Tank position, help needed

[Jeffrey Cottrell 2]

Hi Guys
Been all electric for years, so here's a question I have not had to deal with for some time.
Bought a second hand model, a Freestyle Iconic XL. Not much about it on t'internet, but what I found seems to suggest a 54 size 4 stroke.
Mine came with an ASP80, ho hum.
Anyway, the motor is mounted inverted and its position fixed by the cut outs in the cowl. Similarly, the tank position is fixed by the internal structure.
Way back when, I understood the tank centre line needed to be level, or perhaps a little below the carb centre line. With the layout of this model, the tank centre is some 25-30mm ABOVE the carb.
With a full tank, that puts the fuel level some 20mm or so higher still.
Seems to start and run ok, but not really cleanly. Could be due to my lack of experience with 4 strokes, but once, when trying to start it, I got a fountain of neat fuel upwards through the carb.
Not good, says I!
So, first of all, am I right in being suspicious of tank level?
Your thoughts
Jeff

[Denis Watkins]

Yes, it was syphoning Jeff, and filled the crankcase, as it will in the air.

Despite the fixed internals of the model, the tank needs to be lowered, even onto the front floor of the fuselage.

I have just completed the same job on a Tiger Moth, taking out 2 front formers, and building a thin ply new floor, to support the tank and new formers for the fus.

 

[Jeffrey Cottrell 2]

Hi Denis
Always amazes me how much expertise is available on this forum, but even more so how quick members are to share it.
Yours is pretty much the fastest reply, many thanks for that.
So, to horse
Can't really lower the tank floor. The fuselage is fiberglass and the lower section of the nose is moulded higher than the rest. Also this section includes the u/c mount, so that would also have to be moved to somewhere with the same strength.
Attached is a photo to show the layout.
Similarly, can't raise the motor, It would have to go up so much half the crankcase would be above the fuz top.
Couple of thoughts spring to mind.
I think the 80 size motor might be overkill. the model is only 55" span after all. I do have a spare Irvine 53 which I think might be a better match. Mounted inverted, this has the same carb position issue, but I'm wondering about mounting it sidewinder.
Puts the carb at the right height, but means cutting more chunks out of the painted cowl.
Otherwise, I have enough spare gear to go electric with it, but that seems like a cop-out.
Looking for a bit of inspiration for other than these two ideas.
Changing the subject slightly.
Seems to me the model must have been designed this way. With all the outlay in moulds and production processes wonder why no-one questioned tank position.
Still work in progress
Jeff
P.S. Been doing a little research on the forum, and I came across a system called a 'chicken hopper' tank. Don't know too much about this, anyone enlighten me?
J

[Denis Watkins]

That was the next port of call, the chicken hopper. Ideal solution.

Your regular tank can stay more of less where it is, but this can feed a much smaller tank, below carb level, that does not syphon.

There are some diagrams on this site.

[Jon H]

I saw your post on the laser thread but i will reply here. 

 

Chicken hopper tanks can work if correctly assembled so that the lower tank is pressure isolated from the top tank. You will need to modify tanks and its functionality will depend completely on how well it is built. Consequently, i dont recommend them to anyone. 

 

Also looking at the model you have i cannot understand the problem as it seems the tank is well placed for an inverted 80 4 stroke and there is plenty of space to mount it lower. Can you show a photo with the engine installed?

[Nigel R]

1 hour ago, Jeffrey Cottrell 2 said:

I think the 80 size motor might be overkill. the model is only 55" span after all. I do have a spare Irvine 53 which I think might be a better match

 

A 53 two stroke is very close in power to an 80 four stroke.

[Jeffrey Cottrell 2]

Hi Guys

 

Jon

Thanks for your interest. Unfortunately, can't show the engine install. Last time out I had a mishap (not engine related) but broke the engine mount into several pieces. Have done a lash up with an old mount I had, but I had the cowl front as a guide, so I think it's pretty close.

The outside view can be deceptive. Above the moulded section is a piece of 1/4 ply which the u/c bolts to, and then the tank floor is a framework some 10mm above that.

The red line on the fuz is the tank floor. I have a 10oz one, some 50mm deep, so even without any foam padding under the tank, the centre line is some 25mm above the red line. With the tank full that puts the fuel level at some 45mm above carb centre.

Clue for me was on one occasion, trying to start it, I got a jet of pure fuel upwards out of the carb. Sure indication something's wrong.

 

Nigel

That is interesting. Would have thought an 80 4stroke would be more powerful that a 53. Did find a sales picture here which suggests a 54 4 stroke. That would have been underpowered. Wonder if they meant 54 2 stroke. Either way, should be good if I decide to fit my Irvine instead.

