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Indoor Slow Flyer Scale Up: Possible?

NineZeta

Junior Member
#1
Hi everyone, this is my first post and I was wondering if anyone can help me decide if this idea is feasible:


I remembered those rubber band powered indoor balsa planes.

So I was thinking if I could build a bigger one out of more robust materials like carbon fiber rods, monokote(?), etc...
I might use a 1100kv (or less) motor powered by a 220 mah battery. Also I'll be using a 30A ESC. Basically I might just copy the design in the photo (but i probably wont make it a biplane) and just make it bigger (probably something with a 5ft wingspan). If I want it to be a super slow flyer, I guess I need to make my own propeller. So if I were to scale it up, the size would be 3+ feet or something with an average rpm of 60. So my question is, would the motor burn given that i'll be using a huge homemade prop even if i maintain a "visibly slow"rpm? My only quide is this: if the propeller is spinning and I can't see the blades, it's probably going too fast. Also does anyone think this build will work at all? Any other suggestions would be appreciated as well. Thanks!
 

earthsciteach

Moderator
Moderator
#2
I'd say it is perfectly feasible to build an airframe and prop as you describe. I don't think a brushless motor is what you want to power it with, though. I don't know much about geared, brushed motors (don't know much about brushless, for that matter) but that would allow you to gear down the prop to the speed you are looking for. Is this monster going to be an indoor or outdoor flyer? If outdoors, you probably won't be able to handle much in the way of wind.

There is a whole niche of tiny, indoor rc planes that this would fit within. Here's a video.
 

earthsciteach

Moderator
Moderator
#4
I suppose you could use throttle control. But, I just think there has to be a better way to do it. Have you done much digging around on the interwebs for info? You may find something useful.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#11
Take care bonding monokote to the CF -- CF rods don't like too much heat, especially if they're under tension.

From the Vapor mod thread on RC groups, quite a few people are using contact adheasive (weldwood or 3m 77, sprayed in a cup) on the frame, then applying mylar, shopping bags, or produce bags for the coating. Mylar looks the best, but varies from light to heavy, produce bags lightest, but not as nice, monokote looks nice and comes in average for weight. Personally, I've had good success with the 77 + mylar baloon material and 77+clear mylar wrapping paper from $tree.

You might check out Horizon Hobby and pick up the Vapor or Mini vapor fuselage w/ electronics. For ~$60 you'll get a DSM2/X micro brick w/ 2 servos, a motor mount w/ motor and gear box all mounted on a CF rod (can be remounted as you see fit).

For the prop I'd recommend checking out the 1mm propsaver kit from RCfunlab. It comes with three prop sizes to try out, and is MUCH better than the prop-shaft-that-breaks-inside-the-prop setup HH sells.

If you go with HH's micro-lite gear, it won't be agonizingly slow, but also won't be lighting fast. There are lighter/slower setups around, but few as convenient or cheap.
 

Craftydan

Hostage Taker of Quads
Moderator
Mentor
#13
Ooooh. just re-read the OP. Still possible, but 60 rpm?!? I'm afraid he's going to be reminded that air doesn't scale . . .
 
#14
I really do think the proposed electric set up is all wrong.

A 220mAh battery even at 20C is still only going to produce 4.4A so a 30A ESC is just adding dead weight.

ESC circuitry is not designed to run at very low speeds, most simply stop working below a minimum speed which is usually rather higher than 60 rpm.

Running a prop at 60 rpm requires a specific high torque low speed drive. A direct drive brushless motor does not have these characteristics!

A large extremely light plane is perfectly possible but because of its weight it will have limited power so you have to make the most efficient use of what is available.
As with any flying machine you need to extract the best power to weight performance from each component.
 
#15
You would certainly need a custom high pitch prop and massive gear reduction to pull it off. Not saying it can't be done, just there is gonna be a lot of custom fabrication here.

One bonus though, with massive gear reduction that means you can go with a much much smaller motor to keep the weight down.
 

NineZeta

Junior Member
#16
Thanks everyone for the replies...
Okay so i searched the whole city and I couldn't find any gearboxes so I have no choice but to use direct drive. So I made one with a 1100kv motor that I pulled out from a foam cessna with a 1400mm wingspan and i used a 10 x 4.7 prop with it. The plane I made is pretty much all foam. It had a 10 inch chord, 4-40 style curved plate, had tip polyhedral and its wingspan is 40 inches so it looks like a fat "gumby wing". fuselage style is a basic tube design and that's it. It flies. The problem is, it's too fast for the park I fly in. In fact they banned me there because I lost control of the plane and it almost hit someone. I just really want a gentle flying plane. Obviously it's the weight so does it mean that making a larger version of this plane will effectively make it a lighter plane in relation to motor size? can anyone suggest basic design principles/ or measurements such as fuselage length, fin height etc if i were to create a slow flyer powered by a direct drive 1100kv motor with a 10 x 4.7 prop powered by a 1300 mah lipo? From there maybe I can buy smaller electronics and adjust size accordingly. THank you!
 

NineZeta

Junior Member
#17
oh and may I add a question, somewhat off topic but i can't seem to find a detailed answer. For a conventional monoplane with a tractor stlye prop (no deltas, cannards, etc) is the CG ALWAYS located more or less at 1/3 of the chord from the wings leading edge? I find that I'm basing all my designs on this guide. Thanks again
 

xuzme720

Dedicated foam bender
Mentor
#18
That is usually a good starting point. Feel free to experiment moving back or forwards in little steps to find the sweet spot.