Cutting and reshaping APC electric props - what works?

[Brett Buck]

   It's usually a big mistake, or at least not worth the effort, to cut down APCs, for much the reason Mark mentions. You generally *don't want highly efficient props*, but you want to use the low efficiency to your advantage (less pitch or less diameter),  not just taking a low-drag tip and cutting it off. That does make it less efficient, but just by creating a bunch of drag that gains you nothing compared to running a more correct prop that is inefficient in a useful way.

   Fortunately, there are plenty of people making appropriate props to try. And despite any analysis (marks or mine or anyone else's) there's still a huge amount of trial and error required, because while we sort of understand what matters and how it works in general terms, we don't know and certainly can't calculate what prop it going to work the best for any situation. It's probably less critical with electric but, you still don't know for sure.

   Also - given some of the description of the problems from above, you might want to take careful note of Paul's comment above, because some of your issues sound a lot more like trim issues than power to me, too. Or at least you might be trying to cover up trim problems looking for something in the motor or prop.

    Brett

[Mark wood]

Actually Brett what I am saying isn't to make the prop more efficient in terms of performance. What I am doing is using the efficiency curve as the illustration of what is transpiring in my setup. Between two pitches the more efficient propeller pitch for power performance is not the one I select. Look a the thrust curves. The prop I select has a steeper slope than the other due to being run past peak efficiency or in our typical uses further past. Larger diameter increases the magnitude, not the slope. Smaller diameter reduces the magnitude not the slope. It's the fine pitch not the courser pitch the later being the more efficient power wise. On this point we agree. Perhaps that is what you meant.

My doubt is the rigidity in space angular momentum side. I can't make the moment of inertia of a 3 blade prop in the diameters we use converge to being lower than a 2 blade. Assuming the definition of turns better is the property of angular momentum ie resistance to turning or precessing during a turn that results in the doubt. For every trick that can be applied to a three blade can be likewise applied to a 2 blade prop. Yes, I have done the basic calculations. There's a few discussions on this in the world as well. A basic rule of thumb when we compare power factors between 2 and 3 blade propellers the 3 blade propeller will be roughly 0.9 times the 2 blade diameter. If you simply add a blade, the moment of inertia increases by a 1.5 factor. Reducing the diameter also reduces the moment of inertia but the difference is very small. Making the blades thinner helps which puts the change more like 1.3 times more.

The key is the solidity used in the 3 blade to recoup the loss of thrust due to the diameter reduction. The greater solidity of the 3 blade prop recoups the thrust but in terms of off point thrust gain the diameter is a larger driver. What Dennis is doing is turning the solidity thrust knob value like the wide blade rev ups. Diameter is better for this but in this case the moment of inertia scales with the square and the larger diameter propeller does take a hit momentum wise. So Dennis is looking for better thrust by using solidity and the inertial impact of that knob dial is comparatively minimal.

That not withstanding the three blade propeller does work and does look very cool. Dude, I am totally on board with that aspect. But in the end the thrust gain is much greater from diameter than it is from solidity. In the full scale arena we see a lot of three blade props in the aerobatic realm and it is easy to conclude, and many do, that the reason is they work better. They are definitely smoother, no question. But if you pull back the hood what you will find is that the 3 blades props are generally on 6 cylinder engines which have 120 degree firing order.

Two blade props on 120 degree firing order 6 cylinder engines have alternating out of phase pulses causing serious in plane vibrations. This results in no continuous operation RPM restrictions and shorter time between overhaul periods. On turboprops they are used because of the horsepower and tip speed limitations. One of my AG operators went to a 4 blade prop and realized significant increase in take off thrust. This is because the three blade prop could not absorb all of he horsepower the engine had to give. Ground clearance was the limiting element. A similar case is the C130. The late model C130 have 4,700 horsepower engines vs the original 3,500 horsepower. Between the two engines on the wing , the diameter is limited. The only option is to add more blades to the propellers. And more chord as well to absorb that extra horsepower.

