[khamai]

We'll never know what could have become of the full potential of the Rutan + Chapman collaboration. The Lotus engine was a marvel, with fewer moving parts and lighter weight. Most ultralight engines of the time required the crank to be geared down to roughly half speed. Chapman realised that by driving the prop off the cam (which turns 1/2 speed) that they could simplify the entire mechanism and shed weight!

Cheers,
Kiyoshi

 

[thruthefence]

Chapman may have realized it, but TCM built an engine with the prop driven off the cam, (Tiara, O-405, six cylinder, '6-285' '6-320', 285-320 horsepower) nearly 40 years ago. Never found much acceptance in the marketplace, however.

here's a cutaway:

http://www.flightglobal.com/airspac...es/5674/425x324/continental-tiara-cutaway.jpg

 

[tesprit]

I admire the innovation of the Lotus attempts with aircraft engines, but after owning a couple Lotus engines and comparing them to the reliable engines in the airplanes I have owned, I am really happy they never made it into production aircraft. It's one thing when your Esprit engine blows up or quits running while you are driving, but another thing altogether when your airplane engine fails as you are flying over the Rocky mountains.

 

[fitfan]

...yea - driving the prop of the cam on an avaition engine - will in theory may make weight sense .... is not a good "reliability and durability is #1" idea.

anyways- the elite was built per a boat, then chapman got into planes, true it would have been really fascinating if the lotus personal sport plan had seen the light of day. eventauly liabilty lawsuits would have ruined lotus no doubt - but he, and rutan were/are pure genius!

i heard rutan owns an elise?

 

[thruthefence]

On an aircraft engine, with each firing pulse, there is a sort of kick-back, or "snatch" induced back into the crankshaft by the propeller blade. "Designed for aircraft engines" dampen this with movable counter weights on the crank throws. Rudd was a sharp cookie, and would surely have known of this effect, and designed for it, but it makes you wonder why the idea didn't catch on. Maybe it feeds back into the valve train?

 

[tesprit]

On an aircraft engine, with each firing pulse, there is a sort of kick-back, or "snatch" induced back into the crankshaft by the propeller blade. "Designed for aircraft engines" dampen this with movable counter weights on the crank throws. Rudd was a sharp cookie, and would surely have known of this effect, and designed for it, but it makes you wonder why the idea didn't catch on. Maybe it feeds back into the valve train?

I have a feeling it has more to do with the stress the propeller would put into the drive for the camshaft. It is a lot easier to make a beefy counter weighted crankshaft to deal with the pulses than a gear drive system and a beefed up camshaft to take the same pulses. I was able to watch a lot of test runs of a prototype two cycle V4 diesel aircraft engine and that thing sheared crankshafts and propeller hubs off at an incredible rate. They then tried going to a gear drive with a drive shaft to the prop. and it continuously shredded the teeth off the drive gears so the next step was a belt drive which saved the shafts from breaking, but the belts would smoke after a few hours of running.

It was rather humorous watching the drive end of the crankshaft shear off and the 4 bladed club style dyno. prop go cartwheeling around the dyno. room while the test crew dove for cover. :crazyeyes

 

[fitfan]

not an expert a&p or engine design guy - but it would seem to me that either the valetrain / cam would fail from the load. or you would have to build it so robust that you might as well go with crank redution gearing.

fundamentally, driving anything with load off the cam, is a bad idea

 

[tesprit]

not an expert a&p or engine design guy - but it would seem to me that either the valetrain / cam would fail from the load. or you would have to build it so robust that you might as well go with crank redution gearing.

fundamentally, driving anything with load off the cam, is a bad idea

Agreed, and this is also why car engines usually make lousy airplane engines. Most car engines don't produce enough HP or torque at low RPM so they have to be run at high RPM to work in an airplane. Propellers don't work well spinning fast (efficiency and thrust drop dramatically when the tips exceed the speed of sound and this can easily be achieved with a long prop. even at 2,500 - 3,000 RPM)) so the only way to drive a prop. with a regular automobile engine is to use some sort of reduction drive to a separate propeller shaft. These drives are usually the weak link and the reason most automobile engines are not converted to aircraft use. Using the cam as the drive for a propeller is just not a good idea and it is better to design a robust aircraft engine that runs with a fat torque band at low RPM from the beginning of the design process.