75% power?

[bushcaddy105]

After a recent discussion about setting 75% power for cruise, I have been reading up on this topic. There seems to be more answers than questions!

 

Some ideas are:- Establish maximum full throttle level flight RPM and subtract 10% to get cruise revs. Set 23 or 24 inches manifold pressure (seems to be more applicable to constant speed prop applications). Read the engine manufacturer's info and use their numbers.

 

The discussion involves a 1-off auto engine conversion driving a ground adjustable prop through a reduction drive. Max power and torque numbers can only be guessed at - no empirical testing has been done. The prop pitch has been set by trial and error to produce an acceptable balance between climb and cruise.

 

What are your thoughts on establishing what is 75% power?

 

 

[facthunter]

Judging power.

 

You have no way of working out the torque, normally, so it is difficult. The engine manufacturer would have conducted bench tests. Fuel usage is a fair indication of power output, if you have a fuel flowmeter.

 

With a variable pitch prop, you can set the RPM, and remembering that the engine is an air pump, the Manifold pressure and RPM will be the major factors in determining what HP is being delivered. Mass airflow is affected by temperature as well, ( this is why you don't do too well with hot air being fed into the carburetter.) so in theory, if the mixture is correct the fuel flow will be proportional to power output.

 

Take-off power is normally over richened , so a chart taking this into account would need to be used. Nev.

 

 

[Yenn]

To get 75% power you need the engine manufacturers chart. Without it all you can do is guess. Fuel flow may give you some idea, but I would expect to use a little less than 75% fuel for 75% power. Comparison with similar engines may help.

 

Why not use whatever setting seems sensible, bearing in mind the temps. developed. If you can get high enough run at full throttle for economy and power.emember the old V.W. which ran all day at full throttle.

 

 

[geoffreywh]

If the engine in question is an auto conversion then the max rpm allowable is decided by the piston speed (probably) or the continuing integrity of the rods or valve train. Aviation concepts of cruise at max rpm minus 10% do not apply. If an auto engine's redline is at say, 6000 rpm you can't run it at 5700 and expect it to stay in one piece for too long. Not a thousand hours plus anyway. Probably an auto conversion entails reprofiling the cam and a smaller carb, plus any other mods to bring the max torque to a more usable range. (for Subaru's it's around 4400rpm I believe) This will not be anywhere near max allowable rpm so max allowable rpm and continuous rpm may well be the same.

 

 

[icebob]

Hi,

 

if it is any help my old VP-1 with a 2100cc VW motor was 3550 rpm max and 2850 rpm cruse with a fixed pitch wooden prop.

 

Bob.