Rubber band propelled plane

[Drunken Deriver]

Hello PF!

I have a project/competition in freshman college Aerospace that calls for a plane that can stay in the air for as long a time as possible while going as far as possible. The requirements don't explicitly say we need rubber bands as a propulsion but pretty much prohibit everything but. The plane will be fairly large and it must be launched into the air via throwing it. My teammates and I are wondering if a rubber band propulsion system would be more helpful or detrimental to the flight. I argue the later, saying that after the coiled rubber bands expend their stored energy the propeller will become nothing but air resistance and cause the plane to slow down faster. Essentially, the air resistance will produce more negative ΔV than the rubber bands with the propeller will produce positive ΔV. Not to mention that having no propulsion system will make the craft lighter. Therefore, such a plane will fly farther and stay in the air longer without a propulsion system. My team mate argues that the propeller will produce more ΔV than the air resistance will produce negatively. So who do you think is the correct one? The design of our plane will be akin to a sailplane (like the DG-800).

Alternatively, is there any other propulsion system we might use? I thought of magnetism but I am not sure how to implement that.

The requirements for the project are...
- no electricity whatsoever
- no metal ouside of an inch from the main propulsion unit.
- 2 Kg mass limit
- limit of 2 propellers
- must carry a payload of unknown weight
- cannot use any explosive or potentially hazardous systems or mass-ejection systems
- cannot use anything to artificially lighten the plane like a balloon of helium
- $50 worth of materials limit

Sorry if I posted in the wrong spot. The idea was that the main question was about just the plane and was not specific to the class.

 

[berkeman]

Welcome to the PF.

I moved your post to the schoolwork forums, since this is a school project.

I'd be inclined to use the best of both ideas -- use rubber bands and propellers for the initial part of the flight, and get rid of them somehow after their energy has been expended. It would also help a lot to have some sort of flight attitude control, but you will have to figure out how to do that without powered actuators on flight control surfaces... Fun project!

 

[Bystander]

Rubber band power plus free wheeling of prop for second stage of flight (veerrryyyy old trick --- examine the set-up some time).

 

[rcgldr]

I'd be most concerned about the payload of unknown weight, what is the range of that weight, and how will it attach it to the aircraft (it would need to be attached so that it doesn't move the center of mass forwards or backwards)?

 

[A.T.]

I'd be most concerned about the payload of unknown weight, what is the range of that weight, and how will it attach it to the aircraft (it would need to be attached so that it doesn't move the center of mass forwards or backwards)?

Also the "stay in the air for as long a time as possible while going as far as possible" part. How are those two parameters weighted? Or is it two separate categories?

 

[A.T.]

Rubber band power plus free wheeling of prop for second stage of flight (veerrryyyy old trick --- examine the set-up some time).

A free wheeling prop could generate more drag, than a locked prop. Ideally you would have prop blades that flip back, once they create resistance instead of thrust, as some motorized glider models use them.

 

[Drunken Deriver]

Thank you all for replying and helping us out with this :)

@berkeman Thanks! Unfortunately, my team believes dropping it off may a be a bit out of the picture.

@Bystander Like A.T. said, I think free wheeling would produce more drag.

@rcgldr We have not been given the mass or material of the weight, unfortunately. However, the payload shouldn't be an issue, we can secure it inside the fuselage in a compartment directly around where the CoM should be. The maximum it could be is 500 grams.

@A.T. They are 2 separate categories for the competition, but we only get one flight so we can't specialize. We aren't sure how they will weigh in on our final grade. The propellors are given to us; we cannot modify them. But I wonder if we could have a separate lock that will activate as soon as the rubber bands unwind.

 

[berkeman]

Unfortunately, my team believes dropping it off may a be a bit out of the picture.

That's unfortunate, especially given the constraints (you have to use their propeller). I can think of a pretty simple way that the prop and rubber band assembly could drop off of the plane when the rubber band goes limp...

 

[Drunken Deriver]

So we could have the benefits of the propeller without needing to worry about air resistance... as long as said assembly doesn't require electricity it could be worth looking into :)

 

[rcgldr]

The design of our plane will be akin to a sailplane (like the DG-800).

- $50 worth of materials limit

Those two items seem like a conflict. Just the materials, balsa wood, covering, glue, to build a sailplane that size is going to cost more than $50. Then there's the expense of tools and expendables like glue.

 

[Drunken Deriver]

Those two items seem like a conflict. Just the materials, balsa wood, covering, glue, to build a sailplane that size is going to cost more than $50. Then there's the expense of tools and expendables like glue.

We aren't building a full sized DG-800 lol. I just used it as an example to give y'all a better idea of what I was talking about. The plane we build will be a helluva lot smaller.

 

[rcgldr]

We aren't building a full sized DG-800 lol. I just used it as an example to give y'all a better idea of what I was talking about. The plane we build will be a helluva lot smaller.

I meant a model DG-800. A model glider around 2 kg, would have a wing span around 2.5 meters or more, that's a lot of balsa wood and covering.

 

[Drunken Deriver]

I meant a model DG-800. A model glider around 2 kg, would have a wing span around 2.5 meters or more, that's a lot of balsa wood and covering.

It's not going to be the DG-800; I only wanted to make sure people knew what I meant when I said sail plane. I was a bit unclear on that. My bad.

 

[CWatters]

Is this a trick problem?

- must carry a payload of unknown weight

Without more info the design of the model is virtually moot. You need very different designs for a 1g payload compared to a 1kg payload.

Perhaps take a look on youtube for examples of rubber powered aircraft. Here is one fitted with radio control...