Will my prop selection from this test fly my plane at a suitable speed

As described in the page entitled "Is my engine is over propped" as a rule of thumb it is not hard to take a reasonable guess as to what speed a certain model would be expected to fly at.

If we take a couple of examples like our Spitfire and our Tiger Moth, we could make a reasonable estimate of the Spitfire speed in normal flight to be around 60 to 70 mph with a maximum of around 100mph.

The Tiger Moth on the other hand would be better suited to a normal flying speed of 35 to 40mph with a maximum of 55mph.

Armed with these typical examples, a fair estimate can be made as to what speed our aircraft is expected to fly at, and then work it out from there.

In the case of the Spitfire as an example, let us assume our engine is at its best at 10,000 rpm. Now if we take 70mph and break that down in stages.

70 divided by 60 (minutes) = 1.16666 (miles per minute).

Times this by 5280 (feet in a mile) = 6159.9996 feet/min.

We now times this by 12 to give inches/min (73919.995) and then divide that by 10,000 rpm = 7.39 inches per revolution so an 8 inch pitch will do.

When the final figure from the above is worked out, it will then need about 15% taking off to account for what is known as "prop slip", this is the amount that the prop effectively uses in order to do its job.

This may sound a bit daunting but with today's calculators, it takes only a few minutes to arrive at this result using the order of calculation and values above.

For our Tiger Moth, we might guess at a 4 stroke engine being used, offering a top rpm of say 7,000, our flying speed needs to be 40 mph. if we use the same method as above, it will give us a final figure of 6.034285, so a 6 inch pith prop will do in this case.