First design - Need input on wing design & boom spacing for pusher

[Duck]

I wanted to try a few ideas so I am going to try to scratch build a design of my own. I had a couple of questions and I could use some input.

My general idea was a twin boom pusher with an F-pack motor. My goals were to:
1) Maximize endurance. Allow adding a slightly larger internal battery then most of the mighty mini designs easily fit. A 2200 would be ideal.
2) Minimize build steps and complex shapes.
3) Be adjustable for different load configurations.

Not really original in any sense but I like bank and yank pushers. Random internet example:
https://www.youtube.com/watch?v=yj2_Esw-FhQ

The nose is going to be a simple box sightly larger then the f-pack firewall with an open front for an all in one FPV camera. Now, the questions!

Question 1: A _lot_ of designs use swept back wings. Keeping with my keep it simple plan, are there disadvantages to a strait wing? I was planning on a single strip cut the length of a single sheet of foamboard. This is a bit shorter then a tiny trainer wing. Are there drag/efficiency concerns doing this? Are these usually just to look more like a real airplane which do have to worry about this at a larger scale?

Question 2: I was planning on running 3s with a 6x4.5 prop. How much clearance from the prop should I keep the booms? I don't need alot of elevator authority so I wanted to keep it the minimum width.

Question 3: I was thinking a flat boom of 2-layers of foam, basically folded over with no gap. My guess is that it won't deal with with horizontal stress well. I wasn't planning a rudder but the vertical stablizers on the corners will induce it. A lot of twin boom planes I've seen just use a peice of wood. Is this why?

Question 4: The longer endurance foam mini's I've seen usually use 2s. Is there an inherent efficiency advantage?

Question 5: Last one, I promise. I know under camber increases lift in exchange for drag. This is great for slow flight, but does it help flight times? Can I actually use fewer watts or just use the same or more to keep lift at a wider speed envelope?

 

[Hai-Lee]

Question 1: A _lot_ of designs use swept back wings. Keeping with my keep it simple plan, are there disadvantages to a strait wing? I was planning on a single strip cut the length of a single sheet of foamboard. This is a bit shorter then a tiny trainer wing. Are there drag/efficiency concerns doing this? Are these usually just to look more like a real airplane which do have to worry about this at a larger scale?

Question 2: I was planning on running 3s with a 6x4.5 prop. How much clearance from the prop should I keep the booms? I don't need alot of elevator authority so I wanted to keep it the minimum width.

Question 3: I was thinking a flat boom of 2-layers of foam, basically folded over with no gap. My guess is that it won't deal with with horizontal stress well. I wasn't planning a rudder but the vertical stablizers on the corners will induce it. A lot of twin boom planes I've seen just use a peice of wood. Is this why?

Question 4: The longer endurance foam mini's I've seen usually use 2s. Is there an inherent efficiency advantage?

Question 5: Last one, I promise. I know under camber increases lift in exchange for drag. This is great for slow flight, but does it help flight times? Can I actually use fewer watts or just use the same or more to keep lift at a wider speed envelope?

Question 1. The wing area is more important than the planform. Sweep back gives a slight dihedral type stability and lower drag, (higher efficiency), at high speed though for an endurance model the difference is negligible.

Question 2. In your case clearance of just over 1/2 inches would be OK. Less if your booms and wing support structure is rigid enough.

Question 3. Rigidity would be one reason for wooden booms as would be ease of construction. As for your FB booms consider making the booms a "+" shape to improve their rigidity especially if you minwax after assembly.

Question 4. Chosen properly there is not real efficiency dividend in using 2S over 3S. Having said that lower pack voltage gives greater current capacity for lower weight. If you select your motor correctly and use the throttle as a valid flight control then there should be no real difference.

Question 5. Yes undercamber profile will be higher drag than other wing designs and this drag can greatly reduce overall efficiency especially at high angles of attack. For wider speed range and longer flight times look at the wing profiles used in gliders and adjust your design accordingly.

Long flight times are best obtained by keeping the weight, wing loading and the drag all as low as is possible and at the same time using the throttle gingerly to keep the craft in a nice slow cruise.

Have fun!