Rotax 912 WOT 5200 RPM

[Burtonport]

Hello. I am a newbie builder of the RV-12iS. I finished the empennage kit and I am waiting for the wing kit.
On another thread there is discussion about the advisability of less than 5200 RPM for the Rotax 912ULS at WOT, (DURING CLIMB). It has been suggested that 5100 WOT (DURING CLIMB) is permissible on the RV-12.
I currently fly a Zodiac 601 HDS with the Rotax 912ULS and I pitch my prop so that WOT is at least 5200 RPM (DURING CLIMB) because that was the recommendation. What would happen if an RV-12 was pitched to 5200 RPM WOT (DURING CLIMB)? Would that cause it to no longer fall within Sport Pilot rules? Thanks.
Regards,
Damien Graham

[Driftdown]

The "sweet spot" for the 912 is 5650 at WOT (measured during straight and level flight). That gives you the best combination for climb performance and cruise.
5200 WOT (measured during straight and level flight) is way, way too coarse (too much pitch/bite). It puts too much stress on the drive train and drives up internal temperatures.
In reality, that engine can easily run at 5500 RPM, all day long.

[AndrewR]

912is has a much more sophisticated engine management system, so I wouldn't assume restrictions on the 912ULS also apply to the 912is. Look for specific documentation from Rotax.

I just checked the 912is manuals... very interesting in light of the other thread. The fuel consumption charts in the Operators Manual page 5-5 show about a 30% increase in fuel consumption going from 900 to 950mbar manifold pressure (26.5 to 28 inches).

So it obviously goes way richer at high MP.

[tomkk]

Originally Posted by Driftdown

The "sweet spot" for the 912 is 5650 at WOT. That gives you the best combination for climb performance and cruise.
5200 WOT is way, way too coarse (too much pitch). It puts too much stress on the drive train and drives up internal temperatures.
In reality, that engine is made to run at 5500 continuously.

I think the 5100/5200 WOT recommendation might be takeoff RPM while the "sweet spot" recommendation of 5600-5650 might be at cruise.

[Burtonport]

I am sorry. I meant to say that I have the prop pitched for at least 5200 RPM WOT DURING CLIMB. I will edit my original post. Thanks.

[Mich48041]

Would that cause it to no longer fall within Sport Pilot rules?

Breaking the 120 knot speed limit for LSA aircraft and Sport Pilots in the USA with the Rotax 912 powered RV-12 with fixed pitch prop is unlikely without risking damaging the engine.

[rvbuilder2002]

Originally Posted by Burtonport

Hello. I am a newbie builder of the RV-12iS. I finished the empennage kit and I am waiting for the wing kit.
On another thread there is discussion about the advisability of less than 5200 RPM for the Rotax 912ULS at WOT, (DURING CLIMB). It has been suggested that 5100 WOT (DURING CLIMB) is permissible on the RV-12.
I currently fly a Zodiac 601 HDS with the Rotax 912ULS and I pitch my prop so that WOT is at least 5200 RPM (DURING CLIMB) because that was the recommendation. What would happen if an RV-12 was pitched to 5200 RPM WOT (DURING CLIMB)? Would that cause it to no longer fall within Sport Pilot rules? Thanks.
Regards,
Damien Graham

Raising the climb RPM requires reducing the propeller pitch.
Reducing the pitch will result in slightly lower speed if cruising at the max cont. RPM of 5500, so doing would not make the airplane exceed the LSA speed limit. It will actually make it slightly slower.

[jnmeade]

After you reach obstacle clearance, flatten your climb pitch attitude and adjust flaps until RPM rises to 5200. IOW, lower the nose.

[Jolly]

You need to remember that the manuals are written for 3 different engines. The 912UL, 912ULS and the 914. The specs are written with the assumption of a variable or constant speed prop and that you can achieve 5800 rpm at take off and usually fly at 5500 WOT rpm. This isn't our LSA market norm. It wasn't written for fixed pitch type props.
Some interpretation has to be done since we only use fixed pitch or ground adjustable props in the US for LSA. There are planes in Europe that have the Rotax that are small aircraft with retractable gear that get over 200 mph.
Since 1990 when the 912 was introduced the huge majority has been using 4800-5200 WOT rpm at take off here in the US. Yes there are some higher and lower. Most over the last 15 years use between 4800-5100 rpm at take off. Prop types do make a difference in static rpms at certain throttle settings. Even between 2 blade and 3 blade, flexible vs stiff and long vs short.
Having a 4900-5050 rpm at take off usually puts you in the 5500 - 5650 range at WOT in level flight depending on the factors listed above. Aircraft hull design plays a major factor in aircraft speed. The 5600-5650 WOT in level cruise at your average altitude is a sweet spot to have a well balanced climb, cruise, fuel economy and engine temp. You may have a special need for a better climb prop and a flatter pitch due to things like short fields all the time, obstacles, floats, constant high DA's, ect.. Being over pitched and under 5500 rpm at WOT has no redeeming qualities. Once over pitched the engine hasn't the HP or torque to be efficient any longer. Being over pitched stresses the engine. Before mid 2006 the over pitching at times would put a 2" long crack on the top of the case and ooze oil. Since mid 2006 there was a new crankcase introduced hence the newer 2000 TBO.
A prop setup to get 5700-5800 rpm WOT will use more throttle and fuel to keep up with a person in an identical plane that is set up at 5600 rpm WOT. The person with a 5650 prop setup will out climb one that only gets 5500 rpm WOT in level flight. There is nothing here that hasn't been proven and shown to be true over decades of test, research and real world use.

[RFSchaller]

At WOT in cruise I see about 5580 RPM at 6000 FT DA. My climb is typically about 5080 RPM and 900 FPM solo at 2500 FT DA. After almost 700 Hours the engine still seems happy (good temps and compression). I think I?ll stick with my prop pitch.