I was taught, when reducing power, always reduce throttle / manifold pressure and then RPM. What you want to avoid are situations with high MP and low RPM, since they stress the engine.
Well, but this is anyway also known to be – within certain limits – an OWT.
It may go against what you’ve been told but it does not harm the engine. As you must have seen from the graph that shows in what ‘area’ of the RPM/MP settings you should operate the engine, when above 5200 RPM, there is no limit on the MP. Like Rotax says, the EGT (insofar as this is so important, according to Rotax it is..) is lower at higher MP settings.
As an aside, I remember from reading Deakin’s columns years ago, that this ‘oversquare’ thing is overblown.
Edit: overlapped with Bosco ;)
andy_flyer wrote:
It would go completely against my training
No, it will not. It is different from operating a Lycoming, but you are trained (or should be trained) to operate any engine and aircraft according to the manuals.
LeSving wrote:
Well, the “Rotax way” That is, do it according to the manual.
Well, I believe that’s exactly what I wrote that I’ve learned. Also I did read the manual carefully. (But I guess I’m exceptional…?)
aart wrote:
As an aside, I remember from reading Deakin’s columns years ago, that this ‘oversquare’ thing is overblown.
If refer to the OWT that the MP (as measured in inches Hg) must not be greater than the engine speed (as measured in 100’s of rpm) then it’s not just overblown but a complete myth. Just check the power setting tables in the POH of any aircraft with VP prop!
Airborne_Again wrote:
If refer to the OWT that the MP (as measured in inches Hg) must not be greater than the engine speed (as measured in 100’s of rpm) then it’s not just overblown but a complete myth. Just check the power setting tables in the POH of any aircraft with VP prop!
I’m not worried about OWT, I’m talking about the general principle of reducing power from climb to cruise, with a constant speed prop. I always heard/read/learned that you first reduce MP and then RPM, and not the other way around. I just went through the Rotax operator’s manual and couldn’t find anything about adjusting MP and RPM, neither one way nor the other way around.
So I’m just wondering, have I been doing it wrong the whole time, or what’s going on here. Apart from the Bristell bulletin I can’t find any reference to which is the correct way to do it on a Rotax, (except for pretty much every article about constant speed props online, which says reduce MP before RPM).
It is true for every aircraft engine I have known, but I have never played with Rotax. For sure, an engine is not normally damaged by pulling back the RPM first (a move equivalent to lowering the RPM while maintaining the torque) but some have a limitation on MP/RPM combinations in a high power regime.
How does that work with a constant speed prop and no prop lever?
andy_flyer wrote:
the general principle of reducing power from climb to cruise
First full disclosure: I am an aerospace engineer who majored in aeroengines but I do not specifically know Rotaxes
Second: a water+oil-cooled Rotax at 5000rpm is no air-cooled Lycosaurus at 2500RPM…the concerns are understandably different.
Third: You have to figure how high you have climbed and whether your engine is TC/TN or NA
We all know power is torque x RPM .
On Rotax the idea is to not use high RPM at low power (ie too low torque)
On Lycosaurus the concern is not to use too much power at low RPM (ie too high a torque value). However, in line with Rotaxes we also have a concern about “props driving engines” or too low or negative torque at low power, especially during descent.
Neither of the concerns must be overblown, but we might want to have an idea of what is going on inside our engines.
The “too much torque” concern is about peak pressure in your cylinders being rather high (read cylinder pressure as being more or less directly linked to torque). This is in order to generate the high power at low RPM. THis causes higher risks of detonation and higher stress overall on cylinders, conrods, crankshaft etc.
The “too low torque” concern on Lycosaurus is about negative pressures in the cylinders being too high and forcing rings out of the normal pressure position, causing oil to be blown into the combustion chamber and on some tuned crank engines higher wear on the tuning parts.
The “too low torque” concern on Rotaxes includes additional matters but I wont go there not being the expert: some explanation above but it seems to be connected to the very low torque and cylinder pressures resulting at low power while maintaining the quite high RPM these engines run at high power.
The point is, if you level off after climbing WOT to 5000ft on any NA engine, ambient and engine induction manifold pressure has already gone down without you moving the throttle and going, within reason, straight into lower RPM is not going to cause a high-torque condition.
A different matter is if you initiate cruise at 1000ft NA, or at a higher altitude with a TC/TN airplane where manifold pressure and power available is 90%-100% of SL, then of course you will be generating a high torque on your engine as you reduce RPM.
Rotax does not seem overly concerned on the high-torque condition but rather the low-torque.
Or just use a jet engine and forget about the whole thing, uh?
I’m not worried about OWT, I’m talking about the general principle of reducing power from climb to cruise, with a constant speed prop. I always heard/read/learned that you first reduce MP and then RPM, and not the other way around.
It is generally a good idea, in order to maximize the margin to detonation on any engine. However if that margin is positive that is all that actually matters and on the engines most of us fly it is not so critical, as reflected in the manufacturers recommendations: on many you can run four or five inches of manifold pressure ‘over square’ even steady state, never mind in temporary transition between power settings.
On old, large, high power supercharged engines with fixed ignition timing you can apparently induce detonation by decreasing RPM first, and so training reflected their minimal detonation margin. That does not mean you can induce detonation by running a couple of inches over square on e.g. a small Lycoming. Even less so on a Rotax, even allowing for low octane fuel.
