jeffie8696

Irecently ran a Thunder Tiger GP.25 with a very tight piston fit and it seemed to run a little hot until it broke in but now it is fine. Lots of oil with plenty of castor is the key to it not coming apart at the seams Ithink.

jib

While I appreciate the "Myth" discussion, for everyone's info, the engine running hot was with a full tank, so the tank position should not be critical as the effective fuel level was above the spray bar due to the tank being full.

From my standpoint, this is a combat plane that does not spend too much time flying level and upright, so I'm not overly concerned about the fuel tank level. I've done everthig else I can to make the fuel delivery optimal; I used a super soft silicone tube in the small tank and a sintered clunk, which tends to draw fuel better in low level situations than a standard clunk.

We'll fly it this weekend. The first few flights will be short, to check the head temperatures. If it stays cool, we'll make some longer flights and see if it's losing power or heating up the head from fuel starvation. If need be, I'll move the engine lugs from the top of the mount to the bottom of the mount. That will drop the enigne 3/8 of an inch or more, but will move the prop center off the Leading edge line, something I was trying to avoid.

Thanks for all the help!!!

Jack

Mr Cox

To feel the heat of the head can be a guide but it is how it performs that really matters. If it can hold full throttle for 5 minutes without sagging then there shouldn't be any problem. Just run it rich.

I'm still a bit curious about these myths, what happened to the mythbuster? [sm=confused.gif]

downunder

You rang?
Uniflow absolutely eliminates the effect of varying fuel head change but can't help with a tank mounted too high or low so centre to centre is the optimum position so it stays the same when inverted.

The unloading/leaning out in flight is a bit complicated though. The carb is a venturi so the faster the air flows through it the more the pressure drops which lets it "suck" fuel better so when the engine unloads it revs higher giving a faster airspeed through the carb (same volume but less time to get into the crankcase). Sounds simple but the drop in pressure ("suck") relates to the square of airspeed the same way lift on a wing increases with airspeed. The complication comes with the way fuel flows through a fixed orifice which is what a needle valve is. The flow rate is related to the cube of the pressure driving it through the orifice. Basically this just means that double the pressure (or double the "suck" if you like) doesn't give double the flow rate of fuel so the mixture leans out. This affect is probably cancelled out a little because the extra revs will give a slight increase in muffler pressure. Phew, sorry if that sounds a bit like Dar talking .

Mr Cox

Well that's good explanation, I have no problem with that.

According to some that is a myth and I'm still curious to hear from NM2K;

jib

Jeffie8696 hit the nail on the head. This engine was still not yet broken in.

This was a used engine. It was in great shape and had been run when I bought it, but I did not know how much. We put it up in the air on Mastercraft 15% nitro fuel, which has 18% oil (2/3 synthetic and 1/3 castor), with a smidge of extra castor added by me, just in case. After one flight, it had cooled down and was running nicely. I strongly suspect the engine was just not fully broken in. It had a shortened muffler to fit in a limited space and the project did not work out, so I suspect it may not have flown at all. I swapped in an full size muffler (thanks Andy - draftsman1) and the first flight was excellent. Great power and it'll get better with time.

Thanks for all the help folks.

Jack

Edit: Seeing as how the engine came back in cool in the first flight, my son flew the plane's second flight all the way to an empty tank with no loss in performance, so I am confident that the fuel system is fine. The tank was essentially dry after the flight, with only a drop or two left in the tank.

NM2K

My Feedback: (14)

I don't see where Downunder's explanation is contrary to anything that I have been explaining. It is more detailed than necessary for the argument, but basically continues my explanation with more detail. The "debate" is about why we tune excessively rich before takeoff. Some folks believe that unloading the load on the engine requires more fuel. I disagree in that it requires less fuel. Engines only need more fuel when they are expected to do more work.

However, because the typical model airplane does not use a uniflow fuel tank set up, the needle valve does need to be set excessively rich in order to have the engine not running too lean towards the end of a flight.


Ed Cregger

Mr Cox

The statement I have a problem with is this one;

I do set the engine extra rich because it "unloads" when the plane moves forward. This is an immediate change that is observed directly at launch before the fuel level has changed at all.

Sport_Pilot

Actually it is the same square difference on the fuel as the air. However, Bernoulli's equation also has density as a variable, and fuel is much denser than air. The flow rate is also dependant on the friction loss and fuel is more viscous than air. Also there is a difference in loss from the Reynolds number. So the fuel will not be able to keep up with the air flow. The old automobile carbs kept up with this by lifting a metering rod when the manifold pressure increased.

http://en.wikipedia.org/wiki/Bernoulli%27s_Principle

Sport_Pilot

It does need more fuel. The engine speeds up requiring more air and fuel, but the fuel flow cannot keep up with the air flow.

blw

My Feedback: (3)

Unloading in the air is another way of saying that the propeller disk has become more efficient. It is that simple. If you are seeing 10,000 RPM on the ground when WOT, you may see something like 11,000 in the air at WOT. It's not due to more fuel being drawn into the carb, it's due to the propeller working better because of the inflow of clean air.

