Diggr

Assembling a GP 40 Size Cub. Instructions recommend mounting engine w/ cyl head to the right, rather than inverted, but don't say why. I've pretty much decided to go with inverted mounting.

Is this a mistake and if so why?

Thanks for any comments.

bikerbc

I am not a big fan of inverted unless there is a reason to do so . It means you need to lower your fuel tank and you can have flooding issues unless you learn how to deal with inverted engines . The glow plug is at the lowest point so it is easily flooded . You can close your needle to keep the fuel from syphoning out of the tank but that means resetting the needle all the time . There are ways of dealing with inverted engines but I would sooner mount with the cylinder head to the side instead if possible . The glow plug is easier to get to and I just find it easier to deal with . How ever inverted does work fine .Several of my planes run inverted . I have glow drivers on them .

biam

I have a G.P. .40 size cub and mounted the engine to the right, but routed the muffler inside the cowl and out the bottom. With the dummy engine provided mounted on the left side it looks good. I agree with the above post also, do it for the ease of operation also. I used a magnum .70 four stroke, more than enough power with great sound! Fly it around at 1/4 throttle fpor scale flight.

Gray Beard

My Feedback: (-1)

Just easier. It helps keep the fuel tank and needle valve set correctly and places the needle valve on the top of the cowl. Cubs have the cylinder heads sticking out on both sides so having a model engine out one side doesn't look too bad. Last Cub I built an after market cowl was used and it had both heads molded in and looked good so I went inverted with a four stroke. There are a few rules you need to follow with an inverted engine but if done correctly the engine doesn't run or operate any different then mounting it on it's side or straight up.

TomCrump

I mounted a .91 Surpass, inverted, in my GP Cub 60. I encountered no problems with tank height, etc.

Diggr

Gentlemen,

Thanks for the replies. Since that initial post, I've been doing additional reading, and went so far as to dummy up the forward fuse cage. As it turns out the engine to be used (OS 40FX) when mounted on stock mount inverted puts the spray bar approximately 1/8" above the centerline of the tank (DuBro S-8). This seems to be at least serviceable. The muffler is completely external to the fuse on the left side and the needle valve exits on the right side about 3 1/2" behind the prop. I'm thinking that routing the fuel/pressure lines through the firewall say 1" above the tank centerline it would help to break any syphoning. That would then leave only the issue of the cylinder loading up fairly easily, but I guess I'll have to deal with that when the time comes.

Thanks again for the help.

jester_s1

The cylinder won't load up if your fuel tank is lower than the spraybar, Personally, I think having a muffler sticking on on the side is uglier than having a cylinder head showing on the other side, and I think most others do too. That's why so many Cubs have the engine side mounted- the muffler is pretty much hidden at the bottom and the head just ruins the dummy engine on one side. If you really want to preserve the scale looks, you can use a Pitts muffler with a 2 stroke or a flex tube muffler adapter with a 4 stroke. That lets you invert mount the engine and also have the exhaust going out the bottom too.

Gray Beard

My Feedback: (-1)

As one of my old instructors told me, it's your plane, you can do anything you want to it. I agree with Jester about the muffler but the last one I built was the CG Cub and the fellow I built it for really loved that cowl with the scale engines molded in. The engine was an OS .91 surpass four stroke and got mounted inverted. I mounted the tank low and there are no problems with flooding or idling. Four strokes do tend to run better then two strokes when inverted but if the tank is located correctly then a two stroke is no problem. Just a choice thing and it's usually easier to mount and engine on it's side for most people. There have been millions of Cubs built and the engines have been mounted in every position the 360 degrees will allow. It's not a deep thinker, just a choice.

Rudolph Hart

You can loop the fuel line before the carby too

JohnBuckner

My Feedback: (1)

Good Day Diggr

Yes I think its a mistake (in your case and setup) and I will tell you specifically why (agine with your setup). All that follows is strictly opinion based upon many inverted installations I have done for all the various reasons. However these days the only fully inverted installations I will still do with a conventional two stroke glow engine is certain all out pylon airplanes in which streamlining is worth all cost and additional hassle (you could loosely translate that word hassle as 'pain in the butt'. Notice the key phase in that previous sentence, Two Stroke.

The bottom line is a two stroke glow engine typically will suffer a much greater Tank/Spraybar Height relationship change than most four strokes because of the manifold setups of most four stroke when the engines are inverted. Now most all the responders are talking bout four strokes, well that something else and really does not apply at all to your setup Diggr.

