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Author Topic: aspect ratio and freewheeling props  (Read 1951 times)
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lincoln
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« on: December 16, 2012, 02:37:18 PM »

I have yet to build a P-30, though I've built some other rubber powered models. However, I've been thinking about some of the aerodynamic issues of freewheeling props (which also apply to, for instance, FAC scale). From what I've picked up here and there, at a particular, usable pitch that is a little finer than what we use, the prop drag is the same whether fixed or free wheeling. That says to me that the drag of many props, especially the Peck's, is comparable when freewheeling. A few crude estimates and calculations seem to indicate that the drag of the freewheeling prop is more than that of a typical wing.* Further futzing around seems to indicate that a really low aspect ratio or a biplane would have a significantly lower sink rate, as the model could glide slower and give up less power to the prop. The glide wouldn't be any flatter, but that doesn't matter. If the span wasn't limited, of course, one could just make a longer wing, although Reynolds number problems may limit that.

Do wider wings like this add so much drag or weight that they kill the climb? I'm thinking maybe not, as Barnaby Wainfan won the Nats with his Swallow design, which looks to have a pretty low aspect ratio.  I'm thinking maybe even a delta wing could be better. Keep in mind that the Reynolds numbers here are quite low, on the order of 30k, so a wide wing might have a much lower coefficient of profile drag, partially making up for the drag of having more area.

These considerations would also be important in any span limited events in scale.

Is it just fashion?


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*The real answer, barring a long search of research papers, would be to make a pivoting arm with a freewheeling P30 prop on one side and a flat plate on the other, and hold it out the passenger window while being driven around at, say, 10mph or so. I could trim the plate until the drag was equal, and then I'd know the flat plate equivalent drag, without having to do any calculations!
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TimWescott
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« Reply #1 on: December 16, 2012, 03:50:25 PM »

Are you sure the model could glide slower?

Two answers immediately spring to my cynical mind:  one, why not build something and see?, and two, there's enough random variation amongst designs that you'd think that by now evolution would have pushed the field in that direction.

A bigger wing (or a bipe) is going to be much harder to build down to the weight limit.  I suspect (but do not know) that the ultimate height you'll reach given the rubber and prop limitations has more to do with the aircraft weight than with wetted drag on the wing.

Another thing to note is that (if I'm not horribly mistaken) the alternatives to the Peck props are all higher pitch -- would this not reduce the drag in freewheel, and throw off your calculations?
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lincoln
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« Reply #2 on: December 16, 2012, 10:01:44 PM »

My calculations include a SWAG for the prop drag, so they are only indications. You're right that the other props probably have more pitch. The first order of business would be to do the testing I mentioned, but I fear I'm already the neighborhood eccentric.

If you have more wing area at the same coefficient of lift, the glide will be slower. That's how it works.

I think the differences are significant but not compared to launching in lift vs. launching in sink.

I am not sure, but I suspect that building a delta wing down to the weight limit might not be very tough, since it has no horizontal stab and might not have a fuselage. If it was a pusher, it might not even need a vertical stab.

I suspect that even without the prop drag, the optimum aspect ratio might be 4 or so, since the Reynolds numbers involved are so low and the profile drag coefficient so high.
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Alexandre Cruz
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« Reply #3 on: February 24, 2014, 11:28:47 AM »

Lincoln

I believe you are right on your concepts. If you can reduce the parasite drag influence as fuselage rudder, pylon and prop in the whole L/D then you can have a better power factor (Cl^3/Cd^2) and sink slower.

The other point is the climb. You use some energy for putting those 50g up, some energy is waste with drag as drag x speed (so speed goes cubic) and a lot in is lost in the prop and mechanisms. Prop efficency is related to how much you acclerate the flow to produce thrust so it is more efficient to accelerate a huge mass of air only a little bit, than accelerate a lot a small amount of air.
However since we canĀ“t change diameter in P30 and have limited pitches the only factor left your speed and rubber strands.
So if you have not a propeller matched to your flight speed efficiency will decrease. At last but not least, you must fly 120s so every second with motor run counts it is not only about going higher....

