Last year, I flew quite a bit with a 3 blade Powerfin propeller. I was not happy with the performance. In hindsight, the issue was our selection of a short 3 blade prop -- we should have tried a two blade.
And so, I had switched to inexpensive 2 blade wood propellers. They were a good, economical solution providing reasonable performance. Their downside is maintenance (their leading edge will erode, especially in heavy grass fields) and inability to adjust pitch.
Recently, I was contacted by Powerfin to consider the usage of their propeller with our aircraft. We received a brand new "B" series propeller, and we bolted it onto our Hirth 50HP engine.
I wanted to know the answers to these questions:
a) Would this propeller provide any increase in efficiency over our most ideal wooden propeller?
b) Would this propeller allow for easier power tuning for Part 103 users?
c) Would it look cool?
The answer to all 3 questions is yes. Here's more of the story about efficiency and prop re-pitching:
I've done power consumption testing on the Hirth, and reported on the results in this blog a couple of months ago. The gas consumption was 3.4 gallons per hour. The U.S. distributor of Hirth engines suggested I might be able to improve on this fuel consumption figure; I was a little skeptical. But the Powerfin propeller appears to have proven him correct.
To test the configuration, we selected a prop length of 65 inches, in the "B" series ground adjustable propeller from Powerfin. This is the longest propeller I've ever thrown on our aircraft. I thought the extra length would help with efficiency. I then selected an initial propeller pitch using an online tool I've found which helps interpolate diameter, pitch, RPM, atmospheric conditions, and engine power. You can find this propeller tool here. In order to really make it work, you have to convert diameter and pitch to an angle setting for the propeller pitch. I use this tool to perform that calculation, just put in circumference and pitch into the calculator, and it delivers an angular setting. (For a 65 inch prop, the circumference is 204.2 inches -- you know how to do that, right?!) [diameter * 3.142 = circumference].
After selecting an angle of 7 degrees, we used a prop protractor to set the propeller 'bite' or pitch, bolted all the bolts to the recommended torque values, and it was time to runup and test power.
As it turned, a quick runup and takeoff showed me that this pitch angle (7 degrees) was too much of a bite, and the engine would not rev up to full power. In fact, not even close. However, it was sufficient to get off the ground and allowed me to enjoy flying a grossly underpowered utralight aircraft, but not for very long. (I landed.)
So the pitch was reset to 6 degrees, and I took another test flight. This time, the engine was still not able to develop full power, but that's OK -- it climbed great, and was already exceeding the FAR Part 103 cruise speed limit by a wide margin. (Cruising at 61 knots, 6 knots too fast, this is about 70mph. If I cut the pitch angle back to about 5.5 degrees, the engine would develop higher RPM and full power, and the Belite Superlite would go EVEN FASTER (probably around 75mph cruise), but that's not my objective. (However, it may be your objective, if you are building a Belite as an experimental N-Numbered homebuilt aircraft.)
I want to be able to cruise at exactly 55 knots -- which is 62mph, which is the FAR Part 103 cruise speed limit. So I'll soon reset the pitch to about 6 1/2 degrees. This should be just the right amount of power. In other words, the adjustable pitch prop is an excellent way to fine tune a big horsepower engine to be FAR Part 103 legal.
I continued my testing of the Powerfin prop. I started with a full tank of gas, and I cruised for an hour at exactly 62 mph, with a few takeoffs and landings mixed in. I then landed and checked fuel consumption with a measured dip stick (I'd also been tracking fuel consumption in flight using our fairly amazing Belite fuel gauge):
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| Belite Fuel Gauge -- works great -- please buy one. |
At the end of the flight, I'd used 3.15 gallons, and I had 1.85 gallons remaining. Therefore, total time to fuel exhaustion is about 1 hour and 35 minutes -- a substantial improvement over the wooden prop I'd been using.
When using the Hirth 50HP engine and the Powerfin prop, this calculates to a range of a Belite Superlite, in FAR Part 103 legal mode, of 98 miles.
And I do like the way the Powerfin looks!
Here's some things to keep in mind:
a) The larger diameter is definitely a good thing. However, proper technique calls for 3 point landings and careful takeoffs to avoid grass rash on the prop. If you want a little shorter prop, I wouldn't go with anything less than a 60 inch diameter on a Belite.
b) As I mentioned earlier, good things come at a price. These composite props aren't cheap -- but they are worth it if you want the most adjustable option combined with the most efficiency.
c) Also, they are little heavier than wooden props.
d) I will be adding this prop to our price list.
-- James
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