Some propeller calculations

[Alan Hahn]

I was debating whether to put this into the electric forum or here--so you see what won out!

I made some calculations a couple of years ago using the PropCalc Software (available at http://www.drivecalc.de/)---at that time I made the calculation for some variations of a 10.5" prop --that's what I thought I'd be using at the time.

Since most of us in electric are using somewhat larger props, I thought I would look at something in the 12" category.  PropCalc doesn't have a lot of built in  props in the software, so I chose a prop (Aeronaut 12.5x6.5") which was in the ballpark of the APC 12-6 prop that I now use. This is actually a folding prop used in electric gliders, but I will ignore that detail!

Basically I just want to see the thrust and power curves vs airspeed at a fixed rpm for three simple variants of this prop--one the original prop, two the same prop repitched to 4", and third, the original 6.5" pitch prop, but with 3 blades.

About the repitching---Propcalc provides a prop window which gives the prop airfoil (in this case an E387) and the blade angles as a function of prop station (20%, 30%....). I took the 70% station--which for this prop gave the actual 6.5" pitch number, and recalculated the pitch for this station to 4". Then I scaled the pitches at the other stations (which were NOT 6.5" by the way!) by the ratio of 4/6.5, and then recalculated the angles.

So for the three cases, I adjusted the input rpm to produce 5.0N of thrust at 24 m/s--the speed I fly at. The 5N more or less matches the power my battery is supplying (according to my onboard data recorder) as long as I take into account the propeller efficiency (Prop Calc claims it is about 70% for the 2 blade 6.5" pitch prop) and my motor efficiency (75-80% according to Drive Calc).. Clearly it could be off by some factor, but I am guessing less than 30-40%.

But anyway this provides a base for the comparison.

The fixed rpm comes from the fact that almost all electrics use a motor governor which maintains a fixed rpm throughout the pattern.

Items to note:

1) the Thrust curves for the high rpm/low(4") pitch prop and the 3 blade 6.5" prop lie almost on top of each other. Both are steeper than the "standard" 2 blade 6.5" prop. This implies that as the airspeed drops in the vertical climb, you will get more thrust out of those two props. Also those two props will give better braking in the dive than the 6.5" 2 blade prop ( the 4" pitch is slightly better in the braking capability than the 3 blade).

I have no clue why the thrust curves for the 3 blade and 4" pitch prop came out so close to each other. I simply chose 4" for a pitch because that qualifies as a low pitch in my book!

2) The standard 2 blade 6.5" pitch prop only needs about ~175 watts of input power (roughly 230 watts from my battery) to maintain the level flight at 24m/s. The 3 blade version needs ~195 watts, and the 4"pitch prop needs 250 watts.

So what this tells me is that a 3 blade prop is about the same as a 2 blade low-pitch/high rpm prop in providing extra thrust in the climb, but is a lot more frugal with my battery watts than the 2 blade low pitch.

The 2 blade 6.5" prop wins the frugal award with power, but loses the thrust battle as airspeed drops to the other two props.

I might mention that I am already maxed out on diameter in my particular example. I also guess that a wider prop would tend to mimic the 3 blade prop. In any case I really would like a 3 blade version of my APC 12-6E!

[Dean Pappas]

Hi Alan,
Okay, so inefficiency produces better braking ... that makes sense.
It sure looks like a very wide 2-blader needs to be simulated next! Maybe with thick airfoils?
Well, the 3-blader looks like a great compromise.
Thanks for doing the work,
Dean Pappas

[John Witt]

I tried a 13-6.5P cut down to 9.75 and it worked very well. The actual power required appeared to drop a little, as I recall, compared to the 11-5.5 I had been using, and it pulled well out of the corners.

I was at the flying field without the computer so I started cutting the tips off to get the lap times I wanted. Ended up with this clunky looking thing that should be on a crop duster. Hey, doesn't a crop duster have the same kinds of speed control requirements that we have?

I think I don't understand what I know about this subject: the trade between pitch, blade width and diameter. Perhaps there's more than one maximum in this solution space.

