Take off rpm

Hi Bert,

I have a Whirlwind three blade prop on the front of my 701, 912UL engine, slats installed, prop pitched to achieve 5,650 rpm WOT static. I need as much power as I can muster out of the engine when taking off from my short sod confined area home strip. RPM remains at 5,650 through the takeoff roll until rotation at roughly 38 mph. The 701 is so draggy my rpm increases just a touch to 5,700 during Vx climb of 47 mph, then increases to just below 5,800 when I lower the nose to achieve Vy once I clear the trees- Vy is only about 10 mph faster than Vx in my 701. When I reach 500 AGL, I typically reduce power to 5,500 and leave it there until reaching my desired cruise altitude. I built my aircraft and registered it as an EAB so it doesn’t have a POH, rather, it has a PIH that I wrote as the aircraft manufacturer- and there’s no requirement to do that, but I did so anyway. My aircraft will certainly exceed 5,800 rpm in level flight at WOT pitched as it is, but there is no regulatory requirement regarding prop rpm for an EAB-registered aircraft in the US. Should my throttle cable fail during climb I won’t notice as I’m at WOT anyway. If I lower the nose to accelerate and power can’t be reduced because the throttle cable failed, I have an emergency and the aircraft now belongs to the insurance company. My emergency procedure is to raise the nose again to keep the rpm at 5,800, lower the flaps, and switch off one ignition circuit. This will slow me down and reduce rpm enough to make it to a landing area that allows for a power off emergency landing without destroying the engine. Well, that’s the plan anyway. 

You can flatten a prop pitch to a point it looses efficiency.

Prop pitch can become inefficient with too much or too little pitch.

I can confirm Sean’s +10 C warmer air sniffed by carbs from underneath the cowling (compared to OAT which would go through an airbox). Measures were taken with a temp-logger during numerous flights with different temp-parameters. I have two parallel readings, OAT and air temp inside the air filter.

10 C seemed little at first but there is a loss of power when warmer air is fed to the carbs. Perhaps I can retrieve the data to be very sure but we are not talking 0.1 hp only. Just picture a TO on hot summer day and compare with “normal” days against that. It is noticeable for sure.

To be honest, I was thinking of installing an airbox myself at the time for performance at TO. However, I did not go for it despite the undoubted benefits because there is more to get from properly pitching the prop. The amount of hp one is simply not using with a TO rev of less than let’s say 5600 to 5700 is definitely more significant. Re-pitching the prop to achieve that sort of rev is a little process of trying out on a particular plane. Give it a try in small steps and go flying about to see how it feels and behaves and you will find a setting rather soon that matches your preferences. To me TO performance is also a matter of safety.

Regards

Peter

Sean Griffin, FWIW, I agree with you about to whom one should refer for information on machinery. Refer to airplane designer for flying information, but engine manufacturer for engine information and the same for the propeller specific repairs and how to.

What you describe concerning setting pitch to limit rpm in case of throttle failure is exactly why the prop pitch is set where it is now. The effect upon TO rpm is as you describe and I have been very concerned that this must certainly damage the engine over time. (That concern was based on experience with non-aviation engines.) I also agree with your concern about compromising TO/CO performance, that is why I continue to try to sort the various recommendations.

The brakes on the relevant 701 will not hold the plane at 4,000 rpm to do the run up so it has to be tied to something for static checks.

I repeat my statement in a previous post (or is it comment or reply?) - This discussion, or thread, or whatever it is, has been the most helpful that I have found. (making allowance for old ones that I haven't found)

Hello all

We should be careful here to separate aircraft types.  A 701 is not typical of the entire fleet  of Rotax powered aircraft.  This is why I do not like to state a static RPM on the ramp.  What is crucial for the engine is we unload the prop to 5200 where possible.  (fixed pitch) and not exceed 5800 (limited to 5 min by the Rotax specs)  I am rather perplexed as to why a lot want to ignore the maximum continuous RPM, it is shown in many places, of 5500 RPM.  This is a limit as well.  Without getting into specific aircraft types, such as a design with high drag, the RPM may very well be very near the same on climb and cruise.  This however does not work if you fly a clean fast aircraft, say a Pipistril for example.  The amount of RPM change is dramatic from static to cruise in those types will unload a lot from runway to climb and more in level flight.  Climb RPM is very dependent on your angle of attack of the blades, it stands to reason that from one prop design to the next can also have large differences.  Here is what a tiny part of CFR part 23 says about fixed pitch settings. 

"§ 23.33 Propeller speed and pitch limits.

(a) General. The propeller speed and

pitch must be limited to values that

will assure safe operation under normal

operating conditions.

(b) Propellers not controllable in flight.

For each propeller whose pitch cannot

be controlled in flight—

(1) During takeoff and initial climb

at the all engine(s) operating climb

speed specified in § 23.65, the propeller

must limit the engine r.p.m., at full

throttle or at maximum allowable

takeoff manifold pressure, to a speed

not greater than the maximum allowable

takeoff r.p.m.; and

(2) During a closed throttle glide, at

VNE, the propeller may not cause an

engine speed above 110 percent of maximum

continuous speed."

