Batteries in wings

[BarrelRoll]

One of the projects I am currently involved in is an electric powered long-endurance mini-UAV. As the weight of (LiPo) batteries is going to be around 50% of the total, there is a suggestion to integrate the batteries in the wing structure. What do you think about the idea?

[SST32]

I have had the same recurring thought. I am constructing an airframe entirely out of carbon fiber, and believe that the wings would be able to support such an idea...I will keep an eye on this thread.

[d_wheel]

One of the projects I am currently involved in is an electric powered long-endurance mini-UAV. As the weight of (LiPo) batteries is going to be around 50% of the total, there is a suggestion to integrate the batteries in the wing structure. What do you think about the idea?

One thing to consider is the possibility of fire while charging. Most prudent users of LiPo's remove them from the aircraft while charging to prevent airframe damage if the battery catches fire during the charge. If you are using a balancer which will terminate the charge when it senses anything out of spec, then you might be all right.

Another down side is that it might be difficult to replace the battery packs if that became necessary. You will have long waits between flights unless you can easily swap battery packs.

Later;

D.W.

[Zor]

One of the projects I am currently involved in is an electric powered long-endurance mini-UAV. As the weight of (LiPo) batteries is going to be around 50% of the total, there is a suggestion to integrate the batteries in the wing structure. What do you think about the idea?

Hi BarrelRoll,

I am new to this part of the forums.
I think the idea is good since the airplane would not be aimed at aerobatics in which the wing mass is ususally kept to a minimum to improve the roll rate.

Most full size airplanes have a good part of their mass in the wings. Fuel tanks, engines (airliners).

Depending on the selection of the batteries and their size, it might be a good idea to have a variable airfoil from root to some distance out of the fuselage. The airfoil could be thicker near the fuselage to accommodate the batteries.

Go for it _ _ _

Regards de

Zor

[Zor]

One thing to consider is the possibility of fire while charging. Most prudent users of LiPo's remove them from the aircraft while charging to prevent airframe damage if the battery catches fire during the charge. If you are using a balancer which will terminate the charge when it senses anything out of spec, then you might be all right.

Another down side is that it might be difficult to replace the battery packs if that became necessary. You will have long waits between flights unless you can easily swap battery packs.

Later;

D.W.

A very good thought.
The batteries, since it would have to be two of equal weight to balance the airplane laterally, could be tailored fit into a structure installed from uderneath the wings a bit like a low wing aircraft is mounted. A bit like aileron servos are often installed.

A couple of hard wood (or aluminium tubing) at one end as inserting pins and a wing screw at the other end would allow for quick switching of battery packs.

Ah well, ideas are a dime a dozen and I am sure you also have many ideas.

Zor

[kd7ost]

There are other points to consider however,

With batteries in the wing you can't use that mass to help balance front to rear. Not an issue if the plane balances well without shifting the packs.

With cells in the wing you need to increase the wiring and connector load inside the plane. This could result in a little current loss making the packs less efficient. That might also not be an issue if the trade off's are worth it.

It would add mass around the wing shifting the center of mass higher. That would increase roll rate and decrease inherent built in roll stability. Heavier wings would also cause the roll to happen slower as you have to shift mass from farther away from center. This means it would be best to keep the packs as close as possible to the root and fuselage. Again not a big factor. Flying wings have no fuselage per se but do require additional work to stabilize. They are very responsive and usually take an outside source like a co-pilot to manage roll and pitch from on board.

The more the small packs are spread down the wing, the more structure needs to go into the wing to handle the loads. That means more weight in and of itself. It seems like the easiest thing to do is to keep the main packs internal to the fuselage and maybe just wire in some smaller packs from the wings.

Fundamentally it would work alright. There are just some changes to be aware of. Seems like additional weight and wiring that might not be the biggest step up in technological advances. But if thats all you can do to add additional cells, what are you going to do?

Dan

[CenTexFlyer]

We bury batteries in our wings. Works like a champ! Mock up the layout and check CG before putting them, then go for it!

[anci3nt]

One of the projects I am currently involved in is an electric powered long-endurance mini-UAV. As the weight of (LiPo) batteries is going to be around 50% of the total, there is a suggestion to integrate the batteries in the wing structure. What do you think about the idea?Barrel, would love to hear more about your project! Any pointers to more information?

[Odysis]

There are other points to consider however,
...
The more the small packs are spread down the wing, the more structure needs to go into the wing to handle the loads.

Not quite. In flight, wings support the fuselage, not the other way around. Hence, the lighter you can make the fuselage, by moving the weight into the wings, the lighter the structure can be. The wing root bending moment is the big determining factor for structural weight. Less weight in the fuse, less bending moment, lighter structure.

