New Prop Test Comparisons

From top to bottom

66" Bolly
68" Bolly
69" Kool
69" Aon
71" Kiev
(The spots on the blades are for measuring pitch at 600mm from centres)

 

For these tests, the props were pitched to give 5200 rpm at best climb rate of 57 kts.
Rotax specifies that rpm at climb should not be less than 5200.
This pitch gives 5900 at wide open throttle (WOT) straight and level with two props. 
This is 100 rpm above the red line, but to pitch for 5800 WOT will give a climb at less than 5200.
We never run WOT straight and level anyhow, but we do climb every time we fly, so I'll set for 5200.

 

The test flights using manifold pressure as the reference, gave incredibly equal results for all props – all within one knot of each other!   One knot is less than the tolerance of the measuring ability.
So effectively, each prop gave exactly the same speed for the same power setting!

Engine power is RPM x Torque, so any testing that only compares at RPM settings isn't testing at the same power output.  ie- If the prop is pitched for 5500 WOT rather than 5800, then cruising at 5000 will show a higher speed, but of course that's at a much higher percentage of total power than if WOT was 5800.  Ideally we should test at equal fuel flow rates, because that truely indicates engine power, but that's difficult to determine accurately in real time.  I've tried fuel flow meters, and don't find them accurate enough, at these flow rates, to be useful for good test comparison.  The only way I've found to get truely accurate fuel flow measurement is to measure the fuel used on a three hour flight at a constant power setting, which gets a bit time consuming for multiple tests. 

 

As I understand it, manifold pressure is quite a good indicator of actual engine power output, provided altitude and air density are equal.   So I've done the cruise speed comparisons at equal manifold pressures, all at 2000' QNH, and all at very similar atmospheric conditions.

 

 

Flight Test Results

 

 

It's interesting to have a look at the differences in profile and helix between different props,

but the performance is very much equal.

          AonProp                              Kiev                                 Kool 

Analysis

Just as in the earlier prop tests, I could find no real difference in cruise speed between any of the props tested, when equal power was carefully applied to each.

So it would seem that when someone claims a big difference with a different prop, they are probably depending solely upon RPM, and are actually applying more less power.

The only difference found was that the smaller diameter Bolly was considerably degraded in climb.
This has been confirmed by two real life cases that I know of, who changed from the 66" Bolly to the 70", and both reported considerably better take-off and climb performance.

 

Kool

It runs as smooth, and stops with much less jolt than the Warp.  Ground observers noted that the noise produced at cruise and full power at 800ft AGL was considerably less than the Warp.  Kool also produces a version with anti-vortex tips to reduce noise even further.  I know of a Savannah that has that prop, and we’ll do a noise comparison when we can get together.

The hub is fairly plain, with a matt black finish.  It uses the standard 75mm bolt pattern.

For 5200rpm in climb, the pitch at 600mm from the centre measured 16.5º. 

I've decided that this is my favorite prop. 

Very strong and smooth running.  Very good L/E protection.
I have about 400 hrs on it now, mostly on grass and dirt strips, and it's still looking good.

Vasili at http://www.airtrikes.net/propellers.shtml in Canada seems to be the main distributor for America, but several customers I know of have had long delays to get supplied....  

http://www.lugaprop.kiev.ua/catalog_en.html has been mentioned on this forum before, with good service. To order again I would go direct to this Ukrainian supplier. Looking at his catalog I reckon the R104/L104 to be the same as the 69" Kool that I tested and still love.

 

The Bolly props are an Australian design and well-known here as a high quality prop.  They tried having them made in China, but weren't satisfied with the quality, so are now manufacturing in Australia again.  They're now under new ownership with big plans to become a world prop manufacturer. 

Hans started with a Woodcomp on his 701, but it snapped off cleanly at the junction to the hub when it nicked the ground while taxiing through a dip.....  (The butt of the blade was seen to be hollow!).  He used his first Bolly  for about 600hrs and loved it, until dropping the nose wheel into a rabbit hole destroyed it.....  He immediately replaced it with a new Bolly.

 The Bolly blades are considerably lighter than the Warp Drive blades, so the rotational inertia is noticeably less.  This makes it smoother running, and considerably less jolt on start and stop.  The blades are wider than the Warp and a more rounded shape as seen in the photos.  The 'Duratuf' leading edge protection is a heat-cured polyurethane, moulded into the leading edge.  It’s resilient and very effective protection against chipping.  The Bolly hub also uses the 101.6mm bolt pattern and drive lug system, particularly suitable for the 100hp engine.  It's very nicely machined, solid, and attractive so it doesn't really need a spinner.

The labeling of this BOS range of Bolly props is a bit strange.....
The so-called 70" prop is really 68", and the so-called 68" is really 66"???

This because they changed the design of the hub and kept the designations the same.

The Bolly blades do fit in a Warp Drive hub, and when using that hub then the actual diameter is the 70" and 68" as labelled. Is that clear now??
So I've used the actual diameter when referring to them in the test results. 

 

 

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Earlier Prop Comparisons

This is from an earlier prop test comparison.
Manifold pressure readings aren't quite comparable due to a different gauge used,
but the results are the same, in that all props gave the same speed for the sme power.
Even props as completely different as the Peszke and the Brolga!

The relatively flat blades give good performance at those speeds, and excellent take-off performance, but I expected that they would run out of puff at higher speeds compared to the high-helix props like the Bollie and the Kool.  But testing shows this not to be the case, in that at the same manifold pressure settings, the Brolga performed at 86kts, within one knot of both the Warp and the Kool!  The pitch blocks only come in one degree steps.  16º is a bit too fine for cruise, giving WOT 6000rpm, but 17º is over-pitched for best take-off.  Since the Brolga is under-pitched, the rpm for the same manifold pressure setting is higher, but the speed was still the same......  I had expected that this large 4-blade prop would be best at take-off and noticeably slower at such speeds, but it seems not to be the case.......  At WOT it gave 100+ knots, same as all the others, which was really surprising!

I’ve used this Brolga for 400hrs, and the only down-side is that, on long trips alongside a Kiev on another Savannah , the Brolga consumes one litre/hr more fuel, probably due to the extra drag of the 4-blade configuration, but I can’t measure a difference in testing.  4-blades is necessary with the Brolga because the three-blade cavitates some on take-off.  Of course the 4-blades made for a smooth running and quiet prop, despite it’s rather ‘agricultural’ look. 

I originally chose the Brolga because the factory is nearby, the price was right, and I have had good performance from them on earlier ultralights for 1200+hrs. 

Unfortunately the Brolga is now out of production.......

 Kool