John

[Alan Hahn]

Hi Alan,
Okay, so inefficiency produces better braking ... that makes sense.
It sure looks like a very wide 2-blader needs to be simulated next! Maybe with thick airfoils?
Well, the 3-blader looks like a great compromise.
Thanks for doing the work,
Dean Pappas

I tend to think about a little differently, although maybe equivalently--we talk about the benefit of a draggy airfoil, which "unloads" as the airspeed drops in the climb, thereby letting more of the prop thrust go toward fighting gravity. In some sense the draggiest item on the plane is actually the prop ---although that drag is being absorbed into the parameter called thrust. So my guess is that is the "inefficiency". Works in a climb the same way as parasitic drag. A little hand waving I agree!

[Alan Hahn]

I tried a 13-6.5P cut down to 9.75 and it worked very well. The actual power required appeared to drop a little, as I recall, compared to the 11-5.5 I had been using, and it pulled well out of the corners.

I was at the flying field without the computer so I started cutting the tips off to get the lap times I wanted. Ended up with this clunky looking thing that should be on a crop duster. Hey, doesn't a crop duster have the same kinds of speed control requirements that we have?

I think I don't understand what I know about this subject: the trade between pitch, blade width and diameter. Perhaps there's more than one maximum in this solution space.

John

Here is the original plot but now with a 50% wider 2 blade prop with 6.5" pitch (light green trace). It's thrust behavior is similar to the 3 blade and the low pitch props, but it is a bit more frugal with input power. Just to recapitulate, the dotted traces are the thrust curves (see left vertical axis) and the solid traces are the power curves (right axis).


I note that the force due to gravity is about 12N for a 44 oz (1.25kg) plane, so it is pretty clear that the plane is going to slow down with any of these props, and the differences of prop thrust are not overwhelming compared to gravity ( I am thinking the reduction of drag as airspeed drops is actually a bigger effect). But I suppose every bit helps!

[Dean Pappas]

Anyone for a B-Y&O 12-6? Of course, there ain't no pusher version, but ...
Hi Alan,
Yeah, let me arm-wave for a bit: it's just the right term.

My assumption, lo these several years has been that a prop that an efficient prop with thin airfoils and fairly wide blades will have a steeper slope of power absorbed versus airspeed at a fixed RPM (pretty much true) and this will cause the governor to change the motor drive effort more dramatically in the uphills/downhills.

The losses inherent in the three-blader do seem to work well at combatting whip-up based on actual flying tests our forum-mates have done. The profile drag of another blade and the vortex due to another tip force us to use more battery to fly and, as a tradeoff, help limit the RPM of the freewheeling propeller. Okay ... the inefficiency has both a good and bad side.

Now I think we are looking for wide, light, large as we can stand diameter, pusher two-bladers. Was the wide two-blader in the simulation using the same percentage thickness airfoils? Maybe wooden 3-bladers ain't such a bad idea. I used to laminate and glue them together for Pattern 60's back in the early '80s! It's not that hard.

Regards,
Dean P.

[Alan Hahn]

Dean,
In PropCalc, it allows you to increase the chord by a proportional amount--so I chose 1.5. Since the airfoil stays the same, I am guessing that the thickness may also get proportionally thicker, but I am not sure. Need to read the help file again!

PropCalc is a nice piece of software. If you know the airfoil used in the prop and the polars, it isn't too difficult to setup a a new prop.

For me the main utility is to try and get a feel for how parameter changes in a prop affect the way it performs. Of course flying is the real proof.

[Igor Burger]

This was analyzed several years ago on compuserve. I will add some thoughts:

- Prop in level flight waste lot of power, because it does not make thrust and thus it does not have output power. But prop still rotates and needs some input ower, so the statement of efficiency can be "reversed" as: more power, giving steeper slope of thrust to speed curve leads to worse efficiency in level.

- The slope of that curve is typically responding to power aivailable at best efficiency - and that is when prop pulls most - more power, steeper slope, (almost) does not matter what prop we have - large with lot of pitch, or small flat.

