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Rotax Bank Angle Limitation


Doug G.

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Posted

In a coordinated level positive G turn the engine doesn't know it is at an angle. The sum of all forces is down through the bottom.

 

Even inverted if at positive G's then engine doesn't know its inverted.

 

How is this limitation useful?

Posted

The manual is a bit vaigue. In the "Bank angle" section, it says the "Max" "Deviation from bank angle" is 40 degrees. What does "deviation from bank angle" even mean?

Posted

I got it!

 

CT's max bank is 60° and 60°+ is where deviation from max bank angle begins and 100° is where the limitation begins.

 

It makes sense, at 60° there is no issue the engine's fluids are motivated to the bottom of the engine. We know 90° is not possible so somewhere between 60 and 90 is the max sustainable positive G turn the air-frame/engine is capable of. Rotax is saying you can effectively tilt the engine some but when performance is limited you can only effectively tilt the engine to 40°.

 

Another way to look at it is an 80° turn is a 20° deviation from max bank angle and within Rotax's limitation.

Posted

CT, that makes as much sense as any other explanation! :)

I agree with Fred what does that statement mean? It could mean that if you have ever had your plane over 40° your Rotax warranty may be void - if Rotax wants it to read that way.

Posted

CT, that makes as much sense as any other explanation! :)

I agree with Fred what does that statement mean? It could mean that if you have ever had your plane over 40° your Rotax warranty may be void - if Rotax wants it to read that way.

 

It means precisely what it says. In aviation the term deviation is used relative to various variables like the LOC.

 

To rotax max(bank-angle) is a variable, from one air frame to another.

 

deviation from [max] bank angle

  • you can only deviate from the max bank angle by exceeding it, otherwise you are not deviating
  • max angle on CTSW = 60°

max deviation from max(bank-angle) = 40 ° = Rotax limitation

 

Rotax limitation for CTSW = 100°

 

==============================================================================

 

Say you put a Rotax on a Challenger LSA and the Challenger was limited to 45°

  • max angle on CTSW = 45°

max deviation from max(bank-angle) = 40 ° = Rotax limitation

 

Rotax limitation for Challenger = 85°

Posted

CT, sorry, but how could the operating limits of the engine change for each airframe? It doesn't make sense. When you have verification from Rotax, then put the theory forward as fact. Until then, I will not believe that Rotax approves use of my 912 in bank angles exceeding 90 degrees.

Posted

CT, sorry, but how could the operating limits of the engine change for each airframe?

 

In My CTSW I can easily bank 60 degrees and stay level, positive, and coordinated. The only effect on the engine is a higher than 1G condition.

 

Put my motor behind a powered parachute and my bank angles become limited. You can get into a steep bank in any aircraft but will it be with positive Gs? Depends on the air-frame and that's why it makes sense to use a deviation from max bank.

 

90 degree level turns require infinite g's so any limitation at 90 or better is the same as unlimited. This limitations is only helpful with aircraft that cannot sustain level steep turns.

 

I arrived at this thinking by realizing the only interpretation that works is a literal one.

Posted

If I had to guess...

 

...they put a 912 on a test stand and rotated it in both directions until there was a problem - either with the floats in the carbs or oil pickup or oil tank or something. Got to at least 40° in both directions before issues.

 

Then poor communication skills got in the way, possibly aggravated by poor translation.

 

All speculation, driven by the fact that in a coordinated turn simple physics keeps bank from coming into play here, and the 2g's in a 60° bank is less than the plane is rated for, so that can't be it.

 

Note: I think this has come up previously.

Posted

Why would you not interpret literally if the literal interpretation works?

 

"Max" "Deviation from bank angle" is 40 degrees

 

It just isn't very ambiguous. I agree they put it on a stand and tilted it till they saw problems but next they translated that, they applied aerodynamics and realized that their 40 degree tilt equaled zero in the air. max deviation from max bank angle is the best variable Rotax has to work with and it simulates that tilted engine on a stand.

 

Ask it this way, with this powerplant / airframe how far can I bank and remain positive? We instinctively know answers that work, I know I can demonstrate 60 degree turns and I instinctively know when I'm negative and how that acts on my fluids.

 

If you worked for Rotax and were tasked with creating a limitation how would you write it?

Posted

Ed,

 

I can't quite grasp what you're getting at.

 

Since our engines are (fairly) rigidly mounted, how can there be any deviation from bank angle?

 

If the plane is level, the engine is level - no "deviation".

 

If the plane is at 60°, the engine is at 60° - no "deviation".

 

I think you're reaching on this one, but I admit I may just be dense.

Posted

I think it means the engine should be mounted so that it is within 40° of horizontal.

 

Maybe, after all some engines are mounted vertically (as in helicopters) and work fine.

 

I think someone needs to get a response from ROTAX - I do not agree that it's unambiguous. If it was unambiguous, we wouldn't be bebating it's ambiguity, now would we?

Posted

This discussion reminds me of a Mark Twain quote, "There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact". Not that interpreting Rotax manuals is science, mind you.

Posted

Ed,

 

I can't quite grasp what you're getting at.

 

Since our engines are (fairly) rigidly mounted, how can there be any deviation from bank angle?