Thanks for that

 

Appreciate all your help

 

Jeff

[Nigel R]

14 hours ago, Jeffrey Cottrell 2 said:

Hi Guys

 

Jon

Thanks for your interest. Unfortunately, can't show the engine install. Last time out I had a mishap (not engine related) but broke the engine mount into several pieces. Have done a lash up with an old mount I had, but I had the cowl front as a guide, so I think it's pretty close.

The outside view can be deceptive. Above the moulded section is a piece of 1/4 ply which the u/c bolts to, and then the tank floor is a framework some 10mm above that.

The red line on the fuz is the tank floor. I have a 10oz one, some 50mm deep, so even without any foam padding under the tank, the centre line is some 25mm above the red line. With the tank full that puts the fuel level at some 45mm above carb centre.

Clue for me was on one occasion, trying to start it, I got a jet of pure fuel upwards out of the carb. Sure indication something's wrong.

 

Nigel

That is interesting. Would have thought an 80 4stroke would be more powerful that a 53. Did find a sales picture here which suggests a 54 4 stroke. That would have been underpowered. Wonder if they meant 54 2 stroke. Either way, should be good if I decide to fit my Irvine instead.

Thanks for that

 

Appreciate all your help

 

Jeff

Can you remove the tank floor and rest it on the UC plate?

 

On a tangent. Have you checked that all is well with the carb? O rings good, etc. 

[Jeffrey Cottrell 2]

Hi Nigel

Good thought, thanks, but I don't know how much it will help. The tank floor is only some 10mm above the u/c mounting plate, and also the captive nuts sit on top of the plate so it would need some sponge packing below the tank to prevent the nuts chafing on it.

In all, I figure I could only lower it about 5mm, which wouldn't be much use.

Haven't done much with the motor, apart from give it a good clean. Will be putting it on the test bench soon, give it a good wringing out.

Have to decide on the way forward with the model first.

Jeff

[Don Fry]

How about, putting an additional plate below the undercarriage plate, then drill so the captive nuts are below the plate, then use a low profile wide tank, or even bolt together two such tanks side by side to the full width of the fusalage.

Point is, bottom 10 mm down, top 10mm plus whatever you can save in hight. Bit more adventurous, make a tank in epoxy glass, the you don’t need to mess with the undercarriage, just built knobs in the tank floor to allow the tank to sit on the undercarriage plate. 

[Paul De Tourtoulon]

"If" you were up to fiddling about, maybe an extension to the carburettor inlet pipe to bring the carburettor up a couple of centimetres.

 

 

 Think outside the box.

Edited September 19, 2022 by Paul De Tourtoulon

[Ron Gray]

Just a thought, can you move the tank back and put it under the wing, behind the U/C?

[Eric Robson]

Another thought, why not mount the engine sideways, that would raise the carb and it still would not stick out as far as a 2 stroke exhaust.

[Jon H]

28 minutes ago, Eric Robson said:

Another thought, why not mount the engine sideways, that would raise the carb and it still would not stick out as far as a 2 stroke exhaust.

 

this is the simplest solution by far

[Paul De Tourtoulon]

1 hour ago, Eric Robson said:

Another thought, why not mount the engine sideways, that would raise the carb and it still would not stick out as far as a 2 stroke exhaust.

That's what i do in most of my planes but it does look ugly.

[Nigel R]

sounds like a practicality vs aesthetics call

 

(practicality often wins for me)

[Andy Stephenson]

Is there anyone who thinks like me on this subject. I don't really care where the tank goes within limits because when in the plane is in the air, the fuel is at all sorts of different orientations to the engine even in a relative non-aerobatic plane. Once the engine is started I can't see that it cares about tank position provided it's not too far away to draw fuel. Take the example of a plane in a prop-hang you see pilots hold this for minutes on end without the engine stalling. There may be some seepage of fuel from a full tank that's high but a simple fuel tube clamp will take care of that. If it's low a good prime will get the engine started and drawing fuel.

 

 

[Eric Robson]

I bought a used CML Me 109 that had previously been fitted with an Evolution petrol engine with wrap round silencer. The bulkhead had been cut away to accommodate the silencer, I fitted a Saito 125 and had the tank in the original position, I could not lower it easily as the cut out went well back and was well made with ply and then fibre glassed. The outcome was that on filling and starting the engine flooded and ran backwards at times, once running right it did fly ok but I would have sooner went for a lower tank position. It did finish life with a petrol engine, end of problem. 

[Nigel R]

Thoughts;

 

Lots of IC powered 3d things are quite big and take pumped petrol motors. A pump and regulator can of course solve almost any fuel tank location issues.

 

Add some 'g' positive or negative, and any tank position problems become bigger tank position problems.

 

Some engines draw fuel and cope with a wider range of pressure (due to fuel level/tank pos) better than others.

 

Your results may vary, etc.