Let me digress for a minute, Jester is the only one that hinted at the answer to your first question as to why most all kits and arfs of cubs in 40/60 sizes show a side mounted engine with a PICTURED two stroke. Most of these designs date from a period when two stroke glow is the norm and four strokes were rare. Indeed these designers were not stupid people and knew of the trouble most would have with an inverted mount in a cub with a two stroke glow and as Jester described Any model of a Piper J-2, J-3 or J-5 is gonna need a cylinder like appendage hanging out both sides and with a side mount you are half way there.

Also would like to explain just why it is that any typical J-2, J-3 or J-5 model seem to have such a problem with this fuel tank/spray bar relationship when using two stroke glow engines more than just about any other popular RC model:

The Cub has a very, very deep windshield and accordingly shallow firewall and of course cowl. This nose section behind the fire wall is also very short . The bottom line of the cowl on the full scale rises sharply right up to the bottom of the engine crankcase and this will allow simply no room to lower the fuel tank sufficiently when using a glow engine inverted on a model.

Lets take about your exact installation Diggr. Your engine an OS .40Fx (an excellent engine and fine choice by the way) you have checked you said and found you can have the spraybar even with the middle of the tank, Ok now lets talk about this height relationship thing agine. This relationship is affected by Degree, in other word things can get progressively worse or better depending on the degree of this change from the ideal. I think you will find that ideal and even most OS manuals (of two stroke glow engines) recommend somewhere with the spray bar around level with the top of the tank or just a little higher.

So what does that mean for you with your spraybar level with the middle of the tank? Well that means your engine may run just fine! It also means you may or may not have to use Hemostats on the fuel feed line between and especially after fueling, It means you may or may not have to always be attentive and carry your ship nose up as well as informing others to do so who may pick up your ship. It means you may or may not need to keep a rag over the engine/nose when sitting in the hot sun after fueling. This lat one is a fun one that no one ever thinks of and most definately can initiate a syphon quicker than snot, that fresh cool fuel in a full tank just heats right on up and starts a syphon right on up and little loops of fuel line seldom prevent this.

So its all a matter of degree an what you are willing to put up with.

Yup all the above just that old opinion thing agine John

Bozarth

My Feedback: (15)

What does that do?

Kurt

Rudolph Hart

Kurt it kind of works like the 's' bend does in toilet fittings,in that instance it helps control the flow (or not) of water.

a70eliminator

A loop places an anti siphone high point in the fuel line, proved the loop places line above the tank as in an overhand loop, it's not as much the "loop" that stops siphoning but rather the line location, an upsidown U in the line also as long as it puts the line above the tank.

Bozarth

My Feedback: (15)

But does it compensate for a tank that is higher or lower than it should be?

jester_s1

It does not. It will only prevent a siphon from getting started if you leave some air in the line. It won't affect one that's already going at all though. Once you have fuel in the line all the way to your carb inlet, a loop will still allow siphoning.

Bozarth

My Feedback: (15)

jester_s1,

100% agree. It is the delta in pressure that floods an engine, regardless of the path.

Kurt

Indy Park Flyer

I had a PC-9 ARF I finished last year that called for the engine to be inverted and I ran into many of the same problems that are described in this thread. The engine was constantly flooding and would not run for very long. When I mentioned this problem to a friend of mine he put on "Fuel Filler" and it solved all of my problems. The Fuel Filler is installed in your cowl. I just ordered a few of these from Airborne Models to keep on hand. The link to the fuel fillers is below.
http://ecsvr.com/abm/ShopDisplayproducts.asp?Search=Yes

Indy Park Flyer

Diggr

Just an update on this question. JohnBuckner's comment on tank placement is spot on. I went back and read the engine manual for what must have been the 1000th time and sure enough there is the information on page 12. If I had the cash available I would mount the engine on it's side and loose the stock muffler in favor of a Pitts style exhaust out the bottom of the cowl. But lacking the funds I'm going to press on with the inverted mounting and bring the fuel lines through the firewall as high as practical and the install a filler/fuel dot, probably on both the fuel line and the pressure line as well. The idea is that it will allow me to open up the line(s) and vent to atmosphere to break the siphon. As someone already pointed out, once that line is completely filled, it will continue to flow until the level drops even with the spray bar.

But again, thanks for the feedback. Having a fresh pair of eyes on a problem is always a good thing in my mind. Y'all take care.