WOW MAN! Like, how do i get-like, MAXIMUM WARPAGE from my plane man!??

Ok, but seriously, I'm in something of a pickle.

I'm an ARO (areospace engineering) majoring freshman at Cal Poly Pomona and on a whim decided to join a Design-Build-Fly club at my school where we :ahem: design, then build, and then fly a plane in a competition hosted by the AAIA. Since it turned out that I had the most expirence with electric motors (from my expirence with ground-hugging vehicles) the whole matter of propulsion for our plane has dropped into my lap. This includes what electric motor to use, which batteries and yes, prop choice.

This year's competition requires a huge, heavy plane to be powered by a max of 3lbs of batteries and by a motor that is limited by a 40amp fuse. :O

The most difficult mission we're going to fly requires us to load two 4.4lb waterbottles, fly around a short 2000ft course, land, load another waterbottle, fly a lap. land, load another bottle and so on to 5 water bottles for max points. THats 22 lbs of water. We figure our design (a high-wing dual boom looking thing, looks exactly like the plane from Flight of the Pheonix) is going to weight about 8 lbs. Add water to that and you get 30lbs max. THe design currently has a 9.5 ft wingspan.

We can't use LiPoly's either.

I'm seriously not sure what motor, batteries or prop i shoudl go with. Since were not caring at all about speed, I guess a huge propeller would be good, The fueselage's diameter is going to be about a foot n a half. With landing gear figured in, we should have clearance for onemonster of a prop.

With something that large, using an electric motor would probably require gear reduction? Maybe? Is it possible to create about 7lbs of thrust (what the seniors say we'll need) with a 40 amp limit and around a 26 volt limit? (I get 26 volts from being limited to 3lbs of batteries, which means a max of 24-26 Sc cells).

I'm sorry I don't have an actual striaght foreward question, I'm just digging for any information. Any ideas and or suggestions would be greatly appreciated.

Thanks alot!

Some thoughts.

1. Over 40 amps equal simulated catastrophic failure.
2. Last flight payload is 13 pounds greater than first flight payload.
3. There are four distinct flight load design problems.

Clearly you have a current/battery management problem. You will need maybe 40% more power for the last flight than the first.

1. Consider a current limiter to be mandatory.
2. Consider limiting the power at each flight load, which would force the pilot to fly on the wing to conserve batteries. A simple method would be to switch additional batteries into service as additional water bottles are added.
3. Consider a prop change at each increase in load. A compromise prop for 15 pound and 30 pound loads is not likely to be found.

Sounds like an interesting project.

Bill

I'm not trying to be argumentative, but increasing pitch won't necessarily give maximum (or even more) speed, nor will increasing diameter always give more climb ability. Those generalizations are just that -- generalizations -- which are generally true (that's why they are called generalizations ), but often they can be far from the truth.

The pitch/diameter generalization is a reasonable place for SuperAirborne to start, but he is looking for a simple answer to what is arguably the single most complex technical element in model flying. Many (maybe most) modellers have nothing more than a passing understanding of propeller function & its wildly interactive relationships with engine & airframe combinations. That, presumably, is why Jaka is seeking more specific information.

The quickest way to deepen understanding of this topic is through the examination & explanation of specific examples, plus personal experimentation that is based on the knowledge gained from the examples & explanations.

SuperAirborne,
Increasing speed and vertical performance are almost totally opposite things. If you increase your speed you lose vertical performance, and vice versa. There are always trade offs when selecting props. What you need to do is decided which is the most important to you and then select a prop accordingly. As was said before, higher pitch props give you more speed and lower pitch props give you more pulling power for vertical performance.

Ken

Thanks! I know that there are trade offs! To be honest, my question was answered in the very first replies to this post, viz pitch = speed and diameter = verticle, and you basically have to trade one off for the other.

Thanks for all the responses, though!