I started Saturno design using this. Since there is no VIT the idea was to match climb speed and glide speed to prop speed by choosing chord and airfoil and strands. This took a while since torque varies a lot and so does prop speed.

Saturno differs from other models by using a very high cambered airfoil and 98mm chord. I use standard peck props and I like to use from 6x1/8 for wind to 6x3/32 for calm being 10x1/16 my favorite. Most of the data I have is using 6x1/8, in this case the motor will run for 52s and make the model climb something close to 60m. That makes 120s really easy even with some turbulence. Dead air times are around 2:37 in this config being 3min possible with longer runs and very very calm air.
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lincoln
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« Reply #4 on: February 24, 2014, 05:45:29 PM »

Another factor with using a larger chord is that it's easier for the timer, or anyone, to see the model in the air.
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Vern
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« Reply #5 on: February 25, 2014, 12:46:45 PM »

Lincoln,
The Free Flight Quarterly P-30 book has a lot of great info. I have designed & flown a number of P-30's only one dog. Here are my "rules"
1- Build to min weight
2- Wing Chord 4-4.5 inches,I have used wider chord wings but weight & wind conditions are concerns
3- Trim per TOP criteria
4- Reliable Free Wheeler
5- Long run motor
6-Wing DT
7-Wide chord stab

The prop drag is a function of glide speed the slower the glide the better.

Regards,
Vern

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Dave Andreski
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« Reply #6 on: February 25, 2014, 12:53:00 PM »

Another factor with using a larger chord is that it's easier for the timer, or anyone, to see the model in the air.

Depends on the color of the underside of the aircraft.

Dave
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Modelace
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« Reply #7 on: May 06, 2014, 11:40:17 AM »

FWIW, Fred Pearce won the AMA Nats with a P-30 having a 12" chord (Twice).
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Bargle
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« Reply #8 on: May 06, 2014, 07:27:36 PM »

Is there a picture of that thing somewhere on the 'net?
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Dave Andreski
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« Reply #9 on: May 07, 2014, 07:15:27 PM »

Maybe this one?
Dave
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Re: aspect ratio and freewheeling props
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Modelace
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« Reply #10 on: May 07, 2014, 09:34:26 PM »

I will see if Fred still has it. Simple box fuselage, rectangular wing and stab, sub rudder, film covered.
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Ross J
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« Reply #11 on: July 01, 2014, 10:31:15 PM »

I've just read sporadically so if this is off base let me know. There were a few attempts at a large chord P-30, the Window Plane won the NFFS model of the year 15 or so years ago. But in the 2001 NFFS sympo John O'Dwyer did extensive testing on 3 P-30's that differed only in wing / stab area. He glide tested indoors with and without props as I remember, and then did extensive power testing. The result was that the small model was superior overall as it made more in climb than it lost in glide.

In the 2014 Sympo, which will be released at the US Nats, Tapio Linkosalo tests a P-30 model with a Midic designed Low Drag Airfoil (LDA). He finds that reduced drag from an LDA gained 30 feet more altitude in the climb over a similar size model with a USA 5 (standard) airfoil. He had also tried a large model with poor results. He thinks that the prop is such a drag in glide, that trying to improve the glide with more wing area is a waste of time.

Marie and Marie II were also rather small models with long 4 strand motors, sort of a mini-Mulvihill. I have downsized my P-30 in wing and stab area and performance is much better, though I still use a 6 strand motor.

The goal seems to be altitude in the climb which you get from a smaller model.
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lincoln
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« Reply #12 on: July 02, 2014, 02:24:23 AM »

I'm guessing that the "smaller" models with their greater climb are better for windy conditions and that the "larger" models are better when it's calmer. Of course, you could go with a relatively large chord and a smaller span if you really want to climb fast!
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Ross J
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« Reply #13 on: July 10, 2014, 01:30:39 AM »

Tony Matthews put it best: "the most important thing about wing tips is how far apart they are". I wouldn't skimp on the 30" span at all. There is no getting around the drag of the prop in glide, and there is no getting around the drag of the flying surfaces under power or glide. There is a point at which the compromise between the two achieves an optimal flight time however and I think that point is achieved with a smaller wing area.

