Question about the MAP limits in SB-912-079 R1

Hi Brett,

I am disappointed that none of the real experts' have answer your post - I will do my best;

"Previously I had decreased the pitch of the propeller on my RV-12 to less than Van’s Aircraft to ensure that I was running at 5,250 rpm on climb out. "

"The question is if I used unleaded gasoline which typically has a lower octane, would I need to reduce my propeller pitch further or this acceptable since it is a short transient condition?"

It seems to me that Rotax have moved away from their very simple advice on engine RPM (WOT 5200 RPM/ Static) to a more complicated pilot/maintainer range of possibilities (RPM  / MAP / Fuel RON).  

If I were in your situaton I would adjust my prop to give WOT 5200 RPM Static (aircraft tied down). This will mean that your engine will always be operating within a safe RPM / MAP, subject to you climb out attitude ie you as pilot, can control rpm with attitude, raise the nose to reduce, lower the nose to increase rpm. The fuel type/RON will have nill impact at this conservative setting.

Note; During high load situations (Climb)  the original Rotax recommendation was to not go below 5200 RPM,  Max 5800 RPM - for 5 minutes and Max 5500 RPM - sustained/continuous.

It is not uncommon for a lower or higher Static rpm to be chosen, to suit a particular aircraft and its anticipated mission ie short field TO/Landings or maximising Cruise speed.

😈

Sean, I appreciate your response.

I also was surprised that I did not see a response to my question.  Hopefully that response is still coming.

Yes, the earlier guidance that one should one be at 5,200 rpm during a climb was more straightforward than the current Service Bulletin.  My interpretation of the 5,200 rpm guidance was that it was a minimum rpm recommended rpm for climb.  Namely, faster was better that slower.  So, targeting slightly above 5,200 rpm is desired to ensure all conditions are covered (i.e., altitude, temperature, barometric pressure and climb speed are covered).  This is why I targeted slightly above 5,200 rpm for prop pitch in climb.  

As in interesting side note, if I only had analog gages, I wouldn't have the manifold pressure data that I have every second with my digital data logger and the ability to know what else is happening at the same time.  Most likely I would only be reporting the manifold pressure during cruising.  Which I have shown is much lower.

Brett

Hi Brett,

"My interpretation of the 5,200 rpm guidance was that it was a minimum rpm recommended rpm for climb.  Namely, faster was better that slower.  So, targeting slightly above 5,200 rpm is desired to ensure all conditions are covered (i.e., altitude, temperature, barometric pressure and climb speed are covered).  This is why I targeted slightly above 5,200 rpm for prop pitch in climb. "

This is exactly my technique when flying a fixed  pitch (or ground adjustable) propeller. As long as you keep the engine speed below 5800 rpm for 5 minutes and 5500 rpm continuous, you are managing the engine speed correctly. I would usually accelerate rapidly, in ground effect, to 5800 rpm, raise the nose of the aircraft, when a little above best climb speed, engine rpm would drop to 5300-5400 rpm for climb-out.

"......if I only had analog gages, I wouldn't have the manifold pressure data...."

By setting your prop to Static WOT 5200 rpm there is no practical benefit in having a manifold pressure gauge.

If you have a constant speed propeller, MAP is used to set your power (throttle) RPM (engine speed)is used to set pitch.😈

Rotax Wizard;

With regards to this Service Bulletin, it was my previous understanding that the requirement for a 912 ULS with fixed pitch propeller was that the propeller needed to be pitched such that the engine was running be at least 5,200 rpm in a WOT climb.

So, for a fixed pitch propeller, is the 5,200 rpm minimum for a WOT climb still applicable and this Service Bulletin is applicable to cruise or is this Service Bulletin applicable to all operating conditions and the 5,200 minimum climb rpm guideline is over ridden by this Service Bulletin?

Thanks.

Brett

hi Brett

In regards to pitch we get a bit of pushback on how this should be done.  The SB is mostly targeting constant speed.  I fixed pitch we have to address a few issues.  First not to overload the engine and second not to overspeed it.  The prop should be considered a governor in the case of a fixed pitch.  

Start by looking at the two most referenced RPMs in our power curve.  Max RPM, with a 5 min limitation, and max continuous.  These on the 9 series are 5800 and 5500 respectively.  We have to accept that aircraft design will play a lot into the setting in that lift, drag, weight, all play a part.  For the moment don't worry about static trust, that can vary a lot due to the different aircraft or prop being used.  We normally will have a basic idea what you need to have to break in the engine and do the initial ground tests.  Once you start your test flights there are the decisions we have to make.  Start by being sure you have enough load to not overspeed the prop.  With a fixed pitch this means we must be able to go to WOT and control the engine to under 5800/5500 max. I dealy adjust the pitch to hit about 5500 to 5600 with WOT levell flight.  The idea is that allows some small margin for pitch changes and keeps you under the 5800 redline.  

Once you achieve the pitch for level flight WOT you will find out what your static prop load will have to be to not break that barrier.  In some very clean aircraft that static load on the runway may well be lower than what you expect.  These clean aircraft will tend to unload the prop a great deal on takeoff and climb, in most cases it will hit that magic 5200 on a normal climb.  When you drop the nose and let it run out to max at WOT the prop will keep the engine in control and not overspeed.  Remember this will not work well with a turbocharged engine in that the power to the prop load is not in balance.  (the prop at altitude will lose thrust with fixed pitch, the turbo will not lose power so it is then out of balance)

I would recommend any turbo engine to have constant speed or at least an inflight adjustable prop.  Fixed pitch works great set the way I have explained and will give you the numbers you need to see. 

Roger will confirm and add a bit.  The prop and settings from one OEM to another will not be the same.  indeed some props today are designed to twist in flight (blade flex) and therefore you need to test them for your own aircraft and set accordingly.

Cheers