Cutting and reshaping APC electric props - what works?

[Dennis Toth]

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

 

[Mark wood]

On 35 size airplanes (400 - 470 sqin ) I like to use 9x5 or 10x5 APCs. An airplane like a Ringmaster, flying at 50-55 mph, 5.2 lap ish times, requires around 200 watts to fly and this results in the propeller and power train pulling around 240 Watts. I really don't worry much about the current as that falls out in the wash. The smaller the diameter the worse your efficiency will be and the higher the input power must be to meet the required 200 watts ish the airplane needs. The same holds true using higher pitch propellers. We have success flying with 9x5 also for the lower end of the wing area airplanes. Here in KS our humidity is high in the 50% plus zone and summer temps are mid 90 to 100 degrees. For shorter flight times under 5 minutes a 4s 1600 MAH pack works and above 2000 - 2200 packs. I don't have any numbers on propellers running higher pitches but they are definitely running at lower efficiencies which is consistent with your numbers and results. See the chart I attached. Basically we want to be running past peak efficiency not bellow peak efficiency even though the later works. Where the two curves cross is where both propellers run equally. I have this same basic chart in other variations including thrust but this does a better job of illustrating the effect of pitch. A couple things you left out are pack cell count and motor Kv rating. These impact your power numbers also.

[Paul Taylor]

Xacto and sanding stick

[Dan McEntee]

   There is no such thing as a free lunch. You're gonna have to pay a price somewhere. It's either gonna be more blade area or faster rpm. I don't think you're gonna get a prop that pulls like back home at sea level and your over 2300ft now without using more juice. Buy some extra props, get out the pitch guage, knife and sanding tools and start experimenting. If you need more battery, that's the price you gotta pay.
   Type at you later,
    Dan McEntee

[Reptoid]

Not sure what pitch APC you started with but you might try ordering a couple Master Airscrew "Electric Only" in the proper size (like 10-6 or 10-7). They typically have more blade area than the APC. They are also slightly less pitch than APC for same marked sizes. Their electric props are much lighter than the GF series and usually just as light as the APC

[Motorman]

General rule of thumb, anytime you modify an APC prop you usually make it worse. It's not like a Rev Up or a Zinger. APC is designed as a system. When you cut the tip off you're removing the best part and what's left just makes trouble. If you want smaller diameter and high performance I would look at some expensive 3 blade CF props.

MM    

[Dennis Toth]

Flying here in Fletcher, NC is very much like flying at VSC in March, so if any of the Tuson electric flyers can jump in with setups that work there it should work here. I am working on OTS 35 size ships with BadAss 2814 980Kv motors trying to stay with the 4S 1800mah HobbyStar 120C pack as it is only 68mm long and fits right were the IC fuel tank goes. As I said the MAS GF 10x8 de-pitched to 6.7 gives a 4.7 lap time on 62' C to C lines. I have thought about doing wood but because of the grass at the only field that is within reasonable distant it seems I would be going through a lot of props and changing props with the front mounted motor is a bit of work that I really don't like to do at the field but could do if they would work.

Best, DennisT

[Paul Walker]

I live and fly at 2200 feet. Most of the time I compete at sea level, or close to that. The only change to my regular PA planes is to add 2 grams of tip weight going to sea level. No change in prop or speed settings. These planes use the 3 blade props from Igor.

The bomber on the other hand uses 4 APC 8*6 electric props. Flying at sea level or the 900 feet at Muncie I also made no changes to the settings. Flew at near identical lap times as at home.

Are you starting with APC electric props?  There is a big difference between standard APC'S and the same size electric props.  Other than trying to use a standard APC I am not sure why you have this issue.

[Mark wood]

I live and fly at 2200 feet. Most of the time I compete at sea level, or close to that. The only change to my regular PA planes is to add 2 grams of tip weight going to sea level. No change in prop or speed settings. These planes use the 3 blade props from Igor.

The bomber on the other hand uses 4 APC 8*6 electric props. Flying at sea level or the 900 feet at Muncie I also made no changes to the settings. Flew at near identical lap times as at home.

Are you starting with APC electric props?  There is a big difference between standard APC'S and the same size electric props.  Other than trying to use a standard APC I am not sure why you have this issue.

Those results are what you should see when you go from a high density altitude (thin air) to a low density altitude (thick air). This holds for altitudes in between as well.

On the way down, at constant RPM, thrust rises with density. Parasite drag rises with density by about the same fraction, so lap time stays nearly constant. Higher density also means higher (q), so a lower Cl  does the same (G). Maneuvers get easier. A good pilot can miss the whole thing. Practice in the harder thin air, then pick off mortals at sea level.

