• Re: Take off rpm

    by » 9 months ago

    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?

  • Re: Take off rpm

    by » 9 months ago

    Hello again Jim

    Not everyone seems to agree on the starting point.  In certification testing for aircraft with fixed pitch (FAA in the USA following part 23) the critical factor is that the pitch must load the engine to its maximum continuous RPM as denoted by the engine manufacturer.  In our case that is 5500 for all the 9 series engines.  This then allows, in theory, any small pitch changes to limit you below the red line 5800.  You may not achieve this control if you start with 5200 static is all I am saying.  There are a large number of references to this within the CFR code, here is one.  I am a bit uneasy about speculative settings and until you test your particular aircraft for how the prop works at speed.  

    § 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.

  • Re: Take off rpm

    by » 9 months ago

    I see your point, well, except for the takeoff RPM.  The FAR 23.33 data you posted states the prop must limit the engine to maximum allowable takeoff RPM. That’s 5,800 in the Rotax 912. But you stated Maximum continuous 5,500 RPM in your comments? 

  • Re: Take off rpm

    by » 9 months ago

    A check of your operators manual will show both values.  Given you have a fixed pitch the maxium should be 5500 level flight.  if you hit 5500 in the climb when you drop the nose for cruse flight you will overspeed past the redline of 5800.  While it may seem simple to just pull back on the throttle the idea is that the prop is your govenor to cotrol the engine in the event of a broken throttle.  (this is why to pass certification the throttle butterfly is always spring loaded to WOT)  The actual verbiage in certification is any controls must fail to a run position, not idle in this case. 

    The problem goes way back to the early type 914 version engines, turbo boosted, and deep heat cycles were causing crankcase cracking on the engines.  This also appeared on the type 912 100 HP engines with the higher compression ratios.  Most common were aircraft that people selected prop pitch to get fast cruse speeds, common in lots of European areas for some reason.  This was a huge issue for glider tug aircraft that would run full power to climb fully loaded, drop the glider and then throttle off dive back to the airport to pick up the next glider.  The result was deep heat cycles and extreme pressure loading of certain areas of the crankcase.  Generally the damage would be fretted crankcases in the center and sometimes cracks in certain areas of the case.  (see annual condition inspections relevant to any case made prior to mid 2006, 912 and 914 series) These failures lead to this SI on engine loading.  The worst damage, fastest failures, occurs when we run the engine behind the power curve.  Behind the curve is when the engine cylinder pressure is still building up to peak torque.  Once an engine crests over peak torque it unloads the cylinder pressure and between there and up the RPM will still climb due to the drop of the pressure.  Eventually it drops off also as the engine can only "breathe" so much.  The relationship of the intake size, runner length, intake and exhaust valve size and camshaft opening all hits its limit and HP will drop away.  So the factory wants us to use the power in the engine when we are at peak or after for the most part.  High RPM is not really the problem.  The issue here is not to stress the engine internally and allow it breath.  To much pitch is not really nice to the internal parts, not to mention it adds to detonation risks within the cylinder.  

    Sorry, long answer.  


  • Re: Take off rpm

    by » 9 months ago

    My OM shows both values in a para. 4 table applicable to variable pitch props, is this what you refer to in your first sentence above? 
    If Ross adjusts his fixed-pitch prop to achieve a maximum of 5,500 rpm at (WOT?) level flight, he will likely remain where he is now, well below the minimum 5,200 takeoff rpm, and never be able to access the maximum horsepower available from his engine.  As it is now, at 4,800 rpm he is using only 65% of his available engine power on his takeoff roll.  All discussion about manifold pressure and engine stresses aside, this places him in a potentially hazardous situation under high density altitude flight conditions, even with a long paved runway. 
    Me, I’m thankful Part 23 prop pitch limitations do not apply to my EAB aircraft.  I’d rather have maximum horsepower available when I need it at takeoff or go around to clear those trees rapidly approaching me, which under those conditions, due to deck angle/lower indicated airspeeds, would not over speed the engine should my throttle cable fail anyway- and accept the risk of a throttle cable failure over speeding the engine in level cruise flight, where I have more altitude and time to deal with the situation.

You do not have permissions to reply to this topic.