff fresh air fuel mix back into the cylinder.
 

lake_harley

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On the exhaust discussion, there are a couple ultralights with 2-strokes at the airfield where our EAA chapter meets that don't even resemble anything like a tuned pipe. Now granted, they are more than likely not reaching "spec" HP, but they do run and manage to pull the ultralights through the air. Maybe I'm wrong on this, but if a 2-stroke's pipe design is "off" it'll make the engine run at below it's potential HP, or, it will blow un-burned fuel out the pipe. If EGT and CHTs in the process aren't causing damage to the engine in the process, it just results in a engine that's down on performance from what it could be. It might result in excessive engine wear, fouled plugs, and other ailments, but they do run. In reality, any 2-stroke that's being run at an RPM outside the range of a pipe's "tuning" is running below it's performance potential, either wasting fuel or not evacuating all of the exhaust thereby not developing all the HP it could.

There's my opinion on it, and consider it's probably worth the price paid. $0.00 :gig:

Lynn
 

TFF

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Can mufflers for 2 strokes are less peaky over the rpm range. Tuned pipes are good for one RPM. Most use a can now, but they still have to have the right volume and shape.
 

goldrush

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In the 1980s a Patent was filed for a "Tuneable" 2 stroke exhaust system.

Patent US4537279 - Tuneable tuned pipe exhaust system - Google Patents

Not sure if it ever went into production.
Also someone suggested a "trombone" type exhaust.

Halfbakery: Trombone Exhaust


Back to the original.... any "problems" experienced with using "inverted" engines depends very much on the engine... Rotax 503 for example when inverted, seems to cause many, many problems with "oiled plugs" and poor starting....... unless electric start.... in my mind purely due to it's "poor, weak" ignition system at "pull start speeds", whereas my MZ201/202 for example seems ok either way.
My MZ201 is inverted, with an inverted "updraught" carb and has proved perfectly ok.
Unfortunately, I do not know of any inverted Hirth users, but similarly, I have not heard of the Hirth ignition being "weak".
 

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