• Re: Take off rpm

    by » 10 months ago

    Jim Isaacs, this 701 was purchased from the man who purchased it from the builder, so it actually has no POH, but because I knew my friend's intent of getting his PPL, I have tried to search out the TO/LDG recommendations from Zenith and print those for the owner.

    The Zondeer (it's painted with Rustoleum implement yellow) had the slats removed and vortex generators installed and also has electric flaps with a mechanical limit of 15* full flaps. Friend has bought a partially completed 750 kit for which he has a Corvair engine. While the plane belongs to friend, it lives at a 4,000ft paved runway, so the STOL is an aircraft capability to be learned by friend and his instructor. (I'm also working on that.)

    Thank you for the details. I am going to start over on setting the prop pitch when we get the Zondeer flying again. It is my personal opinion that the engine should produce something over the max continuous (5,500) for takeoff and initial climb. Friend purchased the dual throttle kit from Zenith and we are gradually working to install that so it will be a while before the readjustment can be worked out. 

  • Re: Take off rpm

    by » 10 months ago

    Hi all

    If anyone has the interest to read this paper it directly talks about the considerations of selecting a prop for an aircraft with the Rotax 100 hp, (73 kw) At least it is a new study from 2020 and done by reputable people.  Far more complex than almost all care to digest however on second read it starts to make some sense.  They discuss the contradictory requirements of STOL aircraft particularly, trying to get a good TO and get over the trees at the end of the runway as well as the desire to have a decent cruise speed.  

    Sorry, it is a heavy read but I think a few might like it regardless. 



  • Re: Take off rpm

    by » 10 months ago

    Rotax Wizard, I understand that "my" Zenith 701 is not typical of the Rotax fleet and why you need to point that out. It is amateur built and not subject to the rules to which you refer, but what will result from improper operation of the engine is the same.

    I fail to understand why maximum continuous rpm is relevant to a discussion about how to set ground adjustable propellers since "continuous" implies that higher is possible and should be restricted (logically with the throttle). I am open to learning.

    It dawns on me that I have asked about details, but not the guiding principle, so here goes another try. (expecting varying opinions)

    My propeller is set to limit the maximum rpm in level flight to 5,700. (something like 98.4% of 5,800) At that blade pitch, the engine developed about 5,000 rpm or a little less on TO and well less than 5,500 rpm when CO at Vx.  I reset the blade pitch because the previous setting gave at least 5,200 rpm on TO, but in level flight the WOT rpm was over 6,000. 

    So:  When a fixed pitch propeller can either allow 5,200 rpm on takeoff OR limit the maximum WOT rpm to 5,800, which criterion takes precedence? Must the pitch of the prop allow unloading to 5,200 at takeoff and let the pilot (throttle) limit to Red Line and Max Continuous, or should the prop pitch limit the level flight WOT rpm to 5,800.  On N721BD it cannot be both.  I don't care which one is correct, I just need to set the propeller correctly. Again, I am not asking for a particular number, that must be worked out for this airplane/engine, but which is more important for setting the prop.

    FWIW; While preparing this question, I have figured out what I should do, but even if I am correct, it would be more helpful to others if it comes from you.

    Thanks for helping me think.

  • Re: Take off rpm

    by » 10 months ago

    Hi Bert

    No problem.  it is not easy to wrap your head around things like the  prop loading.  The issue for most is to find the balance between what is good for your aircraft to fly well and at the same time give you long engine life.  

    The slower flying STOL aircraft have a much smaller envelop of usable RPM, I think that is understood.  What that says is your static, take-off roll and even into climb will see very little change.  Simply speaking the speed difference is so small that the blade has only a small change in the angle of attack.  Faster aircraft are going to react far differently in that the forward speed is so much different.  the blades "unload" as you move faster.  I get it, the design is great for STOL performance and for most that is exactly why they love the plane. 

    So If I understand you correctly the question is what is the most important since no matter what we do in this case is make a compromise.  I don't know the engine serial number you have, yes that might affect my answer.  if you have an engine made before 2006, one with a continuous serial number stamped into the crankcase (down between the number 2 and 4 pushrod areas, you have the old version case.  If your engine was a case where they split the number, starting with 06.0010, then you have the newer generation case.  (at that time crankcase numbers were changed to have the year 06 with a dot .  the balance of the number is the serial within that year.  so 96,07,08 and so on for year.  The rest is 0010, 0011, 0012 and so on up to whatever was made within that year run. 

    On the old cases I would prefer to see the 5200 used if that is the heaviest loading.  if for example you are going to climb a lot and fly a lot of time in climb configuration where the case is at its highest stress levels 5200 is much better than 5000.  Dont worry about an occasional thing here, we are talking about the general characteristics that the aircraft operates in.  While 5500 is max continuous I would not sweat 5650 or even 5700 as you said can happen in level flight.  This range of RPM will give you the best life of the crankcase.  On fast aircraft they used to fail due to people setting the pitch to less than 5200 WOT in level flight.  The idea was to get great crusie and low fuel burn, it did not work by the way.  Glider tug aircraft would also fail the case with extreme loading by full power climb followed by full off throttle dive back to the airport to tug another glider.....600 to 800 hours later the case is shot and it is an expensive repair.  

    Now in the event you have the newer crankcase i would not be as concerned about climb being just below 5200,, the internal changes are very major and we no longer see case failures since that change in 2006.  The WOT at level flight can be controlled by your throttle , however the security of the prop acting like a governor to control overspeed is gone.  Obviously control cable failure is not common (of the mechanical linkage some use) so this is unlikely in most cases.  

    All things considered in a STOL aircraft I would say it is better to get over the trees and have a short take-off.  The risks of overspeed can most likely be managed in other ways as you have already pointed out.  That magic 5200 is the peak torque point of the engine (actually it is 5252 but don't worry about the small error) Everything from 5200 to 5800 is building HP.  Limit any time over 5500 to less than 5 mins.  in a dive, or even small pitch changes, RPM can climb up quickly.  So if you can cruise at 75% power, that is 5000, you save a lot of fuel and have lots of room to maintain control with pitch change.  The engine as you know loves 5000 and up for performance. As for the climb performance RPM will be affected by the forward speed.  in a STOL that might be very slow however. 

    Not sure that helps, just my opinions on what is best for the engine. 



    37630_2_power by MAP and .jpg (You do not have access to download this file.)

    Thank you said by: Ross Derksen

  • Re: Take off rpm

    by » 10 months ago


    What is your criterion based upon? Why must you choose one or the other?

You do not have permissions to reply to this topic.