Does RPM = Power?

The answer is a bit different depending on whether you have a constant speed prop or not.

If you have a fixed pitch prop, another way to frame this question is, "how much horsepower is required to rotate this propeller at this airspeed at the desired RPM?" The design of the airplane (particularly the size and number of blades of the prop) should be such that with the engine at full throttle in a climb, the RPM is not overspeed, but is clearly on the high end of the allowable speed range. Thus, in the general case, you can conclude that if you are producing the desired RPM, you are obviously producing the horsepower required to do that.

If you have a constant speed prop, you need to be looking at manifold pressure rather than RPM. Like the fixed-pitch prop case, there is a similar relationship between the horsepower produced and the bite of the blades.

Hi All

The formula for HP is T X RPM divide by 5252  = HP.  The 914 engine must get to 5800 and the manifold pressure has to hit the target pressure, I believe for the 914 that is 40 HG.  if you don't get that then you have less than the 115 HP shown.  There are charts in the operator's manual that will show you the variables.  This only works if you have an inflight adjustable prop or constant speed prop.  With fixed pitch you have to compromise so you do not overspeed the engine with changes in load from the different air density at altitude/temperature variations.  

In general with fixed pitch you should try to get no more than 5500 RPM maximum continuous.  The problem with the turbo is that as you climb the engine will maintain pressure inside (torque remains high) and the fixed pitch no longer has enough load to control RPM.

Cheers

Hello Joaquim,

was it your first start. You are a new owner but i thik it was not your first flight. In other conditions you had better performance - it's right? So your question is why you had less power at this situation - is it? 914 at a motorglider - under every conditions there is to less power an indicator that something goes wrong. There are so many reasons possible. Without more details it is not easy to get help. If you know in germany there is curently a surey about power loss of Rotax engines. See at the DAEC.de for this topic. This survey is not an answer - but you see you are not alone. But now: no panic First inspect the engine by yourself. Can you detect loose hoses? Esp. at the suction inlet / airbox / ventlines at the carburator. Are some rubber connectors defect? All connetions from and to the carburator tight? Who is the performance at ground test? At 5800 and less power there is the question about the pitch of the blades.  Something must be wrong. Motorglider - Airbrake vorgotten?? So many reasons.The posts before can also be helpful.

Greetings

Hi,

It’s a Stemme S10-VT, which has only 2 propeller position: take-off and cruise. My description is for the take-off position. My feeling is that at 5.800RPM it doesn’t climb as well as in the past. Less than 10h ago it had the 5-years hose replacement ;done by an Rotax service centre. After this service it started to loose 1 drop of oil after each flight. The Motorglider was not flown for 6 months, and now in the last 2 flights it did not loose this drop of oil. 6 months ago, just after the service I have replaced the turbo servo because it was faulty. I had also a very scary climb before replacing the servo. Unfortunately I don’t speak German. 

So, if understood right, because it’s a fixed propeller (take-off pitch), if it reaches the 5.800 RPM, it’s delivering the 115HP, right? In that case why the feeling of less power? The airbrakes were closed and locked 😉

I was in a hurry and didn’t copy the flight log from the computer (LX9000), but I am very curious to analyse it because it records the climb rate. 

Hi Joaquim

The 2 position should work fine if it is set correctly.  5800 is good as long as you are getting full boost from the turbo.  The hose replacement would have nothing to do with any power loss, that is more about changing parts before they begin to leak.  You would need to have a reading of the boost pressure (MAP) and look to see if you have full boost.  With the cooler weather now in fall you should see much better performace than hot summer weather.  

Cheers