So the above is a short bit of the Part 23 world.  In the ASTM F2245 standards, for Light Sport Aircraft, they have detailed requirements that we cant ignore the maximum continuous speed of the engine RPM....here is an example of what an OEM has to prove out...

"F2245 − 16c maximum allowable speed for the configuration being investigated, and at the most critical power setting and CG combination. 4.5.4.3 Stability shall be shown by a tendency for the airplane to return toward trimmed steady flight after: (1) a “push” from trimmed flight that results in a speed increase, followed by a non-abrupt release of the pitch control; and (2) a “pull” from trimmed flight that results in a speed decrease, followed by a non-abrupt release of the pitch control. 4.5.4.4 The airplane shall demonstrate compliance with this section while in trimmed steady flight for each flap and power setting appropriate to the following configurations: (1) climb (flaps set as appropriate and maximum continuous power); (2) cruise (flaps retracted and 75 % maximum continuous power); and (3) approach to landing (flaps fully extended and engine at idle). 4.5.4.5 While returning toward trimmed steady flight, the airplane shall: (1) not decelerate below stalling speed VS1; (2) not exceed VNE or the maximum allowable speed for the configuration being investigated; and (3) exhibit decreasing amplitude for any long-period oscillations. 4.5.5 Static Directional and Lateral Stability: 4.5.5.1 The airplane must maintain a trimmed condition around the roll and yaw axis with respective controls fixed. 4.5.5.2 The airplane shall exhibit positive directional and lateral stability characteristics at any speed above 1.2 VS1, up to the maximum allowable speed for the configuration being investigated, and at the most critical power setting and CG combination. 4.5.5.3 Directional stability shall be shown by a tendency for the airplane to recover from a skid condition after release of the yaw control. 4.5.5.4 Lateral stability shall be shown by a tendency for the airplane to return toward a level-wing attitude after release of the roll control from a slip condition. 4.5.5.5 The airplane shall demonstrate compliance with this section while in trimmed steady flight for each flap and power setting appropriate to the following configurations: (1) climb (flaps as appropriate and maximum continuous power); (2) cruise (flaps retracted and 75 % maximum continuous power); and (3) approach to landing (flaps fully extended and engine at idle). 4.5.6 Dynamic Stability—Any oscillations shall exhibit decreasing amplitude within the appropriate speed range (1.1 VS1 to maximum allowable speed specified in the POH, both as appropriate to the configuration). 4.5.7 Wings Level Stall—It shall be possible to prevent more than 20° of roll or yaw by normal use of the controls during the stall and the recovery at all weight and CG combinations. 4.5.8 Turning Flight and Accelerated Turning Stalls: 4.5.8.1 With the airplane initially trimmed for 1.5 VS, turning flight and accelerated turning stalls shall be performed in both directions as follows: While maintaining a 30° coordinated turn, apply sufficient pitch control to maintain the required rate of speed reduction until the stall is achieved. After the stall, level flight shall be regained without exceeding 60° of additional roll in either direction. No excessive loss of altitude nor tendency to spin shall be associated with the recovery. The rate of speed reduction must be nearly constant and shall not exceed 0.5 m/s2 (m/s per second) (1 kt/s) for turning flight stalls and shall be 1.5 to 2.5 m/s2 (m/s per second) (3 to 5 kt/s) for accelerated turning stalls. The rate of speed reduction in both cases is controlled by the pitch control. 4.5.8.2 Both turning flight and accelerated turning stalls shall be performed: (1) with flaps retracted, at 75 % maximum continuous power and at idle; and (2) with flaps extended, at 75 % maximum continuous power and at idle (speed not to exceed VFE). (1) Flaps extended conditions include fully extended and each intermediate normal operating position. (2) If 75 % of maximum continuous power results in pitch attitudes greater than 30° for non-aerobatic aircraft, the power setting may be reduced as necessary as follows, but in no case be less than 50 % of maximum continuous power. (a) For flaps retracted, the power setting may be reduced as necessary to not exceed 30° pitch attitude. (b) For any flap extended condition, the test may be carried out with the power required for level flight in the respective configuration at maximum landing weight and a speed of 1.4 Vs1. NOTE 2—If the power setting was reduced to prevent exceeding 30° pitch attitude, then the POH or Flight Training Supplement must note that the aircraft is not approved for pitch attitudes greater than 30°."

is it complex, damn right...the good bit is for anyone with an amature built experimental you can do whatever you want.  You can ignore the RPM limits, it is your experiment.  In general light sport will pitch for level flight to load the engine to maximum continuous.  As noted by a number of you a high drag design might not change much from TO to level flight.  We should not suggest however that is the case for all aircraft.  

Just my view I guess. 

Cheers