OK, so I've got this 30% Paulistinha P-56 ARF and I plan to mount my Seidel/UMS 770 radial. However, when I align the UMS mounting diagram with the lines on the model's firewall (intended for a 50cc gas engine), the top bolt on the UMS mount would be above the top of the firewall.

So there would seem to be two options: 1) somehow extending the firewall upwards or 2) lowering the mounting bracket by about 1 inch. There's room to do this, but I'm wondering what the effect would be on lowering the engine's centerline. Does this affect the aerodynamics?

lowering the engine will make no difference to how it flies,
if using nitro the carb should be in line with the centre of tank / bung
if using petrol it will make no difference as it will pump its fuel

the only difference it will make is to how it will come out of cowl, as it may make it harder ti line up.

Well the cowl won't be a problem since I'm going totally off-track with that and using a round cowl made from an aluminum pot! But I will have to modify the tank and throttle linkage somewhat.

One more solution, add another mount bolt hole such that it fits the firewall. No problemo��

I agree with BobH. You want the engine placement to be correct in the sense that if you had a normal cowl, the thrust washer would be properly centered. Even with a round cowl, if you drop the engine from where the centerline was intended to be, it will likely look funny.

I have an original Seidel 770 and the mount was damaged in a crash such that one of the legs was snapped in half. I just drilled the remaining part of the leg and bolted/JBWelded a new leg on made from some scrap steel. Its been fine ever since. Don't see any reason you could not do something similar in order to move the mounting lug down slightly. As long as you're not moving the lug down so far that you start to lose the 3-leg stability.

Have you thought about fitting engine inverted or 45 deg......as that may help

I don't see where any issues would arise from lowering the thrust line by an inch Abu. If that is the easiest solution then I would not hesitate to go that route. I have no doubt that you are fully capable of getting the engine and your new aluminum "cowl" all lined up properly!

As far as it "looking funny" goes, that's relative. I mean, what should a "cub" with a radial look like anyway? If the radial is centered on the firewall so that it extends equally above and below, then that is below the centering marks for a 50cc gas engine.

Regarding drilling additional mounting holes in the bracket, I'd be hesitant to weaken it, but I'll have a look. Rotating it would be problem unless I remove the exhaust ring.

One matter I do need to consider seriously however is prop clearance once the tail rotates on take-off.

The Pauilistina is, what? A top wing trainer? You're not talking aerobatic model. Lowering the thrust line 1" on a 50cc size model won't make much difference.

The answer is different for an aerobatic model where response purity is more important

The Paulistinha is (was?) a Brazilian-made version of the Taylorcraft and so, yeah, definitely not designed for aerobatics, though the smaller version of the model can definitely do all the basic aerobatic maneuvers.

If you look at the photo above you can see that the "vertical center" of the firewall would be below the hole. The marking lines for the gas engine cross just above the hole. Visually, it seems like having a radial centered on the firewall so that the cowl extends equally above and below the nose would look "least awkward." Bear in mind that the main goals with this model are 1) to have a "test bed" for the engine and 2) to get some experience flying, handling, transporting, storing a large model.

Abu, no problem weakening the mount. As It's pretty stout.

Just to show that I'm not alone in my insanity! The last photo shows a model of the 1931 Kari-Keen.

The third picture is a Piper J3-P I think it was a French engine and proves the point that a Cub with a radial may seem out of place but it's not. Shouldn't matter if you lower the thrust line but if you can re drill the mount(s) then the appearance might be improved. The point is to get some operating time on your Seidel and how it looks may be a moot.
Doc

A Taylorcraft does much better aerobatics than a Cub. Get it close as you can and live with it. Does it matter? Some, but not enough to keep it flying fine. Its not a pattern plane.

Is the prop going to clear the ground?

Initial measurement with a 22" prop seems to be OK, but I'd have to be careful not to let it over-rotate.

Well wait a minute, lowering the thrust line of the engine will increase the thrust moment arm to the center of lift of the wing, so you may need to add some more down thrust to the engine to compensate, if you notice excessive nose pitching up when power is applied abruptly... may not be too noticeable if you're only lowing the engine a small amount... something you can adjust if need be.


John M,

John, I'll keep that mind. I have already mounted the engine about 1" below the center line marked for a 50cc gas engine. On a model this large, I'm not sure how much of a difference that makes.

That looks very good there, mount a Townend ring and pass it off as one of the late 20s or early 30s Travelaire cabin ships.lol
Im sure it will be fine Abu
Doc

This will help explain it, but I don't think you' have any real issues that can't be adjusted or trimmed out,

http://avstop.com/ac/flighttrainghan...stability.html

***EDIT***
Look down towards the bottom of the page



John M,

That's what I was thinking. A full-depth WWI-style round-cowl might look a little odd...particularly since the fuselage has flat sides. Actually, my main reason for wanting a cowl at all is to give some minor protection to the rocker heads and the rocker rods that can easily fall off if one of the rocker arms is depressed, for example even on a gentle nose over.

And, John, thanks for that further information.

Ahhhhh Nr 501 W John Livingstons mount in 1931 at the Cleveland air races. One of the fastest Monocoupes ever in the Shell Speed Dash

A full round cowl or a ring cowl - I agree with Doc - it looks good and should present no problem to the how well the plane flies.

Looking forward to you flight reports.