thePMFC.co.uk: feature

LETS TORQUE ABOUT PROPS.  by Colin Hostler

  Lets take a closer look at those props and engines that we abuse on our aeroplanes. Modern engines produce there highest torque levels at much lower r.p.m. compared with engines a few years ago. The main reason for this is for the demand these days to keep the dreaded noise down. Nearly all engine manufacturers are working hard to produce an engine with high torque and power bands at much lower revs. This means that the props we use may not be large or high pitched enough to make the engine do its job properly.

  I know from information passed onto me years ago by a full size aircraft engineer that a large prop turning slower is more efficient than a smaller prop turning fast. So your screeming “wot not” may not be going as fast through the air as you think because (A) the prop is thrashing a lot of the air away from itself, and (B) the engine has gone passed its torque band. So all you are doing is making a din and screeming the life out of your engine by leaning off the mixture.( less mixture = less oil = more heat = engine wear). As you probably know oil doesn`t just lubricate, when it comes out of your engine it brings heat out with it, but there that’s another topic. (the arrows to the left will help you to scroll through this review)

  I won`t go into making charts for prop/engine ratio`s because I think a prop that is suited to a particular engine in one plane is not always suited to the same engine in another plane. i.e Take a say a cheap 40 size engine in a delta, is quite happy on a 10x8 prop, put the same engine in a vintage aircraft and it won`t be suited on a 10x8, it will need about a 12x6.

  You see the pitch is just as an important factor as the diameter, i.e 8 inch pitch on the delta (hand launched),creating plenty of pull to start off with at the launch and through the flight at lower revs but no loss of speed in the plane because the engine is working hard. At the other end of the scale.6 inch pitch on the vintage, the finer pitch gives a slower flight and landing speed but the 12 inch diameter makes the engine work for a living again.

  Aerobatics, torque the engine up with a larger prop and you`ll enjoy better aerobatics, especially the manouvers that rely on the torque from the engine(smaller prop = high revs = low torque). So, put a larger prop on, bring the revs down, open the mixture screw out a little to cope with extra load and make the engine work as its designed to do.

  Help save our flying sights by making less noise.

Since November 21st 2006