914 Throttle/RPM No-Go Zones?

Notice that all the numbers include a MAP value associated with the RPMs.

Taking 75% power as an example.  75% = 5000rpm @ 26" MAP.

This means that if you are making 5000rpm at 26" MAP you are making 75% power.

This does NOT mean that it is OK to increase the prop pitch so you only reach 5000 rpm at WOT and 28" MAP at Sea Level.

That would be WAY over propped!

The numbers for the 912ULS assume a fixed pitch prop at 5800 WOT at sea level.

With this SetUp, the MAP usually takes care of itself.

26" MAP at WOT probably means that you are at least at 4000 MSL on an average day.

You might see 5000 rpm in a near stall climb and 5400 rpm cruising.

4300 RPM at WOT would be OK at 10,000 ft where only 20" of MAP is available.

As you climb the available power slowly diminishes for every foot you go up.

At 10,000+ Feet you may need WOT just to stay in the Air.

You service ceiling is determined by where the engine is running flat out and no longer has any reserve power needed to climb.

Engine Power is NOT just about RPM,  MAP needs to be factored in every time!

If you are running a TURBO engine and a variable pitch prop, extra attention needs to used to avoid blowing the heads off the engine.

4500rpm and 40" of MAP is a real possibility.  At least for a few seconds!!!

Thanks Bill.

I’m using a 914 Turbo  with a VP prop.

So I’m probably being extremely dense here, but is it manifold pressure that I need to be aware of primarily? For context I live on a high plateau and operate out of a 5300MSL airfield with usual cruise in the region of 7500-9500 MSL for short flights and up to 13500 for long flights.

I usually cruise with the (Airmaster electric) prop in cruise at 5000RPM and with MAP of 30-32” - set according to MAP readout and not throttle position.

Recently I’ve started doing speed rallies and have been using 5000RPM with throttle wide open at around 500AGL (5800-6500MSL) - i don’t recall the MAP readings reached offhand but I take it that it is this that I should be paying attention to primarily. 

If I adjust prop pitch to the climb setting (5500RPM) are there issues with running for 75-80minutes at this setting? Am I correct in saying that this will result in a LOWER MAP for any given throttle setting compared to running at 5000?

I really find this quite confusing. 

Mike

MAP is controlled by the TCU. It senses the throttle position and adjusts the wastegate in order to reach a specific setpoint. If you are setting your throttle lever to max , the TCU will try to command the wastegate to boost at around 40 inches MAP. That can be achieved for max five minutes and at 5800rpm. Now if you push your engine for more time at full boost AND with lower rpm' s ,I think that your motor is not going to love you any more. You can travel all day long with 35 inches MAP and 5500 rpm and you won't hurt your motor. Always remember that rpm and MAP have to be always combined according to the chart that Rotax has in its operating manual. High MAP with low rpm' s are really stressing the engine. Set your prop pitch to get 5450-5500 and adjust the MAP at 34,5-35 and fly all day long. That is the maximum CONTINUOUS power you can have. And at the altitudes you are flying, you should be able to have it delivered by your motor. Above 35 MAP and/or 5500 rpm there is the five minutes restriction.

So Rotax publishes performance data (in 914 Operators Manual) for variable pitch propellers:

Takeoff power (115%, WOT): 39 inches and 5800 rpm

Max continuous (100%): 35 inches and 5500 rpm

and then additional data for 75% (31/5000) 65% (29/4800) and 55% (28/4300).

To me this appears to be data, not limitations. The limitations are "engine speed over 5500 is restricted to 5 minutes.

An earlier post suggests that your setting of 35 inches (100%) and 5000 rpm is over stressing the engine. This may or may not be true. I cannot find anything in the Operators Manual that addresses your question.

There is no question that lowering the rpm from the suggested settings puts more stress on the engine. The question that needs answering (definitively, with actual data) is: "How much can the rpm be lowered below the suggested settings before the engine becomes OVER-stressed?"

Your question is doubly important for those running a 914 without an in-flight adjustable prop. There is a thread on this forum that I think tries to address your question, but unfortunately it does not have any references to any actual Rotax data:

https://www.rotax-owner.com/en/rotax-forum/3-4-stroke-technical-questions/5761-setting-ground-adjustable-prop-pitch-for-914

One would think that Rotax must have addressed this obvious question somewhere.

Ken Ryan wrote:

So Rotax publishes performance data (in 914 Operators Manual) for variable pitch propellers:

Takeoff power (115%, WOT): 39 inches and 5800 rpm

Max continuous (100%): 35 inches and 5500 rpm

and then additional data for 75% (31/5000) 65% (29/4800) and 55% (28/4300).

To me this appears to be data, not limitations. The limitations are "engine speed over 5500 is restricted to 5 minutes.

An earlier post suggests that your setting of 35 inches (100%) and 5000 rpm is over stressing the engine. This may or may not be true. I cannot find anything in the Operators Manual that addresses your question.

There is no question that lowering the rpm from the suggested settings puts more stress on the engine. The question that needs answering (definitively, with actual data) is: "How much can the rpm be lowered below the suggested settings before the engine becomes OVER-stressed?"

Your question is doubly important for those running a 914 without an in-flight adjustable prop. There is a thread on this forum that I think tries to address your question, but unfortunately it does not have any references to any actual Rotax data:

https://www.rotax-owner.com/en/rotax-forum/3-4-stroke-technical-questions/5761-setting-ground-adjustable-prop-pitch-for-914

One would think that Rotax must have addressed this obvious question somewhere.

 

 

This is precisely why I asked the question - there is no guidance on what constitutes overload of the engine. 

So now this leaves two possibilities -either it is impossible to damage the engine with certain RPM/MAP combinations (apart form the hard limit >5500 for more than 3min), or Rotax has been remiss in not providing adequate data or guidance for use of the engine.

Intellectually I understand now why you’d not want to run high MAP against a low RPM but if you read guidance on ‘conventional’ aviation engines there is information about which zones the engine should not be operated in (Red box or red-fin areas).