Going the other way, the same relationships work against you. At constant RPM, density falls with altitude, thrust falls with it, and the airplane slows some. It does not stay as constant as it did on the way down. Lower density means lower (q), so cruise CL rises, induced drag rises, and parasite drag falls some. The same drop in (q) means a higher Cl for the same (G) or a lower (G) at the same Cl. That is the lack of bite.

The fix can be as simple as adding 600–700 RPM and letting the lap time come back to the original TAS. Or more diameter.

[Howard Rush]

Here's a good calculator: https://wahiduddin.net/calc/calc_da_rh.htm Fiddle with it.  You'll see the effect of humidity.  Air density goes down a tiny bit as humidity increases.  The pertinent number is relative density: air density relative to a sea level standard day.  Turning capability is inversely proportional to relative density.   

[Brett Buck]

Here's a good calculator: https://wahiduddin.net/calc/calc_da_rh.htm Fiddle with it.  You'll see the effect of humidity.  Air density goes down a tiny bit as humidity increases.  The pertinent number is relative density: air density relative to a sea level standard day.  Turning capability is inversely proportional to relative density.   

    What I don't understand is that I can fly at the same apparent air density at different locations, and have drastically different effects. We run air densities from sea level to maybe 3000 feet equivalent all in one day - 60 in the morning and 105 in the afternoon, and I have to barely change anything, just screw in the needle 1/16" and no change to the handle spacing. Go to Muncie (980 feet) with air density of 2000 feet equivalent, and the airplane requires all sort of changes, or, if you don't, you damn near have to melt the engine and the control response is absurdly slow.

     I have an air density meter, and it has been my observation that there is something more than just the air density affecting the airplane and engine performance.

     Brett

[Paul Walker]

     I have an air density meter, and it has been my observation that there is something more than just the air density affecting the airplane and engine performance.

     Brett
[/quote]

I have experienced the same thing about RD. Can't explain it, but feel there is something else.

Fortunately, going from my 2200 to Muncie's 900 I clearly don't have to change things much.  I gain 3 to 4 percent RD, but it is not an issue for me.

You can through all your "Q's" around, but I am speaking from personal experience. I have a fair idea if what a stunt plane should feel like and its  response to my inputs. 

Of interest, my IC planes seemed to always need changing going to Muncie. Back then I was going from sea level to 900. Strange that that small of a change in altitude required changes on the plane and now the difference is greater but virtually no changes to the plane.  One difference is now I am using electric power systems. Hmmmmm.

[Brett Buck]

I have experienced the same thing about RD. Can't explain it, but feel there is something else.

Fortunately, going from my 2200 to Muncie's 900 I clearly don't have to change things much.  I gain 3 to 4 percent RD, but it is not an issue for me.

You can through all your "Q's" around, but I am speaking from personal experience. I have a fair idea if what a stunt plane should feel like and its  response to my inputs. 

    Howard has been known to accuse both of us with "princess and the pea" syndrome from time to time. I prefer to think it is my one profound one-ness with my airplane and the universe. Or, alternately, I am so marginal as a pilot that I need everything perfect to have any hope. 

   Another very interesting phenomenon was that the engine run characteristics can change *very drastically" in ways that also doesn't follow the air density. For instance the "break hard on outside corners"/"schnuerle effect" is different from sea level to Muncie/midwest even if the air density is exactly the same. It's much more moderate an effect in the Midwest, to the point Ted and I, in particular, have gotten in trouble for pointing it out because lots of people who only fly back there claim it's not a thing at all and that we are making it up.
 
    Brett

[Mark wood]

What I don't understand is that I can fly at the same apparent air density at different locations, and have drastically different effects. We run air densities from sea level to maybe 3000 feet equivalent all in one day - 60 in the morning and 105 in the afternoon, and I have to barely change anything, just screw in the needle 1/16" and no change to the handle spacing. Go to Muncie (980 feet) with air density of 2000 feet equivalent, and the airplane requires all sort of changes, or, if you don't, you damn near have to melt the engine and the control response is absurdly slow.

    I have an air density meter, and it has been my observation that there is something more than just the air density affecting the airplane and engine performance.

