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OS 40 four stroke, inverted?...

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Foxfan06/09/2019 17:35:23
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824 forum posts
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Hi all,

I have finally secured a small (ish) 4 stroke, an OS 40, which seems to be in excellent condition, but I have read a few comments that they can't be run inverted.

This would rather negate the point of the thing as both aircraft I could have put it in require an inverted installation.

Can anyone shed any experienced light on this issue, please?

Cheers,

Martin

Braddock, VC06/09/2019 17:48:02
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1632 forum posts
82 photos

I've had two of the original 40 fs and one 40 surpass and they ran and idled great when inverted. In fact the former two ran like that without exhaust pressurisation

Edited By Braddock, VC on 06/09/2019 17:48:25

Cuban806/09/2019 17:59:59
2705 forum posts
13 photos

I can't think why there should be some doubt as why the engines can't be run inverted. I have several OS and ASP fourstrokes that are inverted because of the cowl etc and they run without a trace of problem.

 As with any installation, be it upright, sidewinder or inverted, get the tank position correct with respect to the carb spraybar.- Avoid flooding and give the engine a few flicks over with a chicken stick and  without glow to clear any excess before using a starter. Because its upside down, allow the glowplug a while to burn off any fuel that might have accumulated over the element during  priming Get the prime right and they usually start by hand  easily. Even the full size boys pull through their big radials - slightly different cause for doing so but also to avoid hydraulic locks and damage as with a wet glow motor. Not so bad as with an inverted two-stroke without all the valve gear where there's a significant risk of bending a conrod because of a cylinder full of trapped fuel..

 

 

 

 

 

Edited By Cuban8 on 06/09/2019 18:17:06

Peter Miller06/09/2019 18:11:59
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10152 forum posts
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There is no problem with running four strokes inverted. In fact they are far less likely to flood than an inverte two stroke.

Steve J06/09/2019 18:13:24
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1436 forum posts
44 photos

I've never had a problem with an inverted four stroke. Inverted two strokes can be a pain.

Steve

Jon - Laser Engines06/09/2019 19:08:51
4786 forum posts
179 photos

i agree with everyone else. Inverted should be no problem as long as the tank is in the right place. Make sure the tank isnt too high as the air bleed carb on the little OS will not tolerate a high tank

Foxfan06/09/2019 20:33:01
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824 forum posts
6 photos

Thanks, chaps. That was given as the big reason they don't like being inverted...the air bleed carb. Apparently the bigger engines have a twin needle carb., whatever that is!

Cheers,

Martin

Dave Cunnington06/09/2019 21:10:40
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55 photos

So, without wishing to hijack this thread, am I correct in thinking that the fuel tank pipe to the carb should be at approx. the same level as the carb intake ?

I tried to fit an inverted 2 stroke OS and it kept flooding as I think the tank was too high above the carb level, I gave up and turned it upright, ran fine

Presumably if the pipe is a tad below the carb level, the engine should be able to suck in the fuel, aided of course by the pressure from the exhaust

Edited By Dave Cunnington on 06/09/2019 21:15:15

Paul Marsh06/09/2019 21:17:39
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3702 forum posts
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Four stroked run even better inverted. Many models I have with inverted run better than the right way up.

Cuban806/09/2019 21:31:55
2705 forum posts
13 photos

In my experience, having the fuel tank clunk pipe level with the spraybar is the highest that you't want to go, I tend to to aim for slightly lower and don't come across problems. The simpler airbleed carbs in particular need the main jet set correctly (not too rich) at full power and this will translate to giving the airbleed setting a chance to work correctly at idle. Again, don't have the tank too high.

You might have to live with the idle setting being a compromise, i.e a tad rich on a full tank, so a bit sluggish to start with, but much better idling at the end of the flight with most of the fuel consumed so a lower head, and the motor fully warmed up. Does differ between engines and installations - might take a bit of experimentation.

Many years ago, I had  a 60 ASP two stroke inverted in a pusher type jet model - a total nightmare - never ran well and I wasted an entire season messing about with it. Only time I had to admit defeatcrying

Engine ran like the clappers in another model mounted horizontally and as a tractor.

