Drag/Thrust and Flight Times

[DB_Flyer]

Hi everyone,

I have an electric Telemaster 40. I have a project and basically I need to fly the plane as long as possible. We are using most of the equipment (engine, prop, etc) that hobby-lobby recommends when you buy the kit. With the 11.1V 4200mah lipo battery we can fly an average of about 32 minutes. This is flying at the lowest possible throttle our pilot feels comfortable at flying at. Ok, so now to the question. If I were to change the wing, would it be possible to fly longer (at an even lower throttle)? In other words, how can i lower the stall speed? If so, how would I change the wing? What do I want to maximize or minimize? From what I can tell, the airfoil on the Telemaster resembles a "clark y" type of airfoil? For another aspect of the project, it is looking like I am going to have to increase the size of the wing. Would this help me fly slower? And since I am going to have to make the wing larger, I might as well make a whole new wing so are there certain airfoils I should be looking at? Or what properties of the airfoil should I be paying the most attention to? I was reading somewhere about rules of thumb on wing loading where lower wing loading meant lower stall speeds? Is this correct? I am assuming these rules of thumb are coming from the general lift or drag equations? At the throttle setting we are currently at, the airspeed was roughly 11-12m/s giving a Re of about 300000 if that helps at all. Sorry for the random order of questions... just typing them up as they came to me.

Thanks,

DB

[slipstick]

1. Yes
2. More area, less weight, higher lift airfoil
3. See 2
4. See 2
5. See 2
6. See 2
7. See 2
8. See 2

I may have missed a few and I've ignored the bits with question marks at the end that were statements not questions ;).

Simplifying, slower stall speed requires lower wing loading and/or a higher lift airfoil.

Of course if what you really want is to fly longer then a bigger battery and/or more efficient motor/prop combination could easily help a lot more than structural changes ;).

Steve

[Brandano]

try just increasing the wing span. This should lower the wing loading and increase the aspect ratio, that should reduce induced drag but increase surface drag. You can also increase the flight times by drastically lowering the weight of the model, since I expect you won't need to withstand the same kind of stresses the original model is made to survive. Perhaps you could file the inside of a few bulkheads and reinforce them with carbon tow and epoxy.

[HX3D014]

Hi everyone,


I have a project and basically I need to fly the plane as long as possible.
Telemaster 40;
We are using most of the equipment (engine, prop, etc) that hobby-lobby recommends when you buy the kit.
With the 11.1V 4200mah lipo battery we can fly an average of about 32 minutes. (This is flying at the lowest possible throttle our pilot feels comfortable at flying at.)

Ok, so now to the question's

a/If I were to change the wing, would it be possible to fly longer

a.a/If I were to change the wing, would it be possible to fly longer (at an even lower throttle)?

If so, how would I change the wing?

In other words, how can i lower the stall speed?


What do I want to maximize or minimize?

From what I can tell, the airfoil on the Telemaster resembles a "clark y" type of airfoil?

For another aspect of the project, it is looking like I am going to have to increase the size of the wing. Would this help me fly slower?

And since I am going to have to make the wing larger, I might as well make a whole new wing.
so;
Are there certain airfoils I should be looking at?

Or what properties of the airfoil should I be paying the most attention to?

I was reading somewhere about rules of thumb on wing loading where lower wing loading meant lower stall speeds?
Is this correct?

I am assuming these rules of thumb are coming from the general lift or drag equations?

If it helps;
At the throttle setting we are currently at, the airspeed was roughly 11-12m/s giving a Re of about 300000 .

Sorry for the random order of questions... just typing them up as they came to me.

Thanks,

DB

I would be thinking of looking at the best lift/drag ratio. But only after I confirm a thought I am having.

(Cant quite put my finger on it)

I'll Be back (to learn along with this thread)
Bryce.

[fnev]

For the best endurance you have to design at the best L/D.

You have your mass, you have your air speed and you have your altitude: this gives you the minimum required Cl needed for strait and level sustained flight.

You now have to design a wing giving you the minimum drag for this operating point with the required strength and stall behavior.

[DB_Flyer]

Thanks for the responses. The weight has been pretty much minimized as much as possible. For a less drag airfoil/wing could I stick with the clark y shape but basically make it thinner? Is there an airfoil similar to this that I could perhaps put into profili?

Steve - I am pretty much stuck with the motor I have (AXI Gold 2826/12 Outrunner Motor), how would I go about finding a more efficent propeller than the one I have (13x8)?