    Brett

That is tough to decode Brett. I don't know what your pocket meter is but normally they will give you a couple of readings. Pressure altitude, temperature, density and density altitude. Airplanes and engines performance changes with density altitude which is pressure altitude corrected for temperature and a bit by relative humidity. The airplane only cares about the density part. The engine is far more complex in it's operation. Humidity affects it more than the airplane. Temperature impacts the engine and the pipe operation. Way more complicated than my head can keep up with. I'm a turbine guy. They're stupid simple. Electric motors are even simpler. My hat is off to you for sticking with them. I have a couple airplanes with them and I don't fly them much because they're too fiddly for my taste.

Howard, I think your proportionality is backwards. Lift is a direct function of density. I may be wrong but I think that makes G force a direct function of density as well. I'm not clear on what turning capability is as the term is used loosely for many different descriptions but I'm guessing it is related to G / Weight so, I make an assumption that it should be proportional as well.

[Brett Buck]

I don't know what your pocket meter is but normally they will give you a couple of readings. Pressure altitude, temperature, density and density altitude. Airplanes and engines performance changes with density altitude which is pressure altitude corrected for temperature and a bit by relative humidity. The airplane only cares about the density part.

   Except that it doesn't, as Paul and I both explained. I have more than just a meter, I also have an air data app (AirLab) that gives me all sorts of parameters calculated from reported weather station data.  I have yet to find any single parameter or set of parameters that explain the observations, and even more so, why the airplane and engine responds differently to whatever it/they care about differently.   Just like Paul said, I know when something changes in the airplane or it reacts differently, and I have already ruled out changes in the airplane itself.

   Brett

[Howard Rush]

Howard, I think your proportionality is backwards. Lift is a direct function of density.

You are correct.  I was trying to clarify something and muddied it. 

[Dennis Toth]

Howard & Brett,
Interesting calculator, shows that on a hot (95F), humid (75%) day I my 2350' would be at the equivalent of 5350', interesting.

Paul,
At this point I have a good baseline with the MAS GF 10x8 de-pitched to 6.7 (this prop has a little wider blade then their GF 10x7 which is close). Now, what I was looking to do is find a similar performance from a lighter prop that would pull less out of the pack (4S 1800mah HobbyStar 120C, this is 68mm long fits where the tank would go on 35 size ships) with the BadAss 2814   980Kv motor and still have the pull up top.

The APC's electric props have the undercamber and pull more than the flat back's. I have sanded off the cuff at the hub and that save a good amount but they (APCE 10x6) still lose pull at the top as the blade tip is so narrow. Cutting a larger diameter down would give the blade area. Looking at various tip shapes seems to indicate the slight sweep LE with the top of the blade tapered down to the bottom would be the way to go as long as it is flat bottom in that area of the blade. This is why the question to see how others have modified their APC props.

Best,   DennisT

[Paul Walker]

 

The APC's electric props have the undercamber and pull more than the flat back's. I have sanded off the cuff at the hub and that save a good amount but they (APCE 10x6) still lose pull at the top as the blade tip is so narrow. Cutting a larger diameter down would give the blade area. Looking at various tip shapes seems to indicate the slight sweep LE with the top of the blade tapered down to the bottom would be the way to go as long as it is flat bottom in that area of the blade. This is why the question to see how others have modified their APC props.

Best,   DennisT
[/quote]

Have you considered that your trim setup on your plane is not correct?  I ran APC props for several years when I first started. Never had issues with v8, HG or O8 maneuvers..my choice at the time was the 13*4.5 es..very good prop except for the buzz in corners. Flew very well in Bulgaria in that horrible wind through the trees. I have not been successful modifying them to improve other than balance and pitch symmetry.

Try adjusting your trim.

[Dennis Toth]

Paul,
Thanks for the comments. For me the APC props work ok it is just that the prop that works well pulls more amps than the 6.7 re-pitched MAS GF 10x8. As I said the MAS works and it is the baseline but it is a glow engine prop and heaver then the APC. So what I was looking to do was find a way to get the MAS performance in a lighter prop. I am looking at trying wood to see how it does at the field I can fly at, but the APC would hold up better. Still it is all about amps used.

Best,  DennisT

[Ken Culbertson]

Dennis:

I use the APC 12x6EB on my profile and my new PA plane with none of those issues. If anything, I plan to go larger on the profile.  Not enough ground clearance on the other.  I may be in the minority, but the power delivered by the two blade vs the battery used is a good fit for me.  The newer light weight 3 blades may be just a tad better, and I would use them if/when I am going to the NATS but they are not worth the replacement cost for everyday use.  The battery difference from an Igor 3 blade and an APC 2 blade of the same pitch was 2300mah uses vs 1750mah used.  Those differential puts a Li-Ion battery into the droop zone.

