Giant p-38

Squishyp38 · Post by **Squishyp38** » Sat Apr 09, 2011 11:52 am

So, I was looking at the progress of my p-38, and I said " you know what, that looks small."
Apparently, Big isn't big enough for me. So, Here is what I propose:
A double size p-38 (that would be 80 inches!) using the guillows plan, And all Rubber power.
Just a thought.

Thoughts?
Comments?
Questions?
Death rights?

John G Jedinak · Post by **John G Jedinak** » Sat Apr 09, 2011 1:38 pm

Look for Bill Parker to be your guide. He is the Master of the Biggies!!!

Phugoid · Post by **Phugoid** » Sat Apr 09, 2011 1:44 pm

two words.... Reynolds Number

supercruiser · Post by **supercruiser** » Sat Apr 09, 2011 2:17 pm

Phugoid,
I know what Reynolds Number means,... hey, I even know what phugoid means.
Where are you going with this?

Squishyp38 · Post by **Squishyp38** » Sat Apr 09, 2011 2:28 pm

Yes..... Do elaborate....... Please..........

BillParker · Post by **BillParker** » Sun Apr 10, 2011 10:43 am

A diagrammatic representation of a fixed-wing airplane in phugoid

I tried real hard to figure out what Reynolds Number means and came to the conclusion that mayhaps he means that built big, this project would lack stiffness, and the tail might catch up with the nose.... (prolly wrong, but that's my story and I'm sticking to it...)

IMHO, the Guillow's P-38 is the wrong one to blow up, as it has too many plastic parts. The Comet 38 however is ripe for making big... I blew one to 90" out of 1/4 inch pine ply. The kid's right tho, you could double it, and buid with balsa, and go rubber all day.

I wish you hadn't brought it up tho, young man... Now you got me thinkin'...

bp

BillParker · Post by **BillParker** » Sun Apr 10, 2011 11:11 am

Found this:

Title: Re: p 38 on ebay
Post by John Webster on Aug 9th, 2008, 3:46am Disclamer: I have not built a rubber powered P-38...yet.

There are two common P-38 kits, Guillows and Comet.

The Guillows kit builds into a -G (1944 or later, deep vertical chins under the props) and is currently available, has die cut parts and vaccu formed cowlings, canopy and boom radiators. It is intended as a display model and is quite heavy. Significant structural changes would be required to get a rubber powered flying model from this kit.

The Comet kit builds into an E or F (1942-43, shallow slanted chins under the props) and is available on e-bay or as a free plan and printwood set from many of the plan pages we all love. The kit has no vaccu forms and includes print wood and templates for the boom radiators and canopy. It will build up into a flyable rubber powered model with some care. Move the rear rubber pegs to a point slightly forward of the boom radiators, leave the landing gear off, splice in 1/16 stringers aft of the motor pegs and hollow out the blocks at the rear of the booms. Make sure that the horizontal tail is pointed about 3º down in relation to the bottom of the wing and replace the thrust buttons with noseblocks with propshaft bearings pointed about 3º down and balance at about 30% of the root chord to start trimming.

There is also an early Guillows kit which builds into an XP-38 (1938-40, no chin under the props). This is much like the Comet kit and should be flyable given the same care.

Both Guillows kits should have aluminum over all while the Comet kit should be finished in Olive Drab over Neutral Grey.

Phugoid · Post by **Phugoid** » Sun Apr 10, 2011 1:51 pm

ok, so Reynolds Number. This makes things scalable in aerodynamics, and so in basic terms relates the air speed and wingspan (or chord). So what I'm trying to say is that a design that is meant for a relatively small span may not work at a bigger one since the aerodynamic properties and set up won't scale up in proportion. However I like Bills explaination better and I'm happy to go with him on that.

Of an equal worry, is the power/drag factor in terms of scalability, bearing in mind you were mentioning rubber power for your "super model".

Phugoid: A simple exchange of Kinetic and potential energy. This motion has a long period and is the motion that gave rise to the bags in the back of seat backs in airliners..... This is one of the transient motions, like to SPPO (the short period pitching oscillation) and the other strangely named "dutch roll"

You are unlikely to see SPPO on a rubber model but you will likely see dutch roll

Squishyp38 · Post by **Squishyp38** » Sun Apr 10, 2011 3:18 pm

So, phugoid is basically what happens when you go weightless...

What is dutch roll?

And what do you mean when you say

and the tail might catch up with the nose

?

Squishyp38 · Post by **Squishyp38** » Sun Apr 10, 2011 3:23 pm

And you might be wrong, though about being scalable. Bill has shown that a size increase of even bigger than I am attempting is easy and gives no problems....... for U-control, at least.....no one knows about rubber........

THIS COULD GO EITHER WAY.

Even if it doesn't fly, It would make a nice display model.

Phugoid · Post by **Phugoid** » Sun Apr 10, 2011 4:22 pm

I'm not arguing for or against, sorry for appearing to be mysterious with my "two words" but things like the Reynolds number, the power available and weight all have an effect on what is achievable at any particlar scale. You did say rubber power and not U control (in which case we are talking some form of engine or motor) Now ther must come a point where rubber power just aint practical anymore, it's a power density sort of thing. Bill's planes fly because he puts a whacking big power plant in 'em, and i'm not sure you can match that power density with rubber.....

Dutch Roll, is a motion that (apparently) looks like a drunken Dutch sailor, it's sort of a combination of roll and yaw, and is even more puke inducing than the Phugoid motion!

supercruiser · Post by **supercruiser** » Sun Apr 10, 2011 5:29 pm

In a nutshell, thats all called: "scale effect".
I noticed Squish said rubber power, afterwards.

I don't think rubber will be enough, either. Twice the size needs something
like 8 times the power. It could work, but this is way out of my league.

BillParker · Post by **BillParker** » Mon Apr 11, 2011 10:52 am

I just finished another vendor's plane, a P-40, rubber powered, that's 50+ inches wingspan, and it flies like a champ.

with counter rotating props, I can just about guarantee you could be successful with the comet design, especially if you lightened up all the formers.

Take a look at the plans:

http://www.parkerinfo.com/plans/cometp38.pdf

you could easily double it and build with balsa. Any bigger than that tho, and you won't have enuff structure, without going over to heavier wood like I did with the 90+ inch plane...

Phugoid · Post by **Phugoid** » Mon Apr 11, 2011 11:28 am

what did you cover you 50" span plane with Bill?

supercruiser · Post by **supercruiser** » Mon Apr 11, 2011 12:43 pm

The kid has taken the ball and run with it .....

Any idea what it would take to power this thing?
Maybe 8 strands of 1/4" rubber... YOW!

Wear your safety goggles when you stretch wind that honking motor.