Static RPM

Thought some folks might be interested. I finally got around to testing a new UMA tach to replace the Rotax one that I suspected was reading low by as much as 600RPM based on prop balance reading. Seemed impossible to be off that much, but indeed, it appears to have been way off. My static RPM with the Warp prop with old tach was 5200. Now nearly 5800! I suspect full cruise to be over 6000, though I didn’t test that yet. Got some prop adjustments to make! Even though I paid for a prop balance for a prop that was nearly in balance already, the RPM check was worth it.

The 70” Warp prop is strange in that full cruise RPM is only about 300 RPM above static RPM.

Now, Warp Drive 70” 3-blade remounted on 912ULS with new mounting hardware and repitched, trying to get 5650 WOT. I don’t know if the Warp prop is just strange or something else is happening, but I get 5400 static and 5600 WOT sea level. And with warmer weather, it’s more like 5200 static, 5400 WOT. Very little difference. How can this be such a small range? Some days, it’s seems the range is actually 0 (5400/5400).

So, I’m wondering if I’m dealing with a prop issue or something else.

You're not looking to get 5400-5600 rpm WOT static (on the ground). The 5600 is in the air in level flight at your average altitude. Start with 4700-4800 static and then go fly and see what the rpm is in flight. Then make any final tweak of the pitch.

Thanks Roger. My numbers are comparing Static WOT on the ground (5200) with WOT in low altitude cruise (5400). Static=ground, WOT=in air. If I were to set 4800 static WOT, I’ll get about 5000 WOT in cruise. To get 5650 WOT in the air, I’ll likely have to set 5450 static on the ground. Like I said, something is odd in Denmark. BTW, new UMA tach, confirmed with TruTrak.

Reference to Rotax 9 rpm/prop pitch setting;

It would seem that there are two schools of practise on this topic;

(Note: These techniques only apply to fixed and ground adjustable propellers)

STATIC - Rotax recommend 5200 rpm Wide Open Throttle (WOT). Static can only be achieved with the aircraft stationary (usually tied down). To achieve 5200 rpm with WOT, the pitch of the prop must be adjusted/selected so as to limit the rpm. The prop is acting as a governor.

CRUISE - As far as I know this is not a Rotax recommendation. The prop pitch is adjusted/selected, so that in level flight/cruise (at the pilots preferred altitude) with WOT,  the engine speed will not exceed the Rotax max continuous engine speed of 5500rpm (ie is governed to this engine speed). This will also mean that STATIC rpm is below 5200 rpm. 

Pro's/Con's

STATIC -

PRO - The aircraft will have  good take-off (TO) performance, without labouring the engine. Good TO (short ground role and max climb out) for terrain/obstruction clearance is a safety feature.

Cruise altitude achieved quickly, reducing max power fuel consumption.

It also means that in a high density altitude situation the air craft is likely to still have safe TO characteristics (subject to meeting all other related criteria).

The engine is unlikely to be stressed/laboured, at any stage of flight and therefore have a longer service life.

In my personal experience; the unstressed engine will likely have a lower /hr fuel consumption (oil may also be low) and overheating less likely (especially with high ambient temperatures).

CON -  Engine speed must be monitored & controlled, so as not to exceed the Rotax Max Power & Max Continuous engine RPM. Cruise speed is less than optimum, making for longer trip times, higher trip fuel consumption.

Should the throttle linkage become disconnected from the carburettor the engine rpm my rise to Max (runaway rpm out of pilots control). The pilot will be limited to 5 minutes of operation before engine damage may occur. Landing with, engine on, may be problematic. NOTE - this is an unlikely scenario.

CRUISE -

PRO - Higher (optimum) cruise speed, leading to reduced trip times and potentially. Bragging rights (I cruse at??knots).

Should the throttle linkage become disconnected from the carburettor the engine rpm will not rise above the governed 5500rpm.  

CON -  Long TO ground role and shallow climb performance impacting on poor terrain clearance,  therefore reduced safety - further negative  impact on high density altitude situations.

Poor TO performance may also cause the pilot to reduce the aircrafts potential carrying capacity (pax/fuel/stuff) in an effort to improve TO.Climb performance.

Prolonger(shallow) climb out will increase trip time and therefore fuel per trip consumption.

Increased stress (labouring) on engine, may lead to reduced service life and overheating.

Pilots who operate of long sealed/firm surface airfields, with few terrain/obstacle concerns, can afford to opt for reduced TO/climb performance and acheive optimum cruise. However this choice may limit the type/location of airfields they will feel comfortable/safe operating out of.

Pilots who operate out of short, grass, fields with the need to clear obstacles (rising terrain, trees/power lines/ structures/etc) may preferer to be able to achieve their aircrafts optimum TO/Climb performance. This choice will give the pilot/aircraft the flexibility to operate out of all airfields and the occasional paddock/beach.

Some pilots may opt for a compromise setting, somewhere between 5200rpm STATIC and 5500 rpm Cruise

My view CRUISE WOT 5500 rpm has only one real and one perceived  benefit - Real/ higher cruise speed. Perceived/ prevention of  a runaway engine, is so unlikely, it is of no concern (to me). These benefits do not outweigh the potential flight safety and engine wear concerns.