Powerfin Propeller: Recommended! (But choose length and adjust pitch wisely)

I've been doing some testing on a Powerfin propeller on our Superlite.  I've come to the conclusion that if properly selected, Powerfin's composite propeller provides a superior alternative to the wooden propellers which we've been using on our aircraft.  (Albeit at a higher cost.  Most good things cost more.)

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):

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

I spotted 14 deer while loitering in the air

Short post:

I'm not a deer hunter.  I've occasionally mused on the idea; ... maybe some day.

This evening, I spotted a group of 14 deer while flying in the neighborhood of home field.  About half of them had antlers, and one was a very nice buck.  No, I won't tell you exactly where I saw them.

I made several circuits around their field, recounting and noting the number of bucks. 

Awesome.  Awesome!

Even MORE INFO on Stall Speed, Vortex Generators and Ultralight Aircraft

If you haven't been on this blog in the last few days, there are several new posts.  Don't miss the post on landing in a hayfield!...

The following is a discussion which will intertwine some stall speed testing and also some slight discussion on vortex generators.

A couple of months ago, I mentioned that I had put Vortex Generators on a Belite.  I was not able to provide much substantial information to you, my faithful reader, as to how they performed.  I'm still don't have much to offer, but I have begun to experiment with their placement on the wing.


I also recently discussed the FAR Part 103 mandated stall speed requirement of ultralight aircraft.  My blog post on that topic may be found here.


So here's a little more info.

We've had a few days of smooth air, and smooth air is absolutely intentional when performing stalls at absolute minimum airspeed.  Any disruption of the smooth air causes the plane to stall, screwing up my testing.  This is critically important when we are talking about stall speeds of less than 30mph.


Acting on a hunch related to my impression of the high angle of attack on our Riblett designed airfoil, I moved the Vortex Generators closer to the leading edge of the airfoil, which you can see here:

Vortex Generators on an Ultralight Aircraft, EG:  Belite Superlite Dragon

I then did some test flights. But a new problem was emerging:  my airspeed indication was failing completely at the extremely high angles of attack which I was testing.


In looking at the pitot tube, I suspected that the air intake on the pitot tube was 'micro-stalling', due to the non-parallel flow of air hitting the pitot tube obliquely at the high angles of attack.  In other words, the pitot tube needed to match the flow of air, not be parallel to the bottom of the wing.  I mentioned this to Gene, and he simply grabbed the pitot tube and started to bend it down.


"If you do this slowly, you can do it without breaking the tube," he said.  Hmm.  He was right.  


After he bent it a little, I bent it a little more.  The pitot tube was now bent distinctly downwards.

pitot tube bent downward on ultralight aircraft, EG Belite Superlite Dragon

I then performed another test flight in the plane.  After placing the airplane into a minimum controllable airspeed, I captured the following pic:

A picture showing a high angle of attack on a Belite

The above picture nicely illustrates the very high angle of attack.  Due to the fact that the engine is throttled back, it is safe to assume that the actual angle of attack is higher than what is illustrated:  the airplane is descending!  

As I maintained this very low airspeed, I focused my attention on the instrument panel, and captured another picture of the flight condition:

Cockpit panel view during Vortex Generator Stall Test

The instruments are a little hard to read in the photo above, so I've enlarged that part of the pic:

Closeup of instruments

The altimeter is reading 700 feet; the air speed indicator is reading 30mph, the RPM is showing 4700, and I was able to sustain this low flight mode without stalling the airplane.


The 30mph LED is actually a speed range indication:  the actual indicated speed of the aircraft is somewhere between 28 and 32.5 mph.  This is subject to several different errors, such as pitot installation error (EG, the pitot is not located far enough forward of the leading edge; or it is not parallel to the flow of air....), instrument design / calibration error (I try and make a good instrument, but it's not perfect....), etc.  


And if I pulled the nose back just a smidge more, the airplane stalled.  I was able to get the 28mph LED to flicker on a time or two as the plane progressed through the stall.


The aircraft weight, as tested, was around 522 pounds including airframe, parachute, pilot, and fuel.  This is substantially higher than the FAA mandated testing weight --  I would need to lose considerable weight to get down to the mythical FAA weight of 170 pounds. 


TECHNICAL MATH STUFF STARTS HERE...  