 

[Dennis Toth]

Interesting points, and Mark has the idea of where I am going. However, since I have a setup with the MAS GF 10x8 re-pitched to 6.7 that works what I am after is getting the same performance up top with a lighter weight prop. Thus, my interest in cutting a larger diameter APCE prop that would maintain blade wide and amp draw of the baseline system. Mark mentioned the sweep up (the last 1/16" of) the tip (Hoerner tip). I have used this and it works but works best for level flight. In maneuvers it not that strong, but sweeping the top side down to the bottom works stronger. Both of these tip designs need a flat bottom airfoil at the tip to work undercamber kills them and creates lots of drag, noise and amp draw. Also, need to give a little rounded LE and TE (not as much as the APC design) to smooth the corner flow.

The APCE 11x5.5 has a flat tip area within the last 3/4" of the blade so it would work, it has a little less tip blade cord but more in the blade center which might make up for it. I also found at my LHS a Master Airscrew wood 11x7 that has blade shape and width of the GF 10x8 and is much lighter. I need to cut and try both first on the test bench (thrust and amp test cell) then if the amps are close to the re-pitched 10x8 in the air.

Best,  DennisT

[Mark wood]

I have this Sabre Stunt kit on the shelf I keep looking at to build. One of the guys in our CL club in Phoenix, Larry Stephens had one and I always liked the airplane. He was a super scale builder, which some here may remember. Anyway, the way I see it is that airplane really does need a 3 blade prop on it.

[Lauri Malila]

Probably irrelevant with electric motors, but with my .77 IC I have always had the best engine behaviour with square blade tips. I have tried quite many  shapes, and when looking for quieter swept back tips a'la APC, I always suffered from clearly stronger speed-up in wind, and also less breaking in dives.
Second best, and maybe the best compromise with 3-blades is elliptic blade shape.
Also the start/stop of corner is better with square tips. L

[Ken Culbertson]

I have this Sabre Stunt kit on the shelf I keep looking at to build. One of the guys in our CL club in Phoenix, Larry Stephens had one and I always liked the airplane. He was a super scale builder, which some here may remember. Anyway, the way I see it is that airplane really does need a 3 blade prop on it.

And a thicker wing, and a longer tail and a bigger stab and if it is the Jetco kit, new wood.

Ken

[Mark wood]

And a thicker wing, and a longer tail and a bigger stab and if it is the Jetco kit, new wood.

Ken

If I build it, my intent would be to fly for nostalgia. Making one part better begats another. If there were anything I'd actually change, it would be the airfoil. And that is a cascade of changes resulting in a non OTS legal airplane. As it is, it will get an electric motor. The ME109 I have will be the same thing. Build it the way it was. Making it better isn't my objective.

[Ken Culbertson]

If I build it, my intent would be to fly for nostalgia.

I am doing the same with a Brodak kit Stuka.  The lighter wood is 10lb and I am pretty sure it is balsa.  I don't think cutting lightening holes violates the OTC rules.

Ken

[Mark wood]

I am doing the same with a Brodak kit Stuka.  The lighter wood is 10lb and I am pretty sure it is balsa.  I don't think cutting lightening holes violates the OTC rules.

Ken

I'm basically lazy. For me it would be a revisit of my youth. The airplane doesn't have to be a performer. Just fly good enough. As I recall Larry's flew fairly good and looked really cool. About the only change I would actually do would be to substitute 1/4" balsa for flaps and stab/elevator for hinges.

[Dan McEntee]

Guys,
Having moved from Jupiter, FL (roughly 12 - 15' above sea-level) to Fletcher, NC (2350') I have had to do some prop work with my electric ships. Up here the electric motor doesn't care about it, same rpm as always, but the prop is another story. In FL in the summer you had thick air even at 93F cause it was 70 - 80% humidity. In Fletcher (near Asheville) we get some humidity but most of the time it is below 50% (you can tell its very low when you get the static shocks walking across the carpet). The normal props from FL are like bald tires in mud, what I need are props that are like snow tires with studs!! The normal APC electric blades just don't bite as you go up top. Going to larger diameter could work but they pull more amps and ground clearance is a problem in grass. I did get a MAS GF series 10x8 depitched to 6.7 to work but they are heavy props, would like to get something lighter working.

