Take off rpm

Yes interesting discussion here. Way back when (admittedly, when dinosaurs roamed the earth) I was taught to use pitch and power to achieve and maintain indicated airspeeds.  I was taught WOT at takeoff, rotate and climb at specific airspeeds based upon aircraft weight, Density Altitudes, obstacles to be cleared, etc.  No flight instructor I’ve ever flown with ever taught me to fly the aircraft using propeller RPM as a reference- is a Rotax engine different than other engines this regard? I think not. In my simple mind, after ensuring one is not masking an engine problem using prop pitch, just adjust the prop to achieve a minimum of 5200 RPM static at WOT.  That’s a Rotax SI number. Or higher if you need more power at takeoff. There’s no published maximum other than 5,800.  If your brakes won’t hold the aircraft secure it by additional means while you do this. Then go fly the airplane based upon indicated airspeeds and as the pilot in command, control pitch and power to ensure you don’t exceed any limitations, including propeller RPM. 

Jim Isaacs, I must have started flying before there were dinosaurs. I am with you on specified A/S for climb etc. I was attempting to be nice about my concern for using AOA to control rpm, but mainly I am searching for how to set the prop pitch to fly the 701 according to the POH and not break the ROTAX up front.

My limited Rotax experience has shown that with a max static rpm of 5200, the WOT in level flight will exceed 6000rpm. That can easily be controlled with the throttle. I was thinking that the pitch should be set to prevent the only maximum rpm that I knew about, 5800. I have seen that this prop setting results in to low rpm for TO and climb. I plan to try the 5200 static.

Thanks.

Sen Griffin, you are right, max WOT of 5700rpm is not correct, it is low by 100rpm. 5700 rpm is low by 1.724138%.   I accepted that small error because I do not possess a tool with which I can move the blade less than 0.5 degree, especially three times in succession.  My mistake was setting the prop pitch based upon the Rotax stated limit (5800) instead of the Rotax recommendation (5200) and considering the recommendation as only a starting point. You are most likely correct; the max static should be 5200 and then just use the throttle to avoid overspeed.

"the pilot can control WOT engine speed using angle of climb.""In this instance the pilot should be lowering the nose (shallow climb) so that the engine can achieve between 5200-5800 rpm."

No offense intended, but this is not a safe operating practice and certainly should not be taught to PPL candidates (the stated intent of the aircraft owner). A pilot can indeed control WOT engine speed using angle of climb, but a good pilot will fly his aircraft as instructed by the designer through the POH. In other words, a safe pilot will climb out at the Best Angle of Climb airspeed or the Best Rate of Climb airspeed at full throttle until reaching a safe altitude. Airplanes quit flying because of incorrect AOA, most often measured by airspeed in the aircraft powered by Rotax 912 engines.

It is the responsibility of a good, knowledgeable mechanic (which obviously does not include me) to have the drivetrain properly set to safely achieve the performance figures in the POH or advise the owner of the problem or maybe explain that a different engine, mechanic, or maybe both are required. 

I know a fellow who has built five or six airplanes and has put 2,000 hours on 2 or 3 different Rotax engines. I just remembered that he texted me something about setting the pitch. (I'm too old to be messing with this.) I will find that message. 

Bert,

You are not the first and certainly not the last to be perplexed by the Rotax recommendation of 5200 rpm Static (I tie my aircraft to something solid, to do this test ie don't rely brakes alone).

Many will refer to the POH (aircraft manufacturers recommendations) as their authority for setting engine rpm  - I have never understood how the airframe manufactures opinion, somehow trumps the engine makers (Rotax) recommendations.

To complicate things further  - there are many advocates of setting WOT rpm in cruise - the argument usually along the lines of obtaining max cruise speed/reduced fuel consumption and in the (unlikely) event of a throttle system breaking/slipping, the engine will only go to the pre-set WOT of around 5500rpm rather than the screamer of 5800 rpm (or more).

To my way of thinking, the later concept has the potential to compromise TO/CO performance, such that at a high density altitude day, perhaps a fully loaded aircraft and a short field, the aircraft may not clear the obstacle's/terrain ahead. Further unless the pilot is fully aware of the potential to stress the engine by operating below 5200 in TO/CO the engine may have its service life reduced $$$$$.

Engines set to 5200 rpm Static, will be lightly loaded in cruise (5200-5500 rpm) - this should lead to a longer service life and reduced hourly fuel consumption (possibly increased trip consumption, due to lower cruise speed).

I suggest you converse with Roger Lee if you would like to explore the idea of WOT rpm at cruise.

Ross Derksen wrote:

Just want to get some input about air box question asker earlier -

Thanks everybody for your insight

The first thing I am going to try is lowering my climb out angle to achieve 5200 instead of the 5000 I am now getting. I do 5700 WOT Level flight at 2500 msl, so I am wondering if part of my problem is too much warm air going to carbs during climb out? If I had an air box feeding the carbs cold air would that possibly improve power/RPM’s? 
Thanks, Ross

Hi Ross,

It seems you are asking if warm air is significantly degrading your engines performance during climb out.

Its unlikly that it would be making much difference.

You can get a fairly good indication of the temperature your air entering the carby's might be. Just purchase a battery powered fridge/room digital temperature gauge, with a nice long lead to the fridge thermocouple/sensor - they are usually quite cheap. Remove the top section of your cowling. Secure the temperature read out, somewhere easy to see within the cockpit. Run the fridge lead from the cockpit, through a cabin air inlet, forward to the engine bay. Secure the thermocouple/sensor in a position where it will register the air temperature going into the carby's. Use masking tape or similar, to temporally fix the lead in position, on outside of fuselage (you don't want it whipping around in the air blast). Replace cowling top taking care not to damage the lead. Go flying and see what temperature the air entering the carby's reaches.

My last Rotax 912ULS powered aircraft had a fairly consistent +10C above ambient, within the engine cowling - helped to reduce chance of carby ice and had little negative effect on performance.