DB

[nfhill]

You don't necessarily want to fly slower, you want to fly more efficiently. Higher L/D is one part. Drag reduction is also a good way: wing-fuselage fairing, gear strut fairing, wheel pants, etc. A thinner airfoil is also a way to go. Profili has a lot of pre-calculated airfoil data that you could browse thought.

Keep in mind that if you build with sticks and film, the airfoil you get will not be overly close to the airfoil you drew. If you spend a lot of time searcing out the 'best' airfoil, you'll need to use assembly technology that will let you actually build it.

[HerkS]

Recalling my aero theory, I believe that max endurance occurrs at the performance point where aircraft drag is a minimum. That would be very close to the point at which power required for level flight is a minimum.

L/D doesn't have much to do with endurance.

However finding the point at which the drag of the aircraft is a minimum does take some significant theory and math. Then, when you have done all that, it's still just theory and math.

The easiest way would be to find the level flight trim and power setting at which power required is a minimum.

I believe there are some commercial products that will report inflight power draw. If you had the device, you could actually measure the effect of your modifications.

I suspect that increasing span is likely to be most effective - given your other constraints.

[HX3D014]

Recalling my aero theory, I believe that max endurance occurrs at the performance point where aircraft drag is a minimum. That would be very close to the point at which power required for level flight is a minimum.

L/D doesn't have much to do with endurance.

However finding the point at which the drag of the aircraft is a minimum does take some significant theory and math. Then, when you have done all that, it's still just theory and math.

The easiest way would be to find the level flight trim and power setting at which power required is a minimum.

I believe there are some commercial products that will report inflight power draw. If you had the device, you could actually measure the effect of your modifications.

I suspect that increasing span is likely to be most effective - given your other constraints.
I think that is one of the points I was Trying (Mentally;)) to put my finger on. Endurance for time aloft and Endurance with maximum distance travelled for given amount of Fuel energy are different.

It is Exactly true that the Setting of the throttle at its lowest point while still maintaining altitude and some manageability (For us we need manoeuvrability to keep a circuit) will be the maximum Endurance setting.

Very good points there nfhill, about the Parasite drag reduction methods. wheel pants etc.


I am Still Trying to put my finger on a thought I am having about the effectiveness of either increasing the Chord length of a Chosen Airfoil at the root and increasing the Span. (Increasing the span on real birds helps reduce induced flow especially if you reduce the Chord length at the tips) But is this a bit different to RC birds given the Low Re #'s ???

To sum that last paragraph up. I would Choose a Foil of Good lift with low drag and increase the chord length at the root and reduce it at the tip as well as increase the Wing length, but I am not sure if the effect would be noticeable in a Small RC wing. ?? The effect of reduction to tip Vortices. If it would be noticeable. Maybe even add a fence or a wing tip tank. even a little washout would help.

[vintage1]

well its a moot point as to whether the minimum drag for lift=weight is the minimum L/D ratio. :D

Also, its not minimum drag, its minimum drag times airspeed. That's the power needed to stay up.

So the slower the better until the AofA is so great that you start to get turbulent flow and drag is going up faster than speed is coming down.

Experience shows this to be about 10-20% over stall speed.

Or about the natural glide speed or slightly higher.

[DB_Flyer]

Will wing-fuselage fairing reduce drag that significantly? The Telemaster has a somewhat flat windshield and I was basically thinking of making the front more "bullet" shaped as in adding a shaped piece of Styrofoam on top of the hatch. And then also perhaps a spinner. However, at the Reynolds I'm flying at, would it make that much of a difference? Isn't most of the drag coming from the wing? Wheel pants also fall into this category. I get that it makes sense that these modifications would reduce drag (fuselage/windshield fairing, spinner, wheel pants), but I am having a tough time figuring out how much reduction is actually possible, especially in comparison to the drag caused by the wing.

On another note, it looks as if I will need to accommodate a flat area of approx. 12"x90" on top of the wing. Also, it is sounded more like the ultimate goal would be to minimize drag as much as possible to fly more efficiently (and thus longer at a lower throttle setting) Therefore, my idea was to basically make a longer version of the airfoil I have now and make it thinner. Is there much more I can do with this area restriction? Is there a way to enter your own airfoil in profili or should I just keep searching until I find one similar enough to what I am thinking of making?