What I would like to do is take a large diameter APC E prop and cut it down to the 10" I am using on my OTS ships to get the wider blade width. Problem is when I have done this in the past I wind up with noisy tips that kill off the tip pulling power and still use more battery (not as much as the full uncut blade) so it seems that tip shape is important. Has anyone worked with cutting down the APC's and maintaining pulling power without the tip loss? What method have you used, and what did you do to finish the prop?

Best, DennisT

 

   If the Master Airscrew prop is what gives you the best performance, but you want a lighter prop, then the answer is to make a wood copy of the plastic Master Airscrew prop. I have posted several times of wanting a lighter prop for an ARF Nobler with a Brodak .40 in the nose. The "magic" Thundertiger 11-4.5 that seems to work on everything worked well but weighed 1.5 ounces. I wanted to avoid tail weight as the airplane was already kinda porky, so began looking at wood props, and took a Rev-Up 12-5 drone prop that has been turning up in batches over the years, and used it as a blank to make a copy of the TT prop. I marked off the TT prop in sections and measured the thickness at those section. The transferred the blade shape of the TT prop to the Rev-Up, and cut it to shape. Then sanded the thickness down to make the section measurements, shaped the airfoil, re-pitched to 4.5" pitch and balanced. Test flying consisted of getting three flights with the TT prop to check fuel load run times, lap times and take off RPM. The all I changed was the prop. Take off RPM matched pretty closely, run time on the fuel load was within a few seconds and lap times almost exact. It's the blade shape and blade thickness that are important. So, if you want a lighter prop that performs just like a Master Airscrew plastic prop, then make that from any wood prop that lets you make the blade layout and has the proper thickness.
  Type at you later,
   Dan McEntee

[Dennis Toth]

Dan,
That is what I am working on. I picked up a MAS wood 11x7 that has the blade profile very close to the MAS GF 10x8. (depitched to 6.7).  When I cut the tips down to be 10" diameter they are very, very close. I will use the angle down on the tips with a slight rounded corner on the leading edge and very small one on the trailing edge. It will be interesting to see how they compare in bench test amp draw and thrust measurement.

Best,   DennisT

[Dan McEntee]

Dan,
That is what I am working on. I picked up a MAS wood 11x7 that has the blade profile very close to the MAS GF 10x8. (depitched to 6.7).  When I cut the tips down to be 10" diameter they are very, very close. I will use the angle down on the tips with a slight rounded corner on the leading edge and very small one on the trailing edge. It will be interesting to see how they compare in bench test amp draw and thrust measurement.

Best,   DennisT

    The hubs on those wood Master Airscrew props are about a foot thick! The way the machine cut their props is a mystery to me that they have such thick hubs. I would have started with a 11-6 or 11-5 and pitched up. The props would have been a bit lighter if the hubs were thinner start with, since you are wanting to go lighter,  and you wouldn't have to pre-pitch the 6" that much, maybe even just sand in some phillips entry. A Zinger 11-5 might be a good candidate to rework also. Those might be easier to find than even the MA wood props.
  Type at you later,
  Dan McEntee

[Dennis Toth]

Dan,
Interesting point. The MAS Wood 11x7 is 1/2" the Rev-Up is 7/16" the MAS GF is 1/2" (the hub is hollow on the back side). There is a significant difference in the weight of the wood vs the plastic so even with the thick hub wood is enough different to give it a try. A Rev-Up 10x6EW would also be something worth looking at but I only have one so might have to pull a mold first. The CF options always seem to be heavier than the wood, but wood is running around $10 a prop. The APCE's are much cheaper and good enough even with a little tip noise.

Best, DennisT

Advertise Here