Difference between pusher and "pull" configuration?

Thread view:

Enable EMail Alerts

Start New Thread

Thread rating:

Jennifer Smith - 25 Jul 2006 19:29 GMT

Being sooooo partial to my beloved little jets (and just having smashed
one of them to pieces and not enough savings for a replacement lipo) I
thought about building another one and putting an old 15-year-old glow
engine on it - but in front. I didn't find much on that topic on the
'net. Do I have to mess with the CoG? What gotchas are there for
converting from pusher jet to front engine configuration? I'll build the
frame from scratch, so even major design changes would not be a problem.

Also, since I have no documentation on that engine whatsoever - what is
a good way to figure out a usable prop size/pitch? Trial and error?

Thanks again
Jenni

Reply to this Message

Mark Miller - 26 Jul 2006 02:35 GMT

> Being sooooo partial to my beloved little jets (and just having
> smashed one of them to pieces and not enough savings for a replacement
> lipo) I thought about building another one and putting an old
> 15-year-old glow engine on it - but in front. I didn't find much on
> that topic on the 'net. Do I have to mess with the CoG?

To first order, the CG shouldn't change when you go from pusher to
tractor. This is usually good, because any vaguely-conventional
airplane layout with a glow pusher engine will probably need a fair
amount of balance weight; a tractor (hopefully!) won't.

> What gotchas are there for
> converting from pusher jet to front engine configuration?

To my mind, most of the gotchas would be going the other way. It's
awfully easy to have a pusher prop hit the ground; the venturi is in a
bad place and facing the wrong way; the previously mentioned balance
problem; big CG travel as fuel is burned (if the pusher tank is much
farther from the CG than the tractor's tank would be); greater
structural challenges (the engine's the heaviest thing on the plane, and
with a tractor layout, the engine is reasonably near the landing gear
and wing spar), etc.

If it's not a conventional layout, then the story may be different. For
example, if it's an F-18, the nose is so long and the wing so far back
that the pusher might actually be easier to balance.

Of course, the pusher won't tend to get oil all over your shiny
airplane. And pushers look cooler.

> I'll build the
> frame from scratch, so even major design changes would not be a
> problem.

This is definitely a major design change! What airplane is it?

You'll have to figure out how you're going to support the engine in its
new position, where to put the tank and how to get to it. May have to
move most of the radio gear, since the original designer probably had it
stuffed way forward (where you want to put the engine now).

> Also, since I have no documentation on that engine whatsoever - what
> is a good way to figure out a usable prop size/pitch? Trial and error?

In the end, it often comes down to trial & error within a fairly narrow
range. There are good general guidelines. What engine is it?

> Thanks again
> Jenni

Signature

"Whatever will have been, will have been."

- Douglas Adams, "Life, The Universe, and Everything"

Reply to this Message

Jennifer Smith - 26 Jul 2006 19:12 GMT

> If it's not a conventional layout, then the story may be different. For
> example, if it's an F-18, the nose is so long and the wing so far back
> that the pusher might actually be easier to balance.
>
> Of course, the pusher won't tend to get oil all over your shiny
> airplane. And pushers look cooler.

I like mine lots. Up until last week I had three of them. Now only two :/

The main reason for me to try front engine mounting is, as you said, the
problem with ground clearance on takeoff and landing. All my pushers are
electric, which means I can safely stop the engine while landing at least,
and fire it back up if I need to abort. With a glow engine I don't see
killing the engine on final approach as a good idea.

>> I'll build the
>> frame from scratch, so even major design changes would not be a
>> problem.
>
> This is definitely a major design change! What airplane is it?

It's based on a ParkZone F27B Stryker. I've liked them so much that I took
one Strykers airframe measurements, blew it up to 2x its size and built one
from depron, with CF rods for stabilizing and with detachable wings. My
design differs a little from the original: Retracts, (originally) twin
speed 600 engines in rear behind the fins, and I "flattened" the wings
profile slightly. I've also built one with floats instead of retracts. The
original one, with retracts, bit the dust last week because of completely
utterly stupid pilot error. They fly beautifully.

> You'll have to figure out how you're going to support the engine in its
> new position, where to put the tank and how to get to it. May have to
> move most of the radio gear, since the original designer probably had it
> stuffed way forward (where you want to put the engine now).

I figure I'll put the engine in the nose so that the spinner actually looks
like it's part of the nose. I'd _like_ to put the tank right over the CG,
but that's just too far back to be feasible without a pump - which I have
never used, so no experience there. There's plenty of room in the body, so
placement of the components isn't a problem.

The bigger issue will be what to attach the engine to. Depron would
obviously not support it :) I'm thinking a traditional wood frame for the
main body, although I get sick just thinking about cutting that shape.
Alternatively, build only the engine support in wood and then anchor it
with long CF rods to the rest of the airframe.

>> Also, since I have no documentation on that engine whatsoever - what
>> is a good way to figure out a usable prop size/pitch? Trial and error?
>
> In the end, it often comes down to trial & error within a fairly narrow
> range. There are good general guidelines. What engine is it?

I have no idea whatsoever. My guess would be some Graupner engine, I bought
it circa 1992/1993 over in Germany. It used to power a large approximately
2m (~6 1/2 foot) high-wing trainer. I've got _no_ documentation for it
whatsoever, just the engine itself with all the bits and pieces and
spinner - but no prop yet either. The engine has no markings that I can
discern. It's big though, and I do remember that the engine was waaaaay
over the top for that poor trainer.

Jenni

Reply to this Message

Model Forum / Radio Controlled / Air Models / July 2006

Difference between pusher and "pull" configuration?