Take off rpm

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

Jim Isaacs wrote:

Thanks folks, so bottom line, Ross needs to adjust the pitch of his prop to achieve a minimum of 5,200 RPM at WOT on the ground at brake release for takeoff?

I am probably reading a little too much into the above -

My last 912 ULS aircraft, ATEC Zephyr, with ground adjust Fiti  2 blade prop, set for 5200 rpm/static, would not hold on the breaks much above 4000 rpm.

All TO's were rolling. Smooth progressive throttle to WOT. RPM would rise as speed increased. 40 knots, wheels would leave the ground, on grass at about 100m, sealed surface well below this distance.

I would rarely look at the tacho until CO, when I would adjust pitch angle to maintain approximately 5600-5800 rpm, 70-80 kn,  until safe altitude (500ft depending on obstacles/terrain). 

At safe altitude, I would reduce power 5200-5500 rpm, 90 -105 kn, and cruise climb to prefered altitude.

Level cruise, 5200-5500 rpm, 100-120 kn

The only times I flew, in level flight, below 5200 rpm was when "loitering" overhead at around 70 kn

This has been the most informative discussion I have found thus far concerning this topic. Thanks for putting the question out there.

I am trying to get the correct (best, safest) prop pitch worked out for a friend's Zenith 701 with a 912UL and three bladed Warp Drive.

It is currently set for 5700 rpm max in level flight, but the rpm on TO and CO is 5000, if I remember correctly. I have been concerned that the low rpm must be overloading (lugging?) the engine and not yielding max hp or torque. I think this discussion verifies that concern.

Since my friend intends to use the 701 to obtain his PPL, I am unsure about his having to use AOA to control rpm, but I will have to get things working properly and see how things match up with the Zenith recommended airspeeds.

Anyway, I'm grateful for the information provided concerning the different approaches to the problem. I am new to the Rotax world.

"It is currently set for 5700 rpm max in level flight,..."

This setting is not correct according to Rotax recommendation - see below

".....but the rpm on TO and CO is 5000, "

No offence intended but this makes no sense - 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. The problem with this is, there is an increased risk of failing to outclimb terrain/obstacles - see below for corrective action..

"I am unsure about his having to use AOA to control rpm,...." 

If you follow Rotax recommendation for 5200 rpm STATIC your pilot will have WOT, 5800 rpm available for TO/CO for up to 5 minutes.

This does not mean 5800 rpm needs to be used for the whole CO. The pilot can opt for a higher angle of climb (terrain/obstacle clearance?) which will automatically reduce rpm (without changing WOT) however this should not be so steep as to reduce the rpm below 5200.

In cruise (level flight) your pilot will have to reduce throttle to keep rpm at or below, Max Continues of 5500 rpm. This will mean a small loss of air speed but in a draggy STOL type aircraft, like the Zenith 701, this would seem to be  a small penalty for greater flexibility in TO/CO and less stress on the engine.

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