DB

[vintage1]

My best duration models are in fact copies of 50's style duration models..low wing loading and slightly undercambered. They float forever. At the sorts of speeds (18mph or so) the wheels and fairings don't have a lot to say..

However there is a downside to using undercamber. The stall is nastier. And they don't fly upside down worth a damn. Washout helps..but then you lose some of the efficiency.

If you go to sailplane type wings//low aspect ratio tapered or elliptical, then you get a very flat glide BUT you will have really vicious tipstalls doing tight turns at low level..

I and a friend have both crashed these for that very reason. Trying to land in small spaces.

[Flight Engineer]

max endurance will be at Vmd (bottom of the drag curve). This is also L/D max. If you want this to occur at the lowest throttle possible then more wing area, best aspect ratio (although there will come a point where you can't build the wing light and strong and also you may start to loose out on Rn) Best prop for chosen speed. lowest parasite drag you can manage.

[JetPlaneFlyer]

max endurance will be at Vmd (bottom of the drag curve). This is also L/D max.

Can you clarify this point please?
For instance the 'bottom of the drag curve' for a symmetrical airfoil would occur at zero AoA and Cl =0.. Therefore the L/D would be infinitely low at the bottom of the drag curve.
A symmetrical airfoil may be an extreme example but not unique. For instance even the good old Clark Y has minimum drag at zero AoA but best L/D around 4-5 Deg

Or are you referring to the flying speed at which the plane produces minimum actual drag, rather than minimum Cd?

Steve

[DB_Flyer]

I think he means flying speed at which the plan produces minimum drag. Something like figure 4.2 in this link: http://ocw.mit.edu/ans7870/16/16.unified/propulsionS04/UnifiedPropulsion4/UnifiedPropulsion4.htm. Also, what do you mean by "best" aspect ratio? How do I go about finding the best prop for a given speed? Also, to achieve lower parasitic drag, I am assuming make the airfoil as thin as possible? Also, since I am flying at which I perceive to be relatively small Reynolds (low speed) wouldn't I care more about the induced drag? As of right now, I have an airfoil with about 1.5in thickness and a chord that is maybe 11.75in (with the max thickness occurring close to 25% of the chord). Also from the max thickness down to the trailing edge is fairly straight (though not completely). For my new wing, I am thinking of something like a 1in thickness and a chord of about 15in with the max thickness at around 10% of the chord. This airfoil would also be pretty much straight from the max thickness to the trailing edge. Keeping in mind what nfhill said, this will be built with sticks and film as I don't have another way to do it. In that case I guess it is almost a moot point to design a complete airfoil in profili. Instead I am basically trying to figure out with reasoning and equations how the general shape of the wing affects drag and if it will actually be significantly less by making the changes above.

DB

[DB_Flyer]

Also, early on we decided against wingtips (I think I mean winglets?) to decrease wing tip vortices because at the low speed we are flying the vortices aren't that significant. However, aren't wingtip the main (if not only) source of induced drag so would it make sense to have some sort of winglet or specifically designed wingtip?


DB

[vintage1]

Definitely bigger wing area makes a huge difference, as it pulls speed down, and therefore power per unit drag.

The figures I tossed around and compared with what happened in real life, suggest that induced drag is very dominant at the sorts of speeds we are talking about.

Not to say that wheel pants wont help, just not much.

As far as aspect ratio goes, yes a sailplane wing does better, but the downside is a tradeoff in turnability. The dreaded tip stall will get you.

Also structural issues start to dominate and carbon fibre rears its ugly head..;)

[nfhill]

What are the goals/requirements that you're trying to achieve? What are you willing to give up? How much effort/money are you willing to put into it?

I'm sure that you could design & build a new wing that is more efficient than the Telelmaster wing, but harder and more expensive to build would quickly appear.

An inflight data-logger would be useful to evaluate changes: motor watts vs airspeed should reveal a lot.

[DB_Flyer]

Money would be at a minimum, as would effort in the sense of time. I guess I am basically trying to figure out if a thinner yet larger area wing/airfoil would be more efficient than the wing/airfoil I have now. I think you a 100% right in that it could extensive more difficult and expensive quickly. I was looking at the Sky Sailor wing, but I do not have the machinery or the materials to build a wing like that... and building it out of all balsa might be more trouble than it's worth. So in that case I think I would like to stick to a predominantly rectangle wing.