[Jon H]

32 minutes ago, Andy Stephenson said:

Is there anyone who thinks like me on this subject. I don't really care where the tank goes within limits because when in the plane is in the air, the fuel is at all sorts of different orientations to the engine even in a relative non-aerobatic plane. Once the engine is started I can't see that it cares about tank position provided it's not too far away to draw fuel. Take the example of a plane in a prop-hang you see pilots hold this for minutes on end without the engine stalling. There may be some seepage of fuel from a full tank that's high but a simple fuel tube clamp will take care of that. If it's low a good prime will get the engine started and drawing fuel.

 

 

 

Define 'within limits' 

 

The problem is if the tank is too high the engine will have gravity assisting the fuel delivery. It will flood when not running, and go lean when inverted as this gravity assistance is not only gone, but reversed. This causes all sorts of running problems and unreliability. Fuel flow into the engine has 3 parts to it. Gravity, engine draw, needle resistance. 

 

If gravity and engine draw work together as the would in a high tank situation, you need a high needle resistance (closing it) to supply the right amount of fuel. This amount of fuel always remains the same. so if you change the gravity part from +9.81m/s^2 acceleration to a resistance by rolling inverted or doing a prop hang, that is a change of nearly 20m/s^2 of acceleration force on the fuel as the tank is now very low. As a result, the engine will go lean and stop.

 

If however the tank is low in the first place you have gravity fighting the engine draw, so you reduce the restriction on the needle by opening it. Now, if you go into a prop hang there is no change in gravity effect as the tank is already low and so the engine will run just fine. Going inverted will cause a richer mixture, but this is unlikely to cause and engine to stop. 

 

While you are right that the engine has no idea where the tank is, it does know how its needles are set and the degree of tank position change it can tolerate. 

 

Some of the above is an over simplification as there are nuances of tank position, slow run tuning, pipe length etc. but i dont have all day 🙂 

 

 

[Mike Mc]

How is muffler pressure factored in?

 

[Jon H]

51 minutes ago, Mike Mc said:

How is muffler pressure factored in?

 

 

Bundled it with fuel draw. it stays the same no matter the angle of dangle. It does vary with rpm though. 

 

Sorry i was in a hurry and didnt mention it. 

 

If fuel draw from the engine is a -ve number as its a suction, and fuel pressure is a positive, you can just bundle them together. 

 

We are assuming fixed rpm for all this though as its just too flippin complicated to work it all out. If you guys want to do all the measurements and set up the testing feel free, but i aint got time. 

 

Top of the tank in line with the middle of the carb. It always works no matter the engine so just pop it there and forget about it. 

 

See OS diagram. They show the tank a few mm above the carb but note the fuel level is in line with it. As we never leave an air gap like that in a tank its easier to say top of tank, middle of carb. 

 

[Engine Doctor]

There was a simple conversion for this type (OS and copies) of engine when installed inverted that moves the carb to the level you require. I believe it was a topic in one of the magazines. 

It consists of an ali  plate bolted to the engine back plate where carb is originally fitted and the carb is then mounted on the plate at the height you require. A small extention tube needs to be turned to connect the carb to the existing manifold. You will of course need a friendly person with a lathe to turn the tube.

I fitted a os 48 with this mod years ago to git it an a moulded Corsair . It worked brilliantly and engine ran faultlessly.

If I still have a sketch drawing of the conversion I'll post it.

The conversion was a topic in a magazine many years ago and possibly RCM&E but not sure. It's a pity that mags don't include these mods nowadays but then most of the flyers I know are only interested in flying electric foaming.

[Engine Doctor]

Below is a sketch drawing to give the idea of the carb extension for OS and copies of type engines. Measurements will obviously have to be adjusted for different sizes of engines. Sorry for pic orientation. tried to rotate but keeps coming up sideways but gives the idea. When I made mine I also incorperated an "O" ring in the top of the extension tube.

 

Edited September 21, 2022 by Engine Doctor

[Jon H]

While i have no doubt the solution ED proposed can work and he has seen success, i have serious doubts if it will be a success with wide adoption unless it is commercially manufactured. 

 

Like the pressure isolated header tank, its usefulness will be dictated by the quality of the workmanship in building it and this will mostly be dictated by the tooling available. You need a lathe and the skills to operate one. Even a small error in manufacture would create an air leak which would totally scupper any hope of tuning the engine. Flooding could also be a problem and there will be negative impacts from the long intake even if they are acceptable to most. 

 

It is far less effort to just lower the tank. For a start you dont need a lathe! 

 

In the case of the OP, half the issue is the fact that the engine being used is an 80 when the model was designed for a 50. As he has a 46 2 stroke on hand i would side mount that and go for gold. Do not mount the 2 stroke inverted, its more hassle than its worth and both fuel tank and exhaust placement will be much better with it side mounted.