View Full Version : Horsepower/RPM... Me no understand!

4th Jun 2002, 14:03

I have a problem understanding as follows: Take a 150HP Lycoming (the one on, say, a TB-9 Tamp(ax)ico with a constant speed unit, running at a max RPM of , say 2500RPM. Now take a 2000HP BMW with a constant speed unit (the one on the FW190) running at 2500RPM. Assuming equal blade angle, P-factor and torque, WHY oh WHY does the BMW unit give more performance? Surely if the RPM, etc, is the same then the thrust is the same! Is it because of a bigger prop? Can somebody tell me before I go nuts?


Captain Stable
4th Jun 2002, 14:36
Equal speed between two CSU's does not mean equal blade angles.

Increase the power (manifold pressure) on either without changing the rpm and to compensate for trying to go faster, the prop will coarsen its pitch.

A bigger prop is part of is, yes - with a bigger prop you can put more power through it.

7th Jun 2002, 15:40
For a given pitch and RPM, more blades or larger prop absorbs more torque hence more HP (or kW) since HP=torque * RPM

For a given pitch and RPM, a larger prop accelerates a greater mass of air while more blades accelerates the same mass of air but to a greater velocity (less 'slip'). Hence, either will give a greater thrust.

Of course, a constant speed prop complicates things since pitch isn't fixed, but the principle holds.

7th Jun 2002, 18:37

So as I understand it, if the Lycoming is fitted with the FW190 prop, it wouldn't have enough power to drive the prop at the equivalent RPM that the BMW engine could drive it at for the same blade angle. Not enough 'grunt'. Conversely if the BMW engine was fitting with the Lycoming prop, the prop would overspeed due to excessive power even at a high-thrust prop setting. In other word's it's at to do with prop mass, prop pitch and RPM. Have I got that right?


Dale Harris
8th Jun 2002, 08:12
There is also the fact that the BMW engine is geared, i.e the true rpm of the engine and prop combination wiil be very different to the Lycoming example, which is direct drive.

8th Jun 2002, 12:30
It seems to me that the simple guts of the matter is how much air the prop can "bite" as it is turned by the engine. If it bites too much the engine can't turn the prop at the correct speed to make the required torque, if it bites too little the throttle must be retarded to prevent overspeeding and hence unable to make the required torque.
It's torque that does the work, not RPM. Torque enables the prop to accelerate air through the prop disk. A prop blade can usefully be considered as a very large number of tiny wing sections flying horizontally, each tiny section accelerating its own bit of air.
A prop has a designed band of RPM in which it operates efficiently and hence must be matched to an engine having the correct output characteristics. This RPM band is low - 2000 - 2500 is good, otherwise tip speeds cause problems.
A quick, off-the-cuff reply to your question belies the depth of research needed to design an efficient engine-prop combination. It is always a combination - it's not possible to change one much without messing up the other.

8th Jun 2002, 20:10

There are two essential factors to remember when looking at this question.

Firstly, performance is proportional to power to weight ratio. The FW190 with 2000 HP has more than 13 times the power of the TB9 with 150. But the weight of the FW is far less than 13 times that of the TB, so the FW has a much greater power to weight ratio and therefore much better performance than the TB.

Secondly, props do not produce power, but simply convert brake (or shaft) horsepower (Torque x RPM) into thrust horsepower (Thrust x TAS). The best (propulsive) efficiency they will achieve in doing this is about 80%, so the FW actually has about 1600 HP of useful power and the TB about 120 HP.

Even this 80% efficiency is achieved only with the correct combination of propeller, engine, RPM and TAS. If you fitted the FW prop to the TB it would be unable to provide the necessary torque to maintain the ideal RPM. Neither the engine nor the prop would be at their most efficient, so the overall efficiency and performance would decline.

If you fitted the TB prop to the FW it would be unable to absorb the power. You could liken thsi situation to asking a wing to produce 13 times its normal maximum lift coefficient. It would simply stall and waste most of the power.