Edited By Cuban8 on 06/09/2019 21:37:35

Foxfan06/09/2019 22:00:43
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824 forum posts
6 photos

Well, looks like I should be OK with the inverted 40FS then. That puts my mind at rest. I can go ahead with the aircraft now.

Many thanks, gents.

Martin

PatMc06/09/2019 22:53:58
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4197 forum posts
521 photos

To avoid fuel dribbling out of a model between flights when the tank has fuel above the jet level it's best to route the fuel line in an inverted U as per sketch. With the engine stopped open the throttle then raise the model's nose high enough to allow the fuel in the line to run back into the tank before parking the model.

Fuel tube U bend

The sketch is as used in my old Silhouette showing a 2 stroke engine but the same principle holds good for a 4 stroke.

PatMc06/09/2019 23:09:47
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4197 forum posts
521 photos
Posted by Dave Cunnington on 06/09/2019 21:10:40:

So, without wishing to hijack this thread, am I correct in thinking that the fuel tank pipe to the carb should be at approx. the same level as the carb intake ?

I tried to fit an inverted 2 stroke OS and it kept flooding as I think the tank was too high above the carb level, I gave up and turned it upright, ran fine

Presumably if the pipe is a tad below the carb level, the engine should be able to suck in the fuel, aided of course by the pressure from the exhaust

Edited By Dave Cunnington on 06/09/2019 21:15:15

I always tried to position the fuel tank as per sketch in my previous post but the level isn't as critical as many people seem to imagine, after all when the nose is raised in flight the tank can be much lower than the carb. This is especially true of a 2 stroke where the carb is mounted ahead of the cylinder.

Re flooding - so long as you turn the engine over by hand a couple of turns before connecting the plug you should never flood a 2 stroke engine. In fact if you do this there's virtually no chance of causing a hydraulic lock, unlike an overprimed upright 2 stroke.

Jon - Laser Engines06/09/2019 23:57:07
4786 forum posts
179 photos

i would say that the tank in pat's example is in fact too high. I would set it up as cuban suggests and essentially have the top of the tank level with the spray bar. This prevents a change in head as fuel is used.

Dave Cunnington07/09/2019 07:30:24
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112 forum posts
55 photos

Many thanks guys, as ever useful info for a relative newbie

Peter Christy07/09/2019 08:59:18
1540 forum posts

I ran an OS20FS inverted for years in an "Attila". At first I had a few issues due to the tank being positioned as shown on the plan, which was too high. Once I moved it down to the floor of the fuselage, it ran beautifully.

I never had a problem with flooding, but I was always careful to turn it over by hand a few times before applying a starter!

Still got the engine, and an untouched kit in the garage, awaiting their turn on the building board!

--

Pete

ken anderson.07/09/2019 09:02:26
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8432 forum posts
772 photos

hello Dave,ive never had any bother running OS 4/st's upside down....anyway...how do they know if their upside down or right way up when in the air?

ken anderson...ne...1....upside down dept.

Martin Harris07/09/2019 12:12:07
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8790 forum posts
215 photos

Many of my models spend the majority of flight time in anything other than level flight.

Foxfan07/09/2019 13:08:53
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824 forum posts
6 photos

Well, that was my kinda thinking too, Martin. Even as a dutiful trainee doing boring circuits and Fig. 8s, the Irvine 40 is often at quite steep angles and anyway all the guys when starting their kites up lift them to see if the engine will cut or...whatever they lift them up for. At that point I would say the tank position is about as irrelevant as it ever could be!

I should proudly add that I have never yet gone too steep and had the control taken off me

 

Martin

 

Edited to say I have even seen all electric guys lifting their 'plane nose up skywards!<G>  Must be habit from the old oily days, eh?

Edited By Foxfan on 07/09/2019 13:11:30

PatMc07/09/2019 15:14:01
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4197 forum posts
521 photos
Posted by Jon - Laser Engines on 06/09/2019 23:57:07:

i would say that the tank in pat's example is in fact too high. I would set it up as cuban suggests and essentially have the top of the tank level with the spray bar. This prevents a change in head as fuel is used.

Never had a single dead stick, or even poor engine run with this setup.
Virtually the only time the tank was in that position was when the model was flying S&L, which was only a small % of it's quite long life. wink 2

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