Bolly Prop 68"on a 912ULS in a Tecnam Sierra

[DonRamsay]

Any suggestions for setting the pitch?

 

We started with 18 degrees and I suspect it might be a little too fine.

 

Want a cruise setting for best cruise speed as take-off and climb are not the big issues.

 

Obviously don't want to overload the Rotax.

 

 

[rankamateur]

What revs are you getting at full throttle? From my limited understanding if you are getting 5700 you are not overloading the engine, if it was struggling along at 5500 then you would be overloading it.

 

 

[DonRamsay]

Straight and level at 2500 ft AMSL you could wind it off the clock.

 

 

[rankamateur]

Sounds like it needs to bite a bit harder then.

 

 

[DonRamsay]

Yes. We'll try and get some test runs in over the next week or two with a couple of degrees coarser.

 

 

[rankamateur]

Talk to someone who knows what they are doing first. I don't, so I bought an Airmaster, they get them pretty right straight out of the box. They are programmable to tweak once they are installed anyway.

 

 

[Ultralights]

5700rpm, full throttle will get my to VNE straight and level in a Savannah, the prop is set to 22 deg. cruise is 5150rpm at 87 kts.

 

 

[techie49]

Hi Don,

 

I'm using a Bolly 72 x 60 on a Subaru EA81. Where are you measuring the angle on the blade? I seem to recall you should be 75% of the blade length from the hub end. Obviously if you're measuring at a different point the comparison is difficult. I set my Bolly to limit the Subaru to 5600 rpm WOT.

 

regards,

 

Paul

 

 

[rankamateur]

5700rpm, full throttle will get my to VNE straight and level in a Savannah, the prop is set to 22 deg. cruise is 5150rpm at 87 kts.

Do you get 5700 on take off?

 

 

[Ultralights]

Do you get 5700 on take off?

nope, Take off will get to about 5300 to 5400rpm, until airborne then yellow line rpm will be full throttle and about 55kts climb at around 900 to 1000ft/min

 

 

[Dafydd Llewellyn]

The normal certification requirements are:

 

(a) The propeller must not allow the engine to overspeed at full throttle at any speed up to & including the speed for best rate of climb (Vy);

 

(b) The propeller must not cause the engine to overspeed in a dive with the throttle shut at any speed up to & including Vne.

 

I've always thought these to be pretty much common sense.

 

That said, I assume the propeller has three blades? If so, it may have a larger-than-optimum "activity factor" (the propeller equivalent of wing area) for the engine. The best propeller efficiency normally occurs when the local blade angle-of-attack is at the optimum angle for the blade airfoil - which is often in the vicinity of ten degrees (relative to the helix angle of the blade at the flight speed concerned, at around 75% radius). Too much blade area will require the pitch to be fined-off to allow the propeller to rev., and if that results in too low a blade angle of attack, the propeller efficiency will suffer. I once tested a Murphy Renegade to try to discover why it was climbing so poorly; it had a 3-blade ground-adjustable prop; to get the engine to work in its correct RPM range, the blade angle at 70% had to be set as low as 13 degrees. In that case, going back to two blades gave a marked improvement. 3-bladers seem to be "fashionable"; but this does not necesarily mean they are better.

 

 

[Jabiru Phil]

Isn't the saying something like,

 

"Three for show, two for go"

 

 

[Dafydd Llewellyn]

Isn't the saying something like,"Three for show, two for go"

Well, that's been my experience with little aeroplanes, and engines below about 250 horsepower. Also, from a noise point of view, if there's any acoustic effect due to interaction between the propeller and the aeroplane, the more the blades, the higher the pitch of the noise - which makes it much more annoying. This can be important, if it makes you aircraft more noticeable; it becomes "that *!##" aeroplane" - and when that happens, the local complainers really get going.

 

 

[kgwilson]

The more blades you have on a prop, the less efficient they are. The most efficient is one blade but due to counterbalance issues they are not very common. If you want good high speed cruise with the best fuel consumption go for a 2 blade cruise prop especially if you have a slippery air frame. 3 blades will generally be good for climbing even in a cruise configuration but you generally won't get the same top end speed.

 

2 blades have less drag & less weight & have less surface area than 3 blades so are more efficient. On the other hand 3 blades distribute the horsepower across a larger area to provide faster acceleration & improved climb & allow better dissipation of engine vibration. Because 3 blades have faster pulse passes than 2 blades the frequency of noise is shifted up giving less perceived noise in the cockpit (but possibly more annoying noise to those outside). e.g. At 2,700 rpm 2-blade = 5,400 passes, 3-blade 8,100 passes.

 

Me, I'm a 2 blader but OTU.