Ken

[Mark wood]

Well, that is what happens. If you tune your power system for lowest power consumption, the result is lost performance on the aerobatic side. The lack of digging is the same thing as the steepness of the thrust slope in response to slowing or accelerating. Finer pitch running faster at the same speed requires more power to run. The first chart is thrust v speed for three props 10x8, 10x6, and 10x5. Notice the the finer the pitch is the steeper the slope is when the velocity slows. That steep slope for the 10x5 as compared to the 10x8, is what I think you are referring to as "digging". Basically as the airplane slows the thrust increases. The more it does that, the more "bite" it has. The cost of this is power. See the second chart. The second chart shows what you are experiencing. For me, I don't get very worried about the exact power I'm consuming as long as the battery I am using is large enough I can land after the flight at storage voltage. I'm not sure a 10x5 at 4.7 seconds on 62 foot lines will do that for 5 minutes.  We fly 60 foot lines at 5.2 seconds and the 1800 MAH 4s is fine.

[Mark wood]

Dennis:

I use the APC 12x6EB on my profile and my new PA plane with none of those issues. If anything, I plan to go larger on the profile.  Not enough ground clearance on the other.  I may be in the minority, but the power delivered by the two blade vs the battery used is a good fit for me.  The newer light weight 3 blades may be just a tad better, and I would use them if/when I am going to the NATS but they are not worth the replacement cost for everyday use.  The battery difference from an Igor 3 blade and an APC 2 blade of the same pitch was 2300mah uses vs 1750mah used.  Those differential puts a Li-Ion battery into the droop zone.

Ken

After a ton of analysis, ground clearance is best technical reason for using a 3 blade prop. Personally, I go for the coolness factor.

[Dennis Toth]

Mark,
Interesting analyses, one thing with mine the rpm was adjusted to maintain the same lap time on the different props. With the wider blade I went from 10K to 9600 and it had lots of bit up top (here in Fletcher, NC.  2350' at 92F, 70%RH). Like I said this prop works but they are heavy and am looking to see if there is a lighter alternative that's why I'm looking at cutting a larger (like an 11 or even 12" down). The APCE blades have thin airfoils and narrow tips, this is where we lose the bit up top. But going to the wide blade could get the bit and the thinner airfoil and reduced diameter could keep the amp draw down to the 1650 range I get with the MAS. Problem is when you just cut of the tip it starts making noise which is wasted amps not really pulling the ship. Was looking to see if anyone has a tip shape that keeps noise (loses) to a minimum.

Best,  DennisT

[bob whitney]

 when we were flying clown racing we ran nelson 15s and 19s.we are at sea level here in florida trying for the most laps in 15 miniates. we would go to muncie and the motor would quit after removing the battery .we would remove ONE head shim and everything was back to normal . after coming back to miami the same setups would run hard .replacing the one head shim brought everything back to normal. so one .003 head shim made that much difference

[Mark wood]

Mark,
Interesting analyses, one thing with mine the rpm was adjusted to maintain the same lap time on the different props. With the wider blade I went from 10K to 9600 and it had lots of bit up top (here in Fletcher, NC.  2350' at 92F, 70%RH). Like I said this prop works but they are heavy and am looking to see if there is a lighter alternative that's why I'm looking at cutting a larger (like an 11 or even 12" down). The APCE blades have thin airfoils and narrow tips, this is where we lose the bit up top. But going to the wide blade could get the bit and the thinner airfoil and reduced diameter could keep the amp draw down to the 1650 range I get with the MAS. Problem is when you just cut of the tip it starts making noise which is wasted amps not really pulling the ship. Was looking to see if anyone has a tip shape that keeps noise (loses) to a minimum.

Best,  DennisT

That big noise comes from the combination of blade loading and tip speed. The APC propeller profile are designed to have a more uniform flow distribution through the propeller in order to increase their efficiency. It's derived from the mass flow equations for a rotating blade. What we shoot for is a uniform velocity distribution across the diameter. It's not achievable though. When you cut down a propeller this relationship gets muddled up. Noise is a non factor in my consideration bucket.

For blade tips the flight streak style wing tip is as good as it gets for a general purpose airfoil. For a non symmetrical airfoil bringing the lower contour and upper contour to the mean chamber line on the swept profile will get you there. The APC blade tip is an example. Cutting a blade down disrupts the flown distribution and isn't ideal although it works. That is a different thrust knob and the term for it is solidity.