|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Props Comparison
.jpg) Peszke
Bollie
Warp Drive
.jpg) Warp Drive
Kool
Testing Procedure
I’ve long wanted to find some head-to-head comparisons of different propellers. There are many accounts of someone trying a new prop and reporting amazing results but I never found any tests that had been well-controlled to get the base line equal.
The prop I had been using for 400hrs was performing well enough, but I wanted to see if I could do better. Then I had the opportunity to borrow and try several quite different brands of props. I enjoy experimenting, and testing for real results, so took on the project. It’s not as easy at it would seem to get fair comparisons – first, lots of short flights and pitch adjusting to get WOT (Wide Open Throttle) straight and level rpm exactly equal for each. (I sure don’t like thundering along at full throttle, with the ASI off-scale, 100+kts, just under Vne for this
My aircraft is a Savannah, originally with leading edge slats but now with the slats removed, and VGs on the original airfoil. The engine is a Rotax 912ULS, 100hp. All test flights were at 2000’ QNH, about 1600’ AGL here. I installed a digital tacho to get repeatable rpm. Had a manifold pressure gauge calibrated by an instrument shop. Flew 4-way GPS legs with a VSI for level flight (the only way to get true speed comparisons). Kept the fuel weight very near equal. Temperature and atmospheric conditions very near equal (approx 23ºC). Did the comparison runs at first light for most stable air. Swapping the props took only minutes so the conditions were very much equal. Did the whole procedure over and over again to eliminate anomalies and averaged the results. Then found that the results were so much the same for each that I didn’t believe them, so did it all over again. But they came out the same yet again....... After a fuel bill of $500 in one month, I was satisfied with the results as being true and fair.
Some will immediately say they get higher climb rates and better speed than this, and often I seem to get those indications as well in general flying, but repeatable results under same conditions is what I’m after here.
Pitch was measured with a Warp Drive protractor – a very good instrument, worth getting whatever prop you are using. It’s very easy to use and gets repeatable results right down to a quarter of a degree.
Results
The results obtained are very interesting and quite surprising. And somewhat disappointing as well - I had hoped to find some dramatic differences that would get me a prop that’s far superior to others. Not so, the results show that there is very little difference at all between quite different props. When I first got those results that are so much the same, I thought it must have been a coincidence, so had to do it all over again, and then again. It just seemed unlikely that that they could come out so much the same!
A couple of times when I first tried a new prop I got excited by a considerably higher indicated air speed, but each time when the pitch was adjusted to the same WOT reference of 5800rpm, the 4-way GPS speed at 5000rpm came out pretty much equal to the others.
The test flights using manifold pressure as the reference gave even more equal results – all within one knot of each other! As I understand it, manifold pressure is a very good indicator of actual engine power output. Effectively each prop gave exactly the same speed for the same power setting! That’s one test that I just had to do once again last weekend, and the result was the same yet again.....
The only real differences showed up in the static thrust tests. The biggest difference was between the Peszke and the Kool, and still only 12kg out of 190kg, about 6%. But yet, time-to-climb to 1000ft AGL for all of them was very much the same. Time-to-climb from take-off is difficult to compare precisely because it’s so much effected by variations in getting stabilized at climb speed, more chance of variations in lift or sink in the surrounding air, and of course air temperature (even at first light there are often layers of different temperature air at these levels). But after many climbs, all the props showed the same range of times, and averaged pretty much the same from 64 to 70 seconds.
Of course many other users out there will claim they get different performance with their props - some of them almost magical...... But this is what I found, after really careful testing. I have no reason to promote or demote any one prop against another. It's been a lot of effort for no dramatic results, but at least it measures some realities.......
Warp Drive
Warp Drive Warp Drive hub with drive lugs
The Warp Drive props are so widely distributed and known that I’ll use it as the standard for comparisons. I’ll keep referring comparisons to the Warp because it’s the best-known prop of the lot.
The Warp Drive is certainly the strongest of the lot – as others have said, “You could chop wood with them!” I have a broken one from a wrecked 701 that we re-built, and can see that it’s solid carbon fibre and resin all the way through. The lighter props will have foam cores. Of course this makes the Warp Drive heavier and much greater rotational inertia. It takes noticeably longer than lighter props to ‘spool up’ on opening the throttle, and stops with an almighty jolt on a 912S. In my Rans S7 with the older flat-bed engine mounts this is alarming, but there have been lots of Warps on S7s for a long time so it doesn’t seem to cause any structural problem. I believe the newer ‘focalized’ engine mounts on the S7 are much smoother, just as they are on the
The Bolly props are an Australian design and well-known here as a high quality prop. They tried manufacturing in China, but weren't satisfied with the quality, so are manufacturing in Australia again. But they've now bought their own factory in China, and it will be managed by Australian personel on site.
The tips of the Bolly aren’t flat, so can’t measure pitch at the tip. So I used the standard 600mm (2ft) out from the centre, where it measured 19º for 5800rpm WOT.. The Bolly blades also fit the Warp Drive hubs, so I've just ordered Bolly blades to go in a Warp Drive hub that I have on hand - should be a really good combination.
Peszke Peszke hub
The Peszke props are made in
As shown in the photos, the Peszke is certainly the most stylish and sharpest-looking prop of the lot. The metallic silver/grey gel coat really sets it apart from the usual black ones. The blades are slim and gracefully formed, with sexy up-turned tips to reduce vortex noise.
The hub is nicely machined and polished, and matching spinners are available. The 75mm bolt pattern is used.
It’s a very light-weight prop, with the lowest rotational inertia of those tested, which makes it the smoothest running of the lot, and quite noticeably the quickest to speed up on opening the throttle, and less jolt on shut-down. Despite the lesser blade area due to the slim blades, and 68” diameter, it cruised and climbed just as well as the much larger props. Static thrust was 10 kg less than the Warp Drive (but 10 kg out of 190 is only about 5%), so take-off roll was a bit longer, but surprisingly, time-to-climb was very much the same, so the thrust must improve very quickly once the aircraft is moving.
There’s no separate leading edge protection. The factory claims extra strength is built into the leading edge, but extensive dirt-field use might bring some chipping of the surface unless leading edge tape is used....
I’ve lost the pitch settings that I used for the Peszke.....
All in all it’s a very nice prop, and performed really well. I think it's ideally suited for the 80hp Rotax. I think the styling particularly suits the slimmer, sleeker, speedier aircraft.
Brolga
The Brolga is very different construction to the others. It uses a composite hub made by Ultraprop in the
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
The Kool prop is made in
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
The hub is fairly plain, with a matt black finish. It uses the standard 75mm bolt pattern.
Other props
|
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
.jpg)
.JPG)
.jpg)
.jpg)
.jpg)