Let's do the math for predicting our stalling speed, and then compare it with what the LED Air Speed Indicator was telling me:


Using the textbook calculations for stall speed, which I first mentioned several months ago:


Vs=SQRT(2*Weight/(Rho*Area*Cl))

Where the following variables apply (using English units of measurement):

Weight = Weight in Pounds of the loaded flying airplane = 522 pounds
Rho = Density of Air  = .00237 slugs / ft3 (Temperature = 59 degrees, at sea level.)
Area = Wing Planform Area in SF = 101 Square Feet for this test
Cl = Coefficient of Lift = estimated at 2.4

So our equation now looks like this:

SQRT (2*(522) / (.00237*101*2.4)) = 42.63 ft/sec = 29.1 mph predicted stalling speed (at a weight of 522 pounds)


and this agrees with my observation -- the stall speed was above 28mph.


Now let's change the weight to a mythical amount of 478 pounds.  (278 pound airplane with chute and goodies, 170 pound pilot, 30 pounds of fuel).


and let's rerun the math with this new hypothetical weight:


SQRT (2*(478) / (.00237*101*2.4)) = 40.8 ft/sec = 27.8 mph predicted stalling speed (at a weight of 522 pounds) (The FAR Part 103 requirement is 28.0 mph.)


END OF TECHNICAL MATH STUFF -- YOU CAN BREATH A SIGH OF RELIEF


Earl Downs told me anecdotally a few weeks ago that the original Kitfox Lite was designed to barely meet the stall speed requirement of FAR Part 103.  Since our wing is the same aerodynamics, we've matched that characteristic.  And we keep finding that the math (and the actual results) point to a stall speed which is right at or under 28 mph.

This all makes sense -- the highest possible stall speed would also correlate to higher cruise speeds with the least amount of horsepower.  In other words, you can touch both ends of the FAR Part 103 ultralight aircraft flight envelope with a Belite - from 28mph stall to 62mph cruise.

Now some thought kickers for you to consider:

1)  These results were tested at fairly high density altitudes.  What would lower altitudes and temperatures do to the results?

2)  Does FAR Part 103 care that the stall speed is higher as the cabin load is increased?  

3)  What effect, if any did the vortex generators due to the stall speed?

4)  Is it fun to fly around with the airspeed indicator showing just 30mph?  [Answer is YES!]

a Hayfield Landing Highlights Belite's Utility

Long time readers of this blog may be aware that I have had training in mountain flying.  I've been to Idaho many times, and I've enjoyed the skills I've acquired through mountain flying school and through practice into short, odd airfields.  A recent post on this subject concerned several camping trips into the Thomas Creek airstrip.  Another post covered, among other thing, a series wasp bites and and description of the gnarly final approach into the Shearer airstrip, deep in the Selway-Bitterroot wilderness in Idaho.

I enjoy flying into tight, short airfields that curve, hug terrain, pass by trees, and slope uphill.  Ideally, they end in the side of the hill (and offer good camping and fishing nearby).

There is a hayfield next to our primary runway that offers some of these characteristics.  It slopes uphill.  It has a nice collection of trees.  It has bumps.  It has quite a bit of grass and weeds.  It is relatively short.  It is 'unimproved'. 

It is an ideal location for showing the landing capabilities and utility of our airplane.  The rough ground is a great demonstrator for our spring landing gear.

I wanted to demonstrate a nice 3 point landing, uphill, through the hayfield.  I asked my able assistant Gene Stratton to standby and take pictures.  I promised him a series of approaches into landing.  As I 'dialed in' the strip, I was able to establish a final approach of just a few feet over the weeds at the beginning of the field.  (Good mountain flying technique suggests the ability to hit a 50 foot spot... every time.  You can do that with a Belite.)

The photos show the story.

Hayfield Short Final, over the weeds.  I love this picture.

Over the 'numbers', field slopes uphill from here.  Another great pic.

Wheels kiss down on the Hay

Another approach, a little higher

Rollout in the hay, uphill.

Engine shutdown after taxi back

I'm posing by the Superlite

Belite Superlite poses in the grass, by the trees.  Where's the trout stream?

 This flight occured on September 28, about 1:30 in the afternoon.  I tracked down final, adjusting for the crosswind.  (Gene and I talked about my final approach crab angle after we debriefed on the ground, post-flight.)  Winds were out of the north, 12 knots gusting to 19 knots.  Engine power was set at about 35HP, even though the big Hirth develops 50HP when it's fully unleashed.  Landing direction was ENE; about a 50 degree crosswind.  FWIW, Wichita (KICT) Metar weather was:

       KICT 281753Z 01012G19KT 10SM SCT044 
            SCT150 SCT250 24/13 A2998

If you are a potential purchaser, you might compare our Belite with other less capable ultralight aircraft that have stiff gear and no crosswind capabilities and no off-field capabilities and no ChromAloy steel in their structure -- in other words, ultralight aircraft that lack safety, strength, and fun.

A Belite exudes utility and usefulness, and is a blast to fly.

      --  Photo credits: Gene Stratton.