What type of Engine?

Saturday July 25, 2009

Have not posted anything for a while.

This building an airplane is not as simple and straightforward as it seems.

I have been made aware that an aircraft needs an engine to propel it through the air and logic says well use an aircraft engine sheesh until you see the price and overhaul costs that is.

I have started looking around at what is available in the 65Hp-85 Hp segment and more importantly is there something that is readily available as in automobiles that can be used for aircraft. Yeah car engines are cheap and plentiful.

The next discovery is that aircraft engines are direct drive, the propeller flange is part of the crankshaft so the propeller turns at crankshaft speed. Then because of physics the propeller tips can go supersonic making noise but not much thrust and propeller efficiency drops off rapidly. So to stay away from this situation direct drive propellers are limited to about 3000RPM.

This happens to be where all piston aero engines develop maximum torque and power and are able to easily drive a propeller.

Automobile engines on the other hand make maximum torque at 3000rpm-4500rpm and some even higher and their max power at high rpm in the 600rpm -6500rpm range.

This means an Auto conversion needs a reduction drive to reduce the rpm. This opens a whole new box of snakes and spooks.

Any engine attached to a drive via a gearbox suffers from Torsional vibration and design parameters have to be met so that you don’t destroy parts of the engine or reduction drive or even the propeller.
Read a fine article here
http://www.epi-eng.com/propeller_reduction_technology/torsional_vibration_issues.htm

Direct drive ie prop attached to crankshaft does not suffer from his phenomenon. To such a critical extent.

Turns out that the humble VW beetle engine makes a fine aircraft conversion IF and I stress IF you cool it properly at all times.

I have learned that the engines in our Automobiles are the Long Distance Athletes of the engine world, able to putter along at 25% power at highway speeds in 5th gear.

Aircraft engines on the other hand are more the Sprinters of engine being asked to supply 100% power for take off and then running at 75% power during cruising. So they work really hard and hard work produces more heat so cooling is very important to aircraft engines. The majority of aircraft engines are air cooled.

So to get back to the humble VW beetle engine it is being asked to do things it was never designed to do. This leads to reduced reliability and much shorter times between overhauls of critical components like the cylinder heads and especially the exhaust valves as they take the most heat.

You can read more about VW engines converted to aero engines here
http://www.greatplainsas.com/
http://www.aeroconversions.com/
http://www.revmasteraviation.com/products/r2100_engine/index.htm
http://users.lmi.net/~ryoung/Sonerai/Engines.html