- Effective blade aspect ratio based on summary blade width (number of blades * width of blade) tells how much it loads the shaft in slippage (when prop pulls). Means wider blade or more blades load more because of induced drag on tips and thus we use narrow blades on IC engines (without governor) when loading makes ill effects while climbing. Since that time we got governors, so blade width does not have too much influence (beside consumption), but if possible, it is still better to have higher aspect ratio than width of the blade (or number of blades) because of reason in next ppoint. 1.5 x wider blades are almost the same like 10% of the diameter (or 3 blades act like +1" on diameter on our props)

- The lift coefficient comes with angle of attack with linear function cl=something + 0.11 * angle. So while airfoil works in good conditions, does not matter what airfoil it is, the slope of the thrust vs speed is the same. But sometimes we go off that conditions, flow separates and we can see results of tricks like we used on piped motors with chambered airfoils, that they have lot of drag when prop is at low slippage. It prevents overrewing, but we do not need it with governors, so we can use usual airfoils with good gliding ratio from 0 AoA to max thrust. I am not really sure if R/C props like ACPE are the best for us, we will see in some years :-) ... The same can happen at high slippage, prop can reach critical AoA and it can stall, that is reason why it is better to have longer than wider blade.

[Dean Pappas]

Hi Guys,
Yes, Igor: the dCl/dAlpha does not appreciably change with airfoil thickness (or even airfoil type!) but the profile drag (or inefficiency) in level flight does. This is similar to what you say in your first paragraph!

The question behind my question is this: when you have added all the diameter you can (either because the airplane's turn is compromised or because the landing gear is too short) then is it best to add a 3rd blade or widen the same diameter two-blade. I am leaning to the latter for efficiency reasons alone, and I wanted to know the answer so that I could better evaluate Alan's simulation of the wide-bladed 2-blade prop.  The wide 2-blader could easily have even more efficiency with thinner percentage airfoils because the rigidity will be sufficient. That would throw the balance toward choosing the wide two-blader over the 3-blader. Of course flight tests end the speculative B.S.

On the other hand, I have always in the past looked at this as a matter of maximising the thust versus airspeed slope at constant RPM - given that I want to also minimise power consumption. I am beginning to see another side to the coin: thicker airfoils and extra blades mean that the whip-up may be less in the wind because there are more drag losses even in braking.

Strangely, my experience has shown that airframes that suffered from a loss of corner with large-diameter wet props did not suffer the same problem with similar diameter, lower RPM, electric props. Until I do see that loss of corner, diameter is king, but once I do, itwill be interesting to see if width or a 3rd blade is the better answer, in general.

Fun talking to you,
Dean

[Igor Burger]

>>>when you have added all the diameter you can ... then is it best to add a 3rd blade or widen the same diameter two-blade.<<<
I would say that the third solution will be even better - lower pitch and higher rpm :-)

but if to choose from more blades or wider blade, I would say that to some dimeter you must add blade width, becayse of reynolds number, especially if you want thicker airfoil which will allow more slippage. 3 blades is better on larger props, especially because of good mass distribution (2 blader shakes in corners).

Aerodynamically, number of blades does not change lift and drag if the total area is the same (with the same area distribudion on blade diameter and airfoil %) and induced drag at tip depends on aspec ratio: drag = something / AR .. if you have 3 blades, istead of 2 then you have 1.5x more drag (torque on shaft), exacly like if you use only 2 blades and 1.5x lower AR (1.5x wider blade).

>>>The wide 2-blader could easily have even more efficiency with thinner percentage airfoils because the rigidity will be sufficient.<<<
Yes, but only if the rigidity (and not critical AoA) is the reason for that thickness.

[Alan Hahn]

Igor,
I guess I am not completely following your arguments.

All I did was to look at the variations within an easily available software program. As far as I know it does a reasonable calculation. What I was interested in were the changes in the thrust as a function of airspeed, just to try and see where tradeoffs might be. So as much as I could, I tried to vary only number of blades and width--assuming (which is true for me) that diameter is maxed out. I also chose a baseline thrust (5N) which seems to be ballpark (maybe a little low) for the planes I am flying.

Now if the calculations are incorrect, I am not capable of judging. If they are basically ok, then I think you can get an idea of what might matter in making a choice.

