BOTTLE ROCKET

At school we were persuaded to follow through the proof, that for maximum range of a projectile, the launch angle should be 45 degrees, (as has already been reported, so practice seems to confirm the maths).

As soon as you add wings, the lift / drag ratio intrudes into the maths and aerodynamics

The propulsive effort on is based on the pressure difference between the contents of the bottle and the atmosphere outside, so maximum range, I would suggest maintaining that force at as high a level as possible greater than the drag forces.

This seems to have happened already judged by the structural failures!

The optimum would seem to be just strong enough to withstand the drag and lift forces, so the aim should be to minimise drag from any source, whether shape, surface finish, aerofoil section or angle of incidence.

Less parasitic drag ought to allow a reduced angle of incidence, and so less drag from theat source. Could be a virtuous circle, but don't believe in perpetual motion or anything like that1

I used to do this with students to help get the idea of f=ma across...never tried wings though. We took some video using a camera with a slow motion facility and the observation was that all the water has been expelled by the time that the bottle has done 5 metres or so. We pumped them up to 100psi and never had a bottle failure. We got plenty of cold showers! I presume there is an optimum ratio of air and water in the bottle. More water and less air would give thrust for longer, but the pressure will fall off as the air expands.

FAI free flight power models used to use high wing, on a sort of pylon, to help with the trim changes associated with the enormous change in speed and attitude between climb and glide. It's not just the CoG shifting, under power the plane is going much faster, so there will be more lift, and since the climb is nearly vertical, the lift is not in a direction you want. (A typical FAI power model used to be able to climb vertically to about 100 metres with a 6 second engine run, then glide for nearly three minutes from that height.) You don't necessarily need to fold the wings away provided you can control the trim.

On the other hand, how complicated are you allowed to get? Instead of using water for reaction mass, just have the bottle full of air and use it to run a small motor like the CO2 motors that used to be used for model planes. That should give you a pretty long flight time, well into minutes of duration. Should be able to cover a bit of ground in that time.

John

Kitchen cling wrap, they call it Gladwrap here, shrinks on heating. Ian S C

Fantastic ideas and suggestions. I've managed to design and laser cut thin plywood wings. Which should be stronger and better shaped without compromising too much on weight. However before I secure the wings to the main frame, I was wondering if there's a formula that calculates the optimal position of the wings(fore and aft) with regards to the balance between the centre of mass and centre of lift. That the plane neither stalls or nose dives. I imagine that it's application in this instance may be difficult to configure since acceleration is a consideration.

Really, many many thanks,

Jacob

I had disregarded the mass of the water since I want the rocket to glide to the tune of the weighted nose using the momentum of the takeoff. And also the water will be expelled in literally less than a second.

Still a consideration?

Jacob

Jacob,

As a rule of thumb for gliding the C of G should be about 1/3 of the wing chord (front to back distance) from the leading edge. For stability you will need about 1/4 the wing area as a tailplane a good 4 chords back from the C of G. Of course all broad generalisations are wrong so expect to experiment.

I'd go with a folding Rogallo wing that releases after a delay, myself. No stabiliser needed, suspend the bottle from it so c of g variations caused b y retained water won't matter.

Neil