 

If the plane is level, the engine is level - no "deviation".

 

If the plane is at 60°, the engine is at 60° - no "deviation".

 

I think you're reaching on this one, but I admit I may just be dense.

 

Oh Eddie,

 

Your looking for deviation in all the wrong places.

 

Since our engines are (fairly) rigidly mounted, how can there be any deviation from bank angle?

 

You are looking for deviation where none can exist and asking how can it be? It would be truly nonsensical to use the term in this context.

 

Let me use it in context.

 

The pilot deviated from the maximum bank angle of 60 degrees and demonstrated a 75 degree turn. In this case he deviated from the 60 degree maximum by 15 degrees well within Rotax's 40 degree limitation.

Posted

So maybe someone needs to ask Rotax what it means. I don't know how important it is, but when I don't understand something I look to you guys for an answer.

Posted

Let me use it in context.

 

The pilot deviated from the maximum bank angle of 60 degrees and demonstrated a 75 degree turn. In this case he deviated from the 60 degree maximum by 15 degrees well within Rotax's 40 degree limitation.

 

Let me just say, I don't think that's what they're getting at.

 

Let's save internet bits and just have Roger or someone contact them for an interpretation.

Posted

The language used in the POH's is a bad translation. It says:

 

Bank angle [operating limitation] Deviation from Bank Angle = 40°

 

The problem with that language is that it compares Bank angle to Bank angle. They both can't be the same and still realize deviation.

 

The CTLSi POH adds this: Note: Up to this value the dry sump lubrication system warrants lubrication in all flight conditions.

 

We know from the above that we are talking about how much the engine can tilt and deliver oil.

 

If we look to and installation manual we get clarification: http://contrails.fre...2006_d03186.pdf

 

The deviation is clearly from the effective vertical but what is effective vertical?

 

From the final Note we can assume that bank angle is the effective vertical if stable, non accelerated flight condition.

 

Conclusion: ​deviation from effective vertical is due non stabilized flight (with acceleration). You can deviate at slow speeds due to uncoordinated and or accelerating flight condition however at a bank angle at or above 60° deviation from effective vertical will begin due to limitations of the power-plant / air-frame.

 

​My assumption / explanation above looks to be true because you will not deviate from effective vertical by 40° without substaintially exceeding 60°.

 

The installation manual concludes: The resulting bank angle ( effective vertical - 40°) may never exceed the max. bank angle (60°)

 

post-6-0-11114000-1377952697_thumb.png

Posted

I know this is not Rotax, but here is an other example. Almost every summer we have a tree trimming helicopter come through the area. They have a saw with 11 saw blades with a diameter of 24". They used to use Rotax 503 engines to power the saw, but switched to a "V" type 4 stroke twin industrial engine. They said the engine could be mounted up to 45° from vertical. When on the ground the engine is off by the limit one way and in the air it is off the other way.

I also think it is a translation issue. I think that it is a limit as to how far off of vertical it can be mounted.

post-40-0-99122800-1377957669_thumb.jpg

Posted

...

I also think it is a translation issue. I think that it is a limit as to how far off of vertical it can be mounted.

 

There is no ambiguity in the Installation manual. The limit is for the differential between effective vertical and current bank angle, the limit is 40°

 

Effective vertical for that saw is effected by both the mounting and the bank angle. Rotax qualifies their limitation by saying: The engine design is for a non-aerobatic, fixed wing tractor or pusher type configuration with the oil return port in the optimum position. With this condition the engine is properly lubricated in all flight profiles.

 

The saw does not qualify for this limitation.

 

This limitation could not be for " how far off of vertical it can be mounted." because that brings another variable into it.

 

If your 912 was mounted at 40° from vertical in your CT then your differential would be 80° in one direction and 0° in the other.

Posted

This limitations is misnamed, its not as much a bank limitation as it is a tilt limitation because pitch factors in with the same weight as bank. 1° increase in pitch gets you 1° closer to the limitation just as 1° increase in bank gets you 1° closer to the 40° differential limitation.

 

If you used a more complex method that considered a tilted engine limitation you could apply it to an engine that is drooped duel to compressed motor mounts. If the engine drooped 1° you would be 1° closer to the limitation ( pitch down only )

Posted

CT, I think we are in the same book, but different pages. The limit has to do with the forces acting on the engine. No more than 40° from the effect of gravity on the engine. This really has nothing to do with the bank angle of the airplane, rather the forces being applied to the engine. 40° in a big slip could excede the limits, but 135° bank with positive loading might not.

Posted

Tom,

 

The question is how to interpret the 40° Bank Angle Limitation, the OMs are not clear on their definition of deviation but the installation manuals are quite clear. There is only one page to look to, the one that provides the definitions and engineering, this one from the installation manual:

 

post-6-0-11114000-1377952697_thumb.png

 

The section name from the installation manual is: 12.Banking of plane deviation from the effective vertical therefore it is about banking of the plane ( including pitching ).

 

Neither of your examples qualify for the limitation, the big slip and 135° examples violate the limitations requirement that the bank be stabilized and without acceleration.

 

I'm not disagreeing with you I'm just saying that the limitation is based on deviation from effective vertical and 60 degrees bank, level pitch does qualify for effective vertical.

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