You can make any theoretical prop you want, either using the airfoils in the database already, or importing polars from other sources.

[Igor Burger]

Sorry, I did not play with Prop calc yet (well I did, but not seriously), so I cannot respond what and how you did.  Just take what I wrote as an inputs from older debating abut it, no necessarily directly related to what you wrote.  :-)

[Dean Pappas]

Yes, Igor.
I am assuming that structural reasons are the limiting factor on how thin the blade airfoils may be.
Dean

[Igor Burger]

Yes Deand it is for sure, but I was speaking about prop in some slippage. Such a prop with helical blades has separated airfouw from hub to sme diameter - it depends on rpm, slippage and pitch. So if you have thin airfoil, say 6% whis has critical AoA under 5deg, the it pulls less that the same prop with 9% airfoil.

I do not know exactly how this affects the prop properties, I did not do the math, but for sure I know that 12x6 prop on ground (speed=0) pulls far far les than 12x3.5 - experience from wet setups when I must keep such a model before start (and clean hands after  )

However there are tricks how to solve it (more pitch on tip etc).

[Dean Pappas]

Hi Igor,
Yes, you are right, and in my experience, this only applies to the 3-D RCers. All of our flying is at airspeeds well in excess of the necessary speed to unstall the prop blade roots. I think it does not matter, and has the small benefit of limiting static current.
I ran with a data-logger on an F3A ship with two 19-12 props: one was simply a narrowed and thinned version of the other. Current consumption while flying the schedule was so close to identical that small differences in piloting technique had to be considered the biggest variable, but the full-throttle static current both at takeoff and after landing were dramatically different. The narrow prop also made a gurgling sound under static conditions that  indicated the presence of the stalled airflow.

Dean

[Dennis Toth]

Guys,
I know this is an old thread but it seems we never got to a conclusion about 2 bld wide vs 3 bld narrow vs either in low pitch high rpm or high pitch low rpm (for our purpose 4" vs 6"). Has anyone done any further work or flight tests?

For my part I have been testing some MAS 3 bld pushers (11 x 7 which come under pitched from factory to about 5"). I like the three bld for turns what I am still trying to determine is what layout is better for pulling in the wind on those tough parts of the pattern where you need grunt (like the first outside corner of the square eight or hourglass). Also what tip shape? The old rule was that square tips gave better line tension is that just because they have more tip area? Anyone do some comparisons or simulations?

Best,    DennisT

[Shultzie]

Neva nuff PLOPPLA TALK...
Why am I having a flashback from my old Boeing 7J7 propulsion wind tunnel testing daze

[Air Ministry .]

 A B- T/R on an 8 x 8 , tas and ST 21/29s  Thought it was in Top or 3rd. Wasnt till it was at 60kts. or so it took an intrest in bing an 'Air Screw.'

Maybe the Old Cox 7 x3 was a 2nd gear on a donkey with enough balls to Manadge that. FIRST bing a 'starting gear'. There being a period where
the propellor is Slipping outside the efficent range.
Moral being 'gear' or AIRspeed , varying ranges . Equateing to Minimum reqd. airspeed (in pattern) and Maximum Reqd. Airspeed in pattern . So we
lokok stupid as its stating the obvious.
Then propellors vary. The top flites were said to be superior vertical (the plane Up & Down ) to the Tornados ( Direct Comparison - TWO Blade ) which
were ADVERTISED as being designed to " Atain Maximum Revs " in the Loop Scavenged Era , generally. 1970 .

The Tornados would slip a bit. At say 5 = alap, FROM 4.5 (not a lot of it at 4.5 a lap) If they DIDNT slip , They were up boggieing. 11 x 6 Tornados .
Th 12 x 6 was said to be " The Prop ". The range of engines, or Quantities 'Overseas' ,That Could turn on AT 10.000 rpm was insignificant. This was
an honest measure. They didnt mean 'sometimes' They Meant " hold 10.000r.p.m. "


liktrikerily this woundnt be exempt. TO establish a BAS LINE H.P. comparison . Needs a  Five foot wing plan , hopefully AT 60 Oz.
and that would b a big classical 'Stunter " , even if it was " NEW " in more ways than one .

