Mini Pegasus - RC Groups
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Feb 12, 2009, 07:26 PM
"the analog man"

Mini Pegasus

I have a 3W-56B2 gas engine that I would like to put on a "mini Pegasus" tug. This would be used to pull up smaller 1/4 scale and lighter wooden gliders. I have a set of Pegasus plans. What would you recommend I do to reduce the scale of the Pegasus to allow the great stability features of the Pegasus to remain, and allow the little twin cylinder engine to provide the thrust. The engine puts out about 5.5hp, and swings a 22X10 prop at 8000rpm. I could just select a smaller scale (like 75%) and shrink it, but am concerned it would loose many of it's good tug flying traits. Any help will be appreciated,

Larry Fitch
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Feb 12, 2009, 09:52 PM
Retired CAD guy
birdofplay's Avatar
Hmmm ...

As long as the power to weight ratios are good ... should it matter.

Pegasus , as I recall, was just a Box fuse with a Hight wing and a Tow release.
Not much "magic" .

I mean, most of the Big Pegasus tugs had ludicrous power so, well ... there it is !

So Larry,
Ya gonna have it ready for the Spring Union Gap tow ?
To sorta take to load off of poor ol' Gene :-)))

Feb 13, 2009, 10:36 AM
"the analog man"

I've convinced myself that it isn't completely straight forward--maybe too much thinking? I doubt (with my speed in the shop) that it will be complete this Summer, let alone in two months. The big one that was stolen took me nearly three years. I guess if the wing and tail areas remain consistent, and the relative lenghts of the lever arms (nose to wing center, and wing to tail center) remain consistent it should fly. But John Derstine really got it right in his design--it almost flys itself---very stable for so much power.

Feb 13, 2009, 01:14 PM
Larry, making it smaller probably will not cause you issues. If you recall, there were two versions of the Peg - one with a bolt on wing and the other plug in wing. While not significantly different in size (wingspan) there was variation in sizing. The impact was not noticable IMO.

I have designed two pegs for CNC cutting - one for 70 to 100cc (100"), the other for 100+ cc (116"). I also have plans but no CNC masks for a smaller 85" version for smaller motors. I think you are over thinking it and as long as you use the same airfoil and keep the same ratios, you should be fine. Just remember to make it strong for the purposes of towing.
Feb 13, 2009, 09:25 PM
"the analog man"
Thanks for the response. I think I have about generated a set of plans for the "mini". Do you know what John used for the airfoil? My original plans show a symmetrical foil, but what one is not stated on the plans. The plans I have are for the Pegasus II with plug-on wings--what I am going to do with the smaller one.

Feb 13, 2009, 10:29 PM
Registered User
k2k's Avatar
Before the Pegasus there was the 8-Ball Special produced by Reid's Quality Models. The specs on that were 85" wing span, 1360 square inchs of area, length 63" and weight 17.5#. Suggested engines were G38 or Q42. It had many similarities to the Pegasus. The airfoil on both planes is semi-symmetrical and looks the same to me but I don't know the designation. Mine flew ok on a 46 and would do 4 meter planes easily but the power to weight ratio was less than 1:1. A G62 was considerably better. Hope that info is of some use.
Feb 14, 2009, 12:19 PM
Where's the lift?
dgliderguy's Avatar

The DA-100 puts out 9.8hp, where your 3W-56B2 puts out 5.5hp, so you are looking at a reduction in power of 56% of the original. If you reduce the weight of your scaled-down Pegasus by the same amount, you should end up with equal power loading (ref. birdofplay's comment above).

I would run a cubic loading calc on the DA-100 powered Pegasus, and use the same cubic loading value at a .56 weight reduction factor to calc your new scale ratio. I don't have the figures in front of me, but if we say that cubic loading is:

weight/ (wing area)^1.5, and 'A' = DA100 Peg wing area, and 'a' = 3W Peg wing area,


W/(A)^1/5 = .56W/ (a)^1.5,

.56(A)^1.5 = (a)^1.5,

and a/A = .56 inv ^1.5, or .6794,

then this would result in an area reduction of 68% of the original. You can figure your new wing span using this reduction in area, and holding the aspect ratio constant.

BTW, I agree with Tony, that the scale ratio is not critical, and close enough is good enough. More important is the AUW. Don't want to build a smaller Pegasus that is too heavy (!!), but you want to keep it robust. Easy on the heavy ply, use carbon where you need it.

Have fun!

Last edited by dgliderguy; Feb 14, 2009 at 12:26 PM.
Feb 14, 2009, 02:22 PM
"the analog man"
Bob, Tony, and Don,
Thanks for the input from all three of you gents---good stuff. While I don't know K2K, thanks to you as well. I think I can proceed to a useful conclusion


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