Petkov
Hi
I am sure most of you guys are really experienced and I came here to look for some help. I am currently in an University competition in my school. As a mechanical engineer, I know a lot about the construction of the plane and have been building quite a few over the years. My issue is with the propulsion, and this is where I figured you guys could help me out.

I need to figure out what kind of batteries, motor, esc and fuse to use.

Fuse:
I am limited to use a 15 amp fuse but this can be a slow blow type. In fact I am trying to get as much over that 15 limit to give me an edge over the competition. Do you guys have any recommendation for a 15 amp rated fuse that can get 17, 18, or even 19 amp averages over 5 minutes without blowing?

Batteries:
They only need to last 4-5 minutes. This includes take off, flight, and landing. But my main constraint is 15 amp current. Unfortunately, I am limited to NiHM and cannot use LIPO ones :/ Do you guys have any advice on how I can determine what batteries would be best for the application?

Motor:
I am expecting to buy something between 300 and 500 watts just based on my power constraint. I am allowed to spend up to $200 for the motor. Once I decide on the batteries and motor I will look at props. For my 2 missions in the competition I will need a prop to bring my plane up to full speed and then carry a heavy payload. For the speed mission I am guessing something around a 12x12 will do the trick and for the payload mission, I was going to try a few in the 13x10 range. I could use any advice you guys have for me on how to select the right motor/prop combination.

Speed controller:
In this category I really don’t know where to begin, I was just going to follow the recommendations of a hobby story sales person.


I am very lost on the whole electrical aspect of this project and I appreciate any help you guys can give me.
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dtribby
Two things (for starters) we need to know to get you a better answer:
1. Is the fuse a REQUIRED element in your project? Fuses are not something we typically use in the hobby. The power circuits we use are pre-planned so as to avoid overdrawing or overheating components. Even then, if one component fails (say, the motor), the remainder (ESC, servos, receiver) are needed to recover the aircraft, so a master fuse is NOT preferable. That's a quick way to lose a bird.
2. What kind of plane are you going to build? Style-wise, that is. A 15A power system is NOT easily going to get you into the 300W+ range. The quick math tells me that you need to operate on a minimum 20V (16-cell) system (300W = 20V * 15A) to make that happen, and that much weight will be a bugger to get airborne. And how much payload are you talking? The same airframe is not going to do both missions well. Even a 19A system on a 3S lipo with a 200W motor is just good enough to get a powered glider into the air with any decent speed. A wing may do better, but more info about what you had in mind, and the ACTUAL limitations/requirements of your project will help us throw you some options.
Yes, I've known that I'm "different" for some time now...
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Newt
It may be difficult to find much info regarding NiMh batteries since LiPos have become so prevalent.
If you don't get much input from this forum you may try contacting Ed at:
https://www.youtube.com/user/ExperimentalAirlines/about
he has a lot of experience.
I think I would start with determining the voltage that you will use/need since it will greatly impact you current (amperage). 12 volts at 100 watts will draw 8.3 amps (ohm's law)
So using the ohm's formula to achieve 300 watts at max 15 amps you would need at least 20 volts!
So you need to decide voltage and find a ESC and motor that will give you the power you need at that voltage. The more voltage you have the less amp draw for the target wattage that you stated.
Quite a balancing act.
Common motors that I use are ~8 volts and ~12 volts and pull a lot more than your limit of 15 amps. I am guessing that the purpose of the 15 amp fuse is to set a limit for competition.
I do not have experience with using 20 volts to power a motor.
A typical NiMh battery that I looked at is 1.2 volts (nominal) so 20 volts would take around 16 batteries wired in series. That would be a pretty heavy battery pack.
As I said, I really have no experience with NiMh batteries just experience in the electrical field.
Good luck!
Newt

certified FRCPowers Kool-Aid drinker


http://www.youtube.com/user/596newt
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Petkov
Thank you for taking interest!

The fuse is required but only as a restraints to make sure everyone has relatively similar power capabilities. I would like to follow the "sports plane" type of model (5:1 or 6:1 aspect ratio of the wings) that way I would have both stability and some speed. I will add a few degrees of dihedral to the wings and a bit of a forward taper. I will probably use a high wing as well.

The two missions we have to perform are: 1) see how many laps the pilot can make around a preset course in a 4 minute limit 2) once the plane is loaded with 6x6x6in 1lb blocks how fast it can compete 3 laps. For the first mission the score is based on the number of laps, for the second it is based on number of blocks and time. The total score is then divided by the weight of the empty plane, so lighter the plane the higher the score.

A have done some basic calculations assuming that the payload will weigh as much as the plane the most efficient design would be to have 3 blocks and 3 lb plane. That would make the total weight 6lbs for mission two.

The complete set of rules is on:
http://www.aiaadbf.org/2014_files/20...es_draft5.html if you guys are interested. There is a lot of crazy stuff they expect us to do.

Thanks!
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dtribby
Focus on the 2nd goal. Just GETTING a 3lb plane and 3lbs of cargo in the air on 15A is going to be a challenge. Mind you, half your weight is going to be battery, because you're going to need WAY more than 20V/16cells to get in the air. Your motor/prop choice is the first place to start. Even with an efficient airfoil, you're going to need at least 32oz of thrust to get in the air with 6lbs of plane. And even then, that's with a roll-out on weight-adding, drag inducing landing gear.

These guys did something similar, but they weren't limited in amperage. Your biggest problem will be in your power system. Get over that hurdle, and you could win. Keep the plane as light as possible, and you don't have to carry so much weight to get your quotient up.

Yes, I've known that I'm "different" for some time now...
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Petkov
Thanks for the video. This will actually help quite a bit. I had been looking at c-27J spartan and Short Skyvan planes but never could find the RC relevant versions. I am surprised that with relatively small wing area and thin airfoil they could get quite a bit weight off the ground. Once I get home later, I will be able to get audio to see if they explain how much power they have in that thing.

edit: I found an RC plane that I would like to model my prototype after. Its the J3 Cub, if you guys are familiar with the model. I has a pretty large wing with a single tractor. Hopefully I could try a few different wings to see if I can get away with a bit less drag after I figure out what my propulsion will be.
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