Small prop, big cowl --> bigger prop, bigger spinner? - RC Groups
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Oct 15, 2003, 11:45 AM
I even seen a house fly!
LedKitty's Avatar

Small prop, big cowl --> bigger prop, bigger spinner?

Hey gang!

I've got another question! *drumroll*

Ok, say you're going to be building a plane from scratch, and you've decided that you're going to be using a direct drive 400 motor with a 6" prop. Now let's also say in the interest of scale that the aircraft you're building is of the radial engined type with appropriately shaped cowl and that at the scale you're planning on building the bird, the cowl winds up to be, say... about 3" in diameter.

Wouldn't that make for a lot of engine cooling and really not all that much useable thrust left to propel the thing?

Now if that is the case, would it not be possible to simply use a larger diameter prop with an oversized spinner to decrease the useable prop area in order to reduce the load on the motor? Or is it a question of a larger diameter giving higher tip speeds that puts the load on the motor?

Despite my aircraft maintenance training years ago, it seems I have forgotten more than I know unfortunately, so if anyone's got answers to this one, I'd love to hear them!

This is the aircraft I'm planning to build:

I started a thread for it in e-zone/power systems called "electric twins and pusher-pullers". Vintage1 and I were yabbering on about it all day yesterday!

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Oct 15, 2003, 02:50 PM
Registered User
Something like 80% of a propeller's "thrust" comes from the outer 20% of the propeller. Those aren't the exact proportions, but you get the idea, I hope...

Propellers also don't work by "blowing air backwards" as is commonly thought. Like any wing, they work by creating a pressure differential; the airplane is literally SUCKED forward by the low pressure area the propeller creates ahead of the plane.

The extreme example I've seen of your perceived problem is a Gee Bee, the R1 or R2 I believe. Most models of this fat, stubby-winged bird have propellers that barely stick out beyond the edge of the cowl, yet they fly fine. That's because the propeller is still doing its job, creating a low pressure area ahead, and a high-pressure area behind. All that really matters is the area immediately ahead of, and immediately behind the propeller. Is it more efficient to have the propeller clear of the cowl. Probably, but it still works this way.

As far as putting a larger propeller on the motor and using a large spinner to "cover up" some of the effective area, it won't work... The prop disc area increases exponentially as the prop diameter increases. If you went to an 8" from a 6" prop, you'd need a 5-1/4" spinner to cover up the extra disc area. Plus, the outer section of prop is moving at a higher linear speed, and is therefore moving more air, creating more load on the motor.

If you want to run a bigger prop, a gearbox makes much more sense.
Oct 15, 2003, 03:10 PM
I even seen a house fly!
LedKitty's Avatar

That's what I thought! Thanks a million for knocking that particularly huge hunk of rust out of my head! I had this nagging feeling that there was something that wasn't clicking quite right! it's all coming back to me now... heh.. punching away at computers all day (that's my day job!) really doesn't help keep my vague memories of physics and aerodynamics straight in my mind!

So Venture1's idea of sticking two direct drive 400s on there with 6X4 and 6X3 Gunther props (front and rear respectively) still works... Cool!

Also, due to the anhedral of the inner wing sections and the mechanical overhead of trying to articulate the twin rudders, I've instead opted to make the twin fins fixed, incorporating the main gear into them, and have a symmetrical airfoil around the nosegear to give me a steerable nosewheel and inverted rudder in one unit! My buddy Eric came up with that over lunch when I told him I was thinking of flying rudderless! I'm quite comfortable with ailerons and elevators alone, but taxying and those crosswind landings can be tricky without!

This is going to be fun...

Thanks again!

Last edited by LedKitty; Oct 15, 2003 at 03:19 PM.
Oct 15, 2003, 03:45 PM
Registered User
vintage1's Avatar
Hey. It depends on how big it is, Going geared will get you a faster climb, at the expense of top speed. I am working on a sp 400 7.2v design for 3 lipoly that will do vertical on 3s1p et-tech 1200: geared 4:1 with 10x7 prop looks a likely starting point.

However, it won't ever fly at more than 40mph.

I just thought that plane looked like it ought to be screamingly fast. And noisy!

perhaps a couple of Cox 020s would do the trick They run both ways ...
Oct 15, 2003, 04:13 PM
Get a brian, moron!
Zephyr's Avatar

Scale to what?

I've got a sneaking suspicion that your plane is from that computer game, Crimson Skies... Nice Choice!
come to think of it, there were some other damn odd but good looking aircraft in that...
Oct 15, 2003, 04:43 PM
I even seen a house fly!
LedKitty's Avatar
As noted in my original thread about the aircraft, it IS in fact inspired by the Crimson Skies aircraft, and was designed to fit in what that look. I suppose that it does from the top look a little like a Devastator with the top wing ripped off, an extra engine crammed into the nose and the cockpit moved forward!

I am in fact in the process of modifying a Devastor miniature to use for the boardgame!

I'm a big big fan of warbirds... even those that never existed!

Don't think for a minute that I haven't comtemplated making an RC Devastator! *lol* It seems the most likely of the bunch to be able to fly! A Fury.. well... that would just be silly!

Last edited by LedKitty; Oct 17, 2003 at 12:00 PM.