Good morning from showery North Yorkshire , England.

A question for the technical experts please .

I have an old ‘plane that performs just fine when the O.S 46 LA spins at 9,500 rpm.
I have re-engined the ‘plane with a Tower 40 , which prefers an APC 10.5 x 4.5.

Is there a formula which would give me an indication as to what RPM I should set the Tower 40 ?

Sorry if this is a daft question. Keep well and take care during this world crisis .

Robin.

Good morning from showery North Yorkshire , England.

A question for the technical experts please .

I have an old ‘plane that performs just fine when the O.S 46 LA spins at 9,500 rpm.
I have re-engined the ‘plane with a Tower 40 , which prefers an APC 10.5 x 4.5.

Is there a formula which would give me an indication as to what RPM I should set the Tower 40 ?

Sorry if this is a daft question. Keep well and take care during this world crisis .

Robin.

  Probably not in any case, there's no real rule or way to determine it, but particularly not when you don't say what prop you were using on the 46.

    In general, I would guess that a smaller engine would have to turn slower on the ground, since it would unload more. But there is far too many other factors to make any blanket statements.

     Brett

Each revolution with the 5 pitch would be 5" minus about 10% slippage. If the engine unloads in the air to 10,500rpm then you're going about 45mph.

For 4.5 pitch you need more rpm and might be dealing with more slippage from less diameter and blade width.
\

   Just for information, if you are going to calculate it, your pitch is much too low, and your "slippage" is much, much more than 10%, more like 40-50%. That's why it's really going more like 60 mph. Any useful calculation using pitch and RPM has to use the "effective pitch" which includes use of the prop airfoil camber, which can greatly increase the effective pitch over the measured pitch (from back-of-blade measurements).

     You are generally correct that reducing the diameter will increase the "slippage". Lowering the pitch will *reduce* the slippage, because the necessary higher RPM will increase the dynamic pressure and thus require lower AoA.

   Bottom line is that it is just a matter of trying it and  seeing what it needs.

    Brett