The next test .       Chop the wings of AT 60 in. + tips , and IF its 60 Oz. put th THREE BLADE Tornado  10 x 6 on . Its a real 5 or so.

I should be able to get a pair of 10 x 6 and 10 x 4 Tornado NEW unused  LEFT HAND  three blade.  for pulling amould . Voulenteers Here !

Thisll get You Schaeffers or Compestellas W/C spec. Tango or Hallmark ? STUNT SHIP. Operated by Ohms and Jules and other tecchnical things

seeing the chap who started the post was talking about that.                         Not all are cogneitant that . . .so excuse the egg sucking .
Tornados Flat bottomed. particularly in th 3 blade and ar more efficnt at revs OVER the Top Flites,or inneficent  at Top Flite revs . NOT AS
Locked in on long runs vetical. But those three bladers ar a diiferent story. T/F mightnt hav ben 'as quick '. kids see THAT.

Two blade for later spec ship, ann 11 x 5 or in calm air a 12 x 5 Top flight. Figures of Equiv. Consumption Veco 45 on a foot of prop . 6 Oz.
NITRO was essntial for tourque essntial for 10.000  r.p.m.  WICH is the b.oo.y TEST.                Max Reqd. Output. (or is that min.)

                                                                                                                                                                  wots all the smoke !?

[Andrew Tinsley]

Hello Matthew,
  Please do me a favour and slow down! I am having great difficulty in following your thread. I am sure that there are nuggets in there, I jusr have difficulty in recognizing them!

[Peter Germann]

Yes Deand it is for sure, but I was speaking about prop in some slippage. Such a prop with helical blades has separated airfouw from hub to sme diameter - it depends on rpm, slippage and pitch. So if you have thin airfoil, say 6% whis has critical AoA under 5deg, the it pulls less that the same prop with 9% airfoil.

I do not know exactly how this affects the prop properties, I did not do the math, but for sure I know that 12x6 prop on ground (speed=0) pulls far far les than 12x3.5 - experience from wet setups when I must keep such a model before start (and clean hands after  )

However there are tricks how to solve it (more pitch on tip etc).

Is it correct to assume that running a as large as possible propeller with a flat pitch (generating a real lot of static thrust) helps minimzing speed bleed-off in corners and climbs?
Recent flight tests with a substantially overweight electric airplane and a big motor turning an APC 15 x 6 E at governed 8000 RPM for 5.3 sec laps seem to indicate this. As I now have a new (yes, much lighter...) airplane ready to go, I'd like to hear expert opinion on the use of the "large & flat" paradigm for RPM governed applications.

Thanks. Peter Germann

[Dean Pappas]

Hi Peter,
Yes, large and flat is good, up until the prop tip velocities cause both noise and energy losses to rise.
The 13 X 4-1/2 APC pusher prop flies wonderfully, especially in wind. Your 15-6 may have this beat!
My rule of thumb in order to avoid this is to keep the multiplied product of diameter and RPM under 130,000 inch-RPMs. This is roughly 2/3 Mach.
When this limit is reached, you either widen the blades or add a third blade in order to absorb more power at low airspeed.
Your original post illustrated this best!
Have you tried the pusher prop yet?

Regards,
  Dean

[Peter Germann]

Thanks, Dean
As 15" seems to be a bit much for the AXI 2826/12 (I flew it on the bigger and much heavier ZS 4020-8 Hyperion), what I plan to do is to try the 14" APC E prop, with pitch adjusted to get 5.3 sec laps when running 8'600 RPM governed. This will allow me to adjust RPM up/down compensating density altitude variations without having to change the prop. I expect level flight current to be in the 30 A range and with peaks in manoeuvres reaching 50 A for a few seconds, which should not be a problem for the 4S 4000 battery. From APC's page I've learned that E prop RPM should remain below 145000/dia and I would hope that a range from 8 - 9 k is safe, both in terms of load and noise.
No, I have not tried pushers yet, the simpler reason being that the choice of tractors is wider and, most of all, our circle here is snowed in...
rgds Peter