Above 5100 RPM for Climb

H. Paul Shuch is a Light Sport Repairman with Maintenance ratings for airplanes, gliders, weight shift control, and powered parachutes, as well as an independent Rotax Maintenance Technician at the Heavy Maintenance level. He holds a PhD in Air Transportation Engineering from the University of California, and serves as Director of Maintenance for AvSport of Lock Haven.

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designrs
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Above 5100 RPM for Climb

Post by designrs »

I've been flying Rotax for several years now and am surprised not to hear about this sooner. Apparently Rotax issued an advisory recommending climbing above 5100 RPMs. Here's how it was explained to me:

Every time a cylinder fires that explosion sends a blast of force through the the motor, gears and to your prop.

Under load above 5100 RPM's there's a fast stroke so that force is distributed through the the entire travel of the piston.

Under load below 5100 RPM's (slower stroke, higher load, more resistance) the force of the explosion is distributed only through part of the stroke, concentrated at higher forces, and putting more stress on parts effected in that partial stroke.

Think of it this way, if the piston were under extreme resistance and almost locked in a near compressed position you basically would be detonating a bomb in your cylinder.

So I was advised to only climb at Vx or Vy initially, or when necessary.
Otherwise climb at 5100+ RPMs.

The motor does feel happier that way, but only produces 300 to 400 ft/min.
My biggest performance gripe is not climbing fast enough.
I want 1,500 ft/min at all times, even on hot days at gross! LOL
SportPilot
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Re: Above 5100 RPM for Climb

Post by SportPilot »

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3Dreaming
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Re: Above 5100 RPM for Climb

Post by 3Dreaming »

designrs wrote: Every time a cylinder fires that explosion sends a blast of force through the the motor, gears and to your prop.
An explosion when a cylinder fires is detonation. If you have that you have bigger problems. When the sparkplug ignites the fuel air mixture it should be a smooth though rapid burn.

It is true that Rotax has wanted this for quite some time. It goes back to when they started having cracking problems with the old style crankcase. With the faster LSA aircraft and not having in flight adjustable props it creates a problem. With the pitch set to get the climb RPM to 5100 you will easily run past 5500 RPM in level flight.

In talking with Dean at Lockwood about this the propeller that is being used is also a factor. A light weight prop like the Neuform used on the CT will cause less problems than say a heavy prop like the Warp Drive.
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designrs
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Re: Above 5100 RPM for Climb

Post by designrs »

Just to clarify: While props are relevant to discussion... I was in no way referring to non-standard prop pitch settings, or pitching the prop for climb bias preferences... just the optimization of RPSs as conditions permit.

So how bad is it if you labor your motor holding Vx or Vy for several minutes in a steep climb?
Mark Gregor
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Re: Above 5100 RPM for Climb

Post by Mark Gregor »

There were some low S/N engines that cracked the cases. Rotax believes this may have been caused by low RPM and high power settings, thus the 5100 RPM recommendation.
All engines built in the last 10 years or so have strengthened cases. I do not know of any of these later serial number engines that have cracked a case. There may be some but not that I know of.

A variable pitch prop would be easier on the engine and one of the reasons many believe the fixed pitch prop is in reality less safe. Another argument against fixed pitch is differing airport elevation and density altitude. Not allowing the engine to optimize for these conditions always results in reduced takeoff performance or an engine that easily over speeds in cruise.

Mark
3Dreaming
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Re: Above 5100 RPM for Climb

Post by 3Dreaming »

jake wrote:There were some low S/N engines that cracked the cases. Rotax believes this may have been caused by low RPM and high power settings, thus the 5100 RPM recommendation.
All engines built in the last 10 years or so have strengthened cases. I do not know of any of these later serial number engines that have cracked a case. There may be some but not that I know of.

A variable pitch prop would be easier on the engine and one of the reasons many believe the fixed pitch prop is in reality less safe. Another argument against fixed pitch is differing airport elevation and density altitude. Not allowing the engine to optimize for these conditions always results in reduced takeoff performance or an engine that easily over speeds in cruise.

Mark
I've heard about a couple of the newer ones cracking, but haven't seen one first hand.
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designrs
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Re: Above 5100 RPM for Climb

Post by designrs »

I've been climbing above 5,100 RPMs or above and am enjoying the flight experience so much more. No more feeling labored on climb... just cruising along and before you know it you're up there. 8,500 over the coast today. Clear and pretty day with unlimited visibility. It was really GREAT!
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designrs
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Re: Above 5100 RPM for Climb

Post by designrs »

8,500 over St. Augustine, beautiful day!
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