desmobob

In a situation where an engine is mounted too low in relation to the fuel tank, is there any difference between the way a two-stroke and a four-stroke engine will run?

Cases in point: I have two P-51D models. One is a Kyosho ARF and the other is a Great Planes kit. In the Kyosho, I mounted a Super Tigre G51 inverted. It is too low in relation to the fuel tank with the carb spraybar being close to the level of the bottom of the tank. I installed a Nelson Intelligent Glow Driver. After a a few dozen flights, I tried it without the on-board glow and it ran just as well. Despite the spraybar-to-tank-level relationship, the engine runs well and is dependable. No dead sticks.

I recently built a Great Plane P-51D kit. I purchased an O.S. .70 Surpass II for the model. When it came time for engine mounting, I just couldn't get myself to hang this large, heavy engine on the skinny little nose of that airplane. I decided to use a G51 mounted horizontally. Even that engine was a little large and the Pitts muffler would require cutting both sides of the cowl. I figured it wasn't a very scale model, so I ended up mounting the engine inverted and running the stock ST muffler out one side... at least one side of the airplane would look good. I knew the G51 ran well in a similar mounting in the Kyosho Mustang, so I figured it would be OK.

The spraybar in the GP P-51 is in about the same position relative to the tank as the one in the Kysosho model. I'm having trouble getting this one to run the way I'd like it to, though. It seems like it needs to be leaned a little on the high and low needles, but when I do, I get dead sticks. (This may be due to the engine not being fully broken in yet, though.) There is a big difference in mixture and engine operation between a full and nearly empty fuel tank.

I was thinking of the possibility of removing the G51 and installing the O.S. .70 after all. But the O.S. would have to me mounted inverted. Would I have the same running difficulties with the four-stroke carb being too low in relation to the tank? Another option is to re-mount the G51 horizontally to get the spraybar up in a better position in relation to the fuel tank.

The last option is adding a Perry pump. I was hoping to avoid that.

Any thoughts?

Good flying,
desmobob

2slow2matter

My Feedback: (1)

From my limited experience, four strokes seem more suseptable to these kinds of issues. For example, I had some glow fuel that got a little stale. I couldn't even get my saitos to start on it, but my OS 40's ran like champs.

One issue to consider is the fact that with two strokes, the plug doesn't get as cold between cumbustions. This is why you need a special "hot" four stroke plug in a four stroke--they stay hot longer because it's longer between combustions than in two strokes.

Another issue is the fact that four strokes are updraft intake--meaning that when inverted, they are actually "downdraft." All fuel that enters the carb will be available to "run down" the intake and into the cylinder. With two strokes, when they are mounted inverted, the fuel has to be drawn upwards into the engine. I don't know if that makes a difference, but it sounded good when I was thinking about it

anyway, the biggest issue is probably with the fact that in two strokes, the glow plug stays hotter--and that is usually the problem with engines mounted below the tank--especially when inverted.

JMO, and everything posted could be dead wrong, but I tried....

desmobob

It all made sense to me... sounded like good information!

After looking the whole situation over one more time, I'm thinking the easiest way out is to just add the darn Perry regulating pump.

Thanks,
desmobob

w8ye

My Feedback: (16)

You wouldn't need a Perry pump. You would need a Cline or Iron bay regulator.

Make sure the spray bar in the GP ST 51 carb is round and does not have flat sides on it. Also make sure that the slit on the engine side of the spray bar is aimed directly down the carb.

Enjoy,

Jim

desmobob

The Perry regulating pump is a pump and regulator. It provides fuel flow at a user-adjustable constant rate for a lot less money than the $60 Cline or $45 Iron Bay.... and besides, I already have one in the shop. [8D]

Good flying,
desmobob

w8ye

My Feedback: (16)

The regulator in a Perry Pump just limits the maximum pressure the pump will pump. Fuel will go through the pump whether it is pumping or not.

The Cline and Iron Bay are demand regulators. You use crankcase pressure to force fuel to the regulator. But, no fuel will go through the regulator until the engine asks for it with the slightest bit of suction in the fuel inlet line for the carb.

The Cline and Iron Bay need to be mounted as close to the carb as possible.

Enjoy,

Jim

gjeffers

a very good Tommy Boy!!!!
and i think you may have something there[X(]

Flyboy Dave

My Feedback: (21)

Something to keep in mind....a two stroke mounted inverted has the carb
mounted way down below the tank. Four strokes usually have the carb
mounted directly behind the cylinder, at a point just about even with the
thrust line. (or so) The four stroke, in my experience doesn't really care
if it's mounted upright....on it's side, or inverted....because the carb doesn't
really move much in relation to the tank.

The two stroke, on the other hand....runs perfectly with the engine mounted
on it's side. However, if the engine is mounted upright or inverted, issues
come into play that can change the operation like night and day.

Upright mounted two strokes experience lean conditions in climbs, and
positive "G" maneuvers. Inverted mounted engines experience richness
in climbs and positive "G" maneuvers, as well as siphoning and flooding
issues. The amount of fuel in the tank can exasperate the problems as well.

FBD.

Spicoli

I mounted a Saito 72 inverted in a GP P51 and it ran fine.No need for pumps or regulators

jp928

My Feedback: (5)

I went through the same thing when planning an inverted older OS 120FS in a mustang. I decided that testing the setup in the Mustang was a VERY bad idea, si I looked around and found the cheapest, simplest ARF I could that would handle the 120fs - a BIG stik like thing. I added an extra ply plate to the firewall to make sure it ok with the heavier and bigger bang 4 stroke. I worked out how low the carb would be below the tank on the Mustang, and arranged the mounts on the stik to give the same height. Having got mix correct on the ground, full tank, started testing. After a few dead sticks on approach I decided I had to add a low throttle plug lighter. Once this was done, sold reliable flights were normal. Since then the test bed Stik has now been used to test flight 8 more engines - Enya 120, Enya 90, OS 120FS SIII x 3, OS 90 (old), something I've forgotten, and is now housing an OS 120FS SP.
jp

desmobob

Ah... a difference that is notable.

How about this theory:
The Perry will supply fuel to the carb at a slightly higher pressure than the fuel tank does on muffler pressure. But importantly, it supplies it at the same rate all the time when the engine is running. So if I set the engine's carb so that it runs correctly at that pressure, it will run correctly throughout the tank of fuel, regardless of fuel level, airplane attitude or G-forces.


Now, I think I'll get out the spare cowl I already bought for the airplane and think about turning the engine to horizontal and using a Slimline or Bisson Pitts. That would still leave the carb a tad low, but probably almost an inch higher than it is now. Decisions, decisions....

Thanks for the input everyone!

Good flying,
desmobob

Sport_Pilot

He never said the engine was so low it was siphoning. If the engine is mounted too low to the center of the tank, but not so low that it is siphoning, then a Perry Pump will help.

I have seen the Perry pump apart and it appears to have a regulator, however as someone said it regulates at a positve pressure, above 1 atmosphere, whereas the Cline regulates at 1 atmosphere. Little differance in how they work, just a differance in operating pressure. Since the Perry pump works at a higher pressure, it allows the fuel to flow at a lower pressure, such as when gravity is causing fuel to flow out the tank and into the engine carb, I.E. siphoning.

desmobob

Thanks for the help guys. I made my decision. I just mounted my G51 horizontally and started hacking up the nose of the airplane. The Slimline Pitts muffler fits well while the Bisson is too wide. Slimline it is. The horizontal mount puts the spraybar right on the carb centerline. Just what I needed.

I'm beyond the point of no return, now. Looks like I'll be cutting and painting that spare cowl.

Thanks again,
desmobob