Also, the telemaster has a dihedral where each end of the wing is about 2 inches higher than the middle. For max efficiency it would be better to have no dihedral, right? But then I would be losing stability and I am already flying really slow so maybe I should keep some sort of dihedral for stability's sake?

DB

[JetPlaneFlyer]

Also, the telemaster has a dihedral where each end of the wing is about 2 inches higher than the middle. For max efficiency it would be better to have no dihedral, right? But then I would be losing stability and I am already flying really slow so maybe I should keep some sort of dihedral for stability's sake?

DB

Dihedral helps the model stay coordineted (minimises side slip) in the turns so there is a negative consequence to removing it. In any case the loss of efficiency due to small dihedral angles is tiny. Efficiency % due to dihedral is easy to calculate it's 'cosine of dihedral angle x 100' this shows that if the model has (say) 3 degrees of dihedral then it's 99.86% efficient as a flat wing in straight and level flight.

As stated earlier a few times... your best approach is probably to use a wing that generally similar to the one you have now but with panels added at the tips to increase span, weight should be reduced as far as possible. These new panels should taper a little to give more optimal spanwise lift distribution, but dont add too much taper or tip stall will become an issue. Making the wing very thin is structurally not such a good idea unless you are happy to use advance construction methods (carbon etc).

Have a look at the wings on sailplaned to get an idea of what you should be aiming at, but remember that your model will be heavier than a similar size sailplane so the wing needs to be stronger.

[DB_Flyer]

Thanks everyone, I appreciate all the feedback.

JetPlaneFlyer - It does look like that is going to be a good direction.. I was heading that way but wasn't fully convinced. So by adding a tapered panel to the end I will have a more optimal spanwise lift distribution (as in lowering the wing loading?), and this tapering will decrease induce drag?

I was looking at sailplanes and it looks like there are 2 general types of tapering. I attached pictures of both. Is the curved/quarter circle style too much taper? Should I go with something more like the other one which seems to be a more gradual taper?

DB

[DB_Flyer]

On the topic of tapering... does it matter if I were to taper the LE vs. the TE? Is tapering the LE mostly for aesthetics? Just curious..


DB

[JetPlaneFlyer]

The glider in the photo looks to have a good wing shape... something similar should work very well. Add a little washout to the outer panels to guard against tip stall. The curved wing tip would do no harm but adds build complexity.

Current thinking is generally to keep the TE straight and sweep the LE back, the tip itself should have a sharp corner at the TE rather than rounded. the glider in the photo ticks all these boxes.

Here is a link to an article on tip design which gives the reasons why most current high performance wings favour this approach: http://www.hippocketaeronautics.com/downloads/TipTopics.pdf

Have fun!

Steve

[Brandano]

I think that the main reason to leave the TE straight is to better fit ailerons on it. fully rounded tips will rob the ailerons of a little span at the largest moment arm, making them less efficient.

[DB_Flyer]

Let me know what you guys think... this is the 2nd wing my team and I have ever built (the first one was the original telemaster wing)

[DB_Flyer]

Just an update:

The wing has been completed... we added another spar about mid span to make sure it would be strong enough. Also, the wing in the picture does not yet have ailerons. The ailerons we have on now are about 4 inches wide and 21 or so inches long. The total length of the wing is about 104 inches. The tapered section is 15 inches (so 3oin for both sides). The wing looks pretty good, but we went out to fly today and it flew like crap. :( We had very little control, and brough it down within 5 minutes of taking off. The ailerons worked (as in they were moving up and down) but did not seem to be doing very much. We also made the elevator larger than the original telemaster (about 4 inches out compared to about 2. We have been told we need to make the rudder bigger as well. However, more major changes are need, I am just not sure what is wrong or why it is flying the way it is. I don't think it would be the tapered wings, but I guess it could be. We also we measured/evaluated the CG and straightened that out before we flew so that shouldn't be the problem either. Our original ailerons were about 1/2" think and extended out about 2 inches. However, our pilot said we would need ailerons at least twice that big, so we added a 3/32" sheet to the top and bottom of the ailerons to extend them out. It was a shoddy job, but that is all we had available to us. Would this make it as impossible to fly as it is right now? Maybe the chord length is just to large? As of right now, we are thinking of cutting off the tapered section and then trying to fly with that, though I don't think the tapered section is the problem... Any suggestions would be helpful. Thanks.


DB

[DB_Flyer]

pics