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lincoln
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« Reply #14 on: July 10, 2014, 04:31:37 AM »

That idea of mine wasn't entirely serious. My intuition says that you couldn't gain enough extra altitude to be worth it.

As far as Tapio's study goes, using the same airfoil for different chord lengths is, IMHO, unlikely to be fair to all the configurations, because the optimal airfoil for each isn't going to be the same. And if the freewheeling prop's drag increases, the increased induced drag of flying slowly matters less and the reduced prop drag of flying slowly matters more. Anyway, you can probably make these studies come out every which way depending on what airfoils you pick.

I was just messing around with Profili and came up with the following:
-P-30 #1, 8:1 aspect ratio, BE5456 with a trip at 25 percent*, constant chord
-P-30 #2, 4:1 aspect ratio, AG08 (!), camber reduced to 1.6 percent, no trip, constant chord, 20 percent flap at -3 degree. The reduced camber and negative flap are to reduce pitching moment.
If we're flying them both at the same speed, induced drag should be the same, so we'll ignore it. First speed will be about 11 fps, or a Cl of 1 for P-30 #1. For P-30 #2, the area is twice as much, so Cl will be 0.5. The drag numbers I report will be normalized to P-30 #1. That means that the Cd for the profile on #2 will be multiplied by 2 so that it's an actual drag comparision, and the Cm will be multiplied by 4** (!).

#1, Cd is 0.037 and Cm is -0.85
#2 Cd x 2 is .042 and Cm x 4 is -.088
So in this condition the trim drag is about the same, and the profile drag (not the coefficient) is about 14 percent higher. Plus the stab needs to be larger, but I'm not going to worry about that just now. Someone else can figure that one out, or maybe I can tomorrow. Probably this can be made up for by flying at a CL of 0.6 or a bit more instead of 0.5. Notice that I'm not accounting for prop drag here, since it would be the same when flying speed is the same.

So, let's assume that we climb at twice the speed, so Cl is 1/4 what it was. In this case:
#1 Cd is 0.032 and Cm is -.105
#2 Cd x 2 is 0.024 and Cm x 4 is 0.04

So the wide chord model has 25 percent LESS wing drag and less trim drag too. Sounds to me like it might be a win, at least if the weight is kept down. Obviously, everything changes when you change airfoils, and who knows what the optimal ones for each are? I'm supposeing the Midic foils might be better for the high aspect ratio designs, but I don't know which one to use. I picked the BE5456 because it seemed to be better than a number of other possibilities I looked at. For the wide chord models, I think a thinned out  5020 airfoil might be interesting.

At the very least, I think this shows that wider chords don't necessarily have to be slower in climb if you pick the faster airfoils that they permit.

BTW, if I wanted to be evil, I missed anything in the rules that says  you can't swing the prop shaft up 90 degrees and stop the prop's rotation when it's parallel to the direction of flight after the motor run is over.  Hmmmm.... I imagine it would get banned pretty quick. But if you don't want people to try stuff like that, these things should be banned AHEAD of time. And perhaps my interpretation that "freewheeling prop" means a prop that's made with a freewheel ramp is off.

Speaking of rules, if the prop diameter rule was relaxed, I wonder if an 8 or 8 1/2 inch prop might give enough more glide time to make up for any possible climb deficiencies?

All this stuff is a bit silly since the first priority is ability to pick air and the second is a model that performs reasonably well. If the maxes were longer, maybe that performance would matter more, but it would still be trumped by picking air.


 


*The BE5456 needs a trip pretty badly at this Reynolds number
** 2 for the increased moment from the wider chord times 2 for the increased area
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