ORIGINAL: FlooredCOBRA
Say you have an 8 x 6 and cut down to about a 6 1/2" diameter. What would the pitch be then after cut down?
Regarding the nominal pitch at the tip after cutting: Unfortunately there's no general rule/receipt for that. In fact prop manufacturer
X uses a certain pitch distribution for his props while manufacturer
Y does apply a different one. Thus after cutting down the prop the final pitch result at the new tip location will sometimes be quite surprising [:@]
In addition the labels quoting dia/pitch often differ significantly from the REAL pitch measured at a certain blade location. Let's take the 8*6 prop. That announced pitch of 6" should be valid at the outer 20% to 10% of the prop radius. But quite often you will notice up to 1” less [:'(] of stock pitch at the tip..
So there's no rule of thumb! It is the process of
trial and error helping to achieve the best result for your speed project.
Another disadvantage of most commercially available props worth mentioning is the airfoil thickness being too much when talking about 'true' speed props – resulting in too much drag and reduced top end performance. Even the props belonging to the APC D1 pylon series are too fat due to the utilized prop material (let’s call it ‘APC-plastic’
).
In conclusion these miserable circumstances 'forced' me to make my own speed props - of course in carbon fiber technology (MUCH better tensional strength and torsional stiffness). Now it is possible to determine the custom design (pitch distribution, airfoil distribution, blade area and blade geometry) right from the beginning - based on experiences from certain airframe/engine/pipe combos during a test stage.