Hi All,

When choosing a prop with regard to the engine only and not the type of plane, what is the most important factor to keep the engine safe? Is it staying within the recommended rpm range? All the charts I have looked at differ slightly. I have a TT pro .61 and the manual suggests a 11x7 prop. Experienced guys at the field had me put on a 14x4w and the plane flies great, but I'm worried the engine could be damaged using a prop so much larger than recommended. With the 14x4w the engine is turning 9700 rpm at WOT, which is well within the recommended 2000 to 16000 rpm range. Is it rpm that will damage the engine or are there other factors to consider when engine safety is concerned.

Tom

First off, many manufacturers use 16000rpm as the speed at which they get the best horse power. It is not a recommended speed. You might reach that speed with a ducted fan, or very small (too small) prop, but you will also burn out the engine in a relatively short time.

A good top-end for a 60-size engine is 10,000 - 12,000, most falling in the 11,000 range.

Having a prop that is too big can put excessive wear on the engine, but 9700 is not bad. However, if you like, you could trim 1/4" off of each tip, re-balance the prop and test it again - maybe even do it a second time.

You should consider a 13-4 or 14-5 if yo want low speed thrust. I assume that is what you want with a large low pitch prop. This would let the engine come closer to its peak power, and a faster spinning prop with even more thrust.

Yes, the charts vary by manufacturer, engine, etc. Props also vary by manufacturer, size, and material used. The best way to approach the job of selecting a prop is realize that you are going to have to buy them and see.

A 2 stroke engine will generally produce the best power at different speeds than 4 strokes. A 2 stroke needs to approach the maximum RPM limit that the manufacturer publishes. That is where the power is.

Perhaps another consideration is that props unload in the air and allow the engine to turn higher RPMs. I could be very wrong, but does a higher pitch prop unload more in the air than a low pitch prop, so greater care must be taken when using a high pitch to not overspeed the engine in the air? Also, does the plane affect the engine's speed in the air, a faster plane allowing more unloading? TIA.

Sincerely,

Richard

A high pitch prop unloads less than a low pitch prop. The plane must reach a higher speed for the prop to unload, makeing some think the prop was stalled when it was going slow.

Ok, I need to suggest reading Dave Gierke's book "AIRPLANE ENGINE GUIDE" page 43 which has the formula
for propeller load factor, PLF=diameter squared X pitch. In your case, 11 squaredx7= 847. Now using this number as a base, calculate all of the props you would like to use, and go through the forumula for each one. Those close to this base number are the ones to consider using. It then become which one makes you plane fly the best with. So far this information has helped me a lot. Now when going to a Kangke SK .91, etc., this formula may not work as the engine porting has been designed for torque at low rpm;s(Ed Moorman RC Report Magazine On Line). Owning the above mentioned book, will give you a lot more information to follow.

aerorich73