PROPELLER ADVICE

[Gannet]

Hi

I know that there are some clever folk out there who know these things. I am looking for some wider opinion on this before I blow more hard-won cash on trying to fix the problem.

I have a Safari, with a 912 95 HP Rotax. It was delivered with an in-flight variable speed 2 blade prop. At 5 000 RPM, 25 inched MAP, it would cruise comfortably between 100 and 105 MPH. The ride was always a little rough, as you could expect from a 2 blade prop. I recently changed it to a 3 blade ground adjustable Arplast prop.

The ride is much smoother now, but the speed is 90 MPH at 5 000 RPM. Nothing else has changed. Climb out at the coast one-up with full tanks is still around 800 fpm. Peak revs on take off are 5 300. In level flight at full rattle, the engine peaks at 5 800 RPM and 105 MPH.

Now I am sure that 3 blades will have more drag, but a 10 - 15 % reduction in speed seems excessive.

My layman's opinion is that the Arplast has a light 'plastic' blade, which is quite flexible. Under full load, the blades will surely cone forwards and flex axially and must therefore lose efficiency.

So - will a stiff bladed Warp Drive, or a 2 bladed PPlank make for a faster cruise? Other Safaris definitely cruise faster, although their engine/prop installations differ.

Surely others have had similar issues to this? How did you solve them?

If fitting a Warp Drive, do I go for the tapered blades, or the constant width blades? What is the difference, apart from the R 9 000 price between the two models? Are there other options?

So far, I am getting to believe that the spinning technology at the front end is not so much a science, but rather a 'suck it and see' mystery of aviation?

Any opinions gladly received.

[KFA]

You should get about 5200 static (what rotax recommends to be the min rpm). That is max you will get out of the prop, anything courser will overstress the engine, so if it still over revs in level flight then the prop is not the correct one. (Shape, make pitch etc etc)

If you still have the old prop then I would put the 2 blade adjustable prop back and take if for some dynamic balancing at a prop shop (R650-R1200) It will be worth every sent and you will still fly faster and take off shorter. The 95hp engine is actually a bit underpowered for the Safari to fly as it should so with the VP you get the max take-off as well as max cruise. I would live with the bit of roughness and enjoy the better performance. My2c

[Boet]

You are a bit late! He swopped the 2 blade IFA for the Arplast. Ideal STATIC refs on the 912 would be between 5250 and 5300. This will give you the best "compromize" pitch, IE, reasonable everything, take-off, climb and cruise. If the blades have been set at 3000ft ASL, it will not give the same static refs at sea level. Density altitude. I flew this aeroplane some with both props, and what a delightfull performer!