This is one of the big reasons aircraft carry fuel in the wings. I've often wondered why model planes, particularly molded glass ones, don't do this...

Odysis.

[TheThomas]

Odysis is right. Spreading out the weight well into the wings makes for a more efficient wing structure. In order to get the lowest drag, your lift distribution has to be elliptical. And if your weight distribution was also elliptical...theoretically, the only structure you'd need would be to fight flutter and operational loads (controls, landing etc).

Stability wouldn't be penalized too much. You can always add dihedral and lower the wings. The added inertia might even make it harder for a gust to flip your plane over.

[kd7ost]

Not quite. In flight, wings support the fuselage, not the other way around. Hence, the lighter you can make the fuselage, by moving the weight into the wings, the lighter the structure can be. The wing root bending moment is the big determining factor for structural weight. Less weight in the fuse, less bending moment, lighter structure.

This is one of the big reasons aircraft carry fuel in the wings. I've often wondered why model planes, particularly molded glass ones, don't do this...

Odysis.

I agree with this point. What I was thinking is that the wings would need to have battery trays put in. But I guess we need to figure out if its a foam or built up wing. I think in a foam wing that ridding foam for batteries and have a ready made tray. You just need to contain the cells for flight with velcro, hatch or whatever and cut in a wire tray.

I usually work with wood though so that is where I was thinking. In an open or thin balsa sheeted structure you need to put in something to hold the batteries in place. Something that won't let the batteries punch through the balsa sheet on a hard landing or what have you. So there wouldn't be much added weight.

He didn't mention putting cells in the fuselage as well. When he said long endurance in the opening post that's what I was thinking. That they needed additional cells. But he didn't say that so my bad. If all the batteries go in the wings it seems like the easiest thing to do would be use a flying wing for drag reduction. That would give more perfomance boost than adding in a double cell count in a conventional plane. A sleek glider would also be a good choice. Of course he also didn't define long endurance for this discussion so I guess we're just brain storming theory.

Thanks for pointing that out Odysis

Dan

[dmgoedde]

Another thought I've had is to mold the cells into the wing during a vacuum bagging process, to seperate the cells individualy, have them in shallow cutouts in the foam right next to the fiberglass. Some thoughts:

1) LiPo cells are somewhat flexible, so could contour to a mold in the bagging process
2) one side of the cell is directly on the surface of wing for maximal cooling
3) lots of room to build the cells here... could conveivably do something like 3s6p
4) Can make fuselage narrow for less drag, diameter set by the motor width

One thing to watch out for is spreading the mass out from the C.G. Sure, you may have all the mass centered upon the C.G., but the moment of inertia is lengthened, and the radii of gyration is increased. I had a plane years ago with long fuselage and proper C.G., but everthing was so spread out it did not maneuaver very well.

I envision this for boutique applications, not a mainline R/C plane design strategy.

[kd7ost]

Good ideas and good points. I agree that spreading them down the wing will surely change the roll rate proportional to the weight and distance. Damping and rolls will be slowed down. Maybe sizing up the ailerons would held but I'm not keen on that either. Using bigger surfaces to change the direction of the increased mass puts additional strain on the servo's, linkage and the batteries that move them. That of course is moving towards less efficient. (This also may not be very evident unless you’re loading cells way down the wing.) Keeping the mass as close as possible to the root will increase roll rate. In and of itself, who cares if the roll rate is faster on a UAV though? The thing here is that you can use smaller control surfaces, less deflection and smaller servos consuming less power to do the job. That is moving to higher efficiency.

Dan

[BarrelRoll]

Thanks for your points.

The UAV is going to look like a glider: a short fattish fuselage with a tailboom and a high AR wing. Slow roll rates (due to increased moment of inertia) is actually a good thing for this particular aircraft. It is supposed to serve as a remote sensing and/or a video surveillance platform, so low and predictable angular velocities of the airframe is exactly what we need. Most of the time the aircraft would either circle an area in continuos turn or fly straight and level.

One of the big problem as I take it is to come up with a quick battery swap method :)

[indtech]

http://www.scaled.com/projects/globalflyer.html

[dmgoedde]

:) So then mold the cells in the wing, and have 2 or 3 wings on hand! Charge one wing while flying another! simple :D

One of the big problem as I take it is to come up with a quick battery swap method :)

[BarrelRoll]

So one of my friends went ahead and put two batteries in the wings of one of his old RC planes with thick wings just to see how it flies.

Subjectively, the aircraft is less jerky in roll and now responds like a full scale Cessna :)
I think, potentially, it can be exploited for some scale planes to make them look more "realistic" in flight.