Mr Cox

Okey, maybe that's where the misunderstanding comes from...

Work is not only spent on driving the plane forward, there are lots of other sources for the work or energy to dissipate. Spinning a prop with zero pitch will require work and the engine also has internal losses due to friction etc. Also we are not talking about actual fuel spent or where it goes, we are talking about a needle setting on the ground compare to in the air. The prop "unloads" as the plane moves forward and this causes a higher rpm for a given throttel setting, which in turn requires a different needle setting compared to what seems ideal on the ground...

Sport_Pilot

Work is the actual movement of an object from one place to another. That can be a plane, or air, but friction is a loss, not work. A prop with no pitch does not move air so no work is being done, just air friction. That said you can use friction to simulate work, such as when using the 0 pitch calibrated paddle props.

Sport_Pilot

Regardless of why the prop unloads the engine is spinning faster with a WOT. That means it is using more air, and it needs more fuel to keep up. So it is using more fuel. If it did not need more fuel it would not lean out.

jib

There is one point that everyone seems to be missing. An engine that is unloaded, i.e., doing less work, can be run at a significantly higher fuel air ratio, especially at high RPM because there is a reduced load on the engine.

I’ve converted a car from carbs to fuel injection (megasquirt) and I set up my fuel tables as lean a 17:1 for low load, high speed cruising and as rich as 12.5:1 for hard lower RPM acceleration. These settings can keep the engine just on the rich side of knocking under heavy load and just this side of missing at high speed lean. I’ve over leaned my engine out in the high speed, low load scenario and found that it’ll miss and sag, so I richen it up slightly, which is what we do by making the high speed needle slightly rich.

I believe that it’s likely that our RC engines see the same effect.

Hey, if you’re going to play in my thread, I’m adding my automotive experience $.02. <grin>

Jack

Sport_Pilot

I have said it before and getting tired of this. An aircraft engine flying at speed has less load on the prop, less torque required, but that does not mean it is doing less work. In fact the engine is wide open and turning the prop at full power and because the RPM is higher it is doing more work and more power. So it needs both more air and fuel. The fuel air ratio requirements do not change significantly from idle to full power, the metering rods, fuel injection compesation programs, vacuum sensors, etc on cars are not changing that as much as bringing the ratio back to the required ratio.

draftman1

Its nice to see you got it running good! Is your boy enjoying the plane?

Mr Cox

I'm sorry, but the friction comes from movement and the required Work = force x distance.
The distance is in this case along the prop arc.

Sport_Pilot

I am not sure what the prop arc has to do with friction. But friction is a loss not work. Since the prop goes back to the same point it is not considered work.

jib

You bettcha! Who knows. I may be "allowed" to fly it this weekend. <grin>

Later,

Jack

Mr Cox

The friction doesn't do the work but work has to be done to overcome it. Air friction was meantioned and the force is along the prop arc. The sliding surface of the crank in the case also rotates in a plane paralell to the prop arc. The crank has a radius where the friction force is present. The friction force can be simplified with a single force acting on one point of the crank and this travels the distance of the circumference for each turn. Work is done in order to turn the engine around and this work is equal to the friction force x distance. The distance is the distance along the path of the force not the difference between start and end point...


I think I will give this thread a rest, on my part.

I tell beginners to set their engine rich on the ground because it unloads in the air, this is enough to get the message across and they get their engines working well...

PacificNWSkyPilot

My Feedback: (19)

Sport Pilot and Ed Creggar and Mr Cox were all correct. You want your Magnums to Bubble/gurgle some at full throttle on the ground, or else they're too lean. Especially a 28, which I used to use for combat a few years back. They wind out in the air and they'll need that extra fuel to allow the prop to unwind "At Speed".......

Likewise, on the ground you may THINK that prop is cooling it, but it AIN'T. That prop cannot simulate 70 MPH wind through the cooling fins. So you're too lean and you're running it where it can't cool.

Set it free! Give it plenty to drink. You'll like the results. And like you said, a rich run harms nothing. Once you get it up there you can tell if it's a bit too rich, and trim it back a touch. Magnums like having enough to run on, and they DO get hot when they don't get it. They'll bake the spray black on their cases very quickly. It does shorten their lives, too. Give them enough to drink and they'll live a good long life.

Jim

NM2K

My Feedback: (14)

I agree with that, but I don't see where that is contrary to my viewpoint. My head hurts! <G>


Ed Cregger

tomlee

My Feedback: (1)

Strange, not one of you said a thing about the instruction manual and what Magnum tells you to do.

I fly several Magnum 91's per the instruction manual and all is good.

fizzwater2

My Feedback: (61)

on the ground, the prop may be going back to the same point.. but the air it is moving is not. Air has mass, moving mass = work.

In the air, the prop is also not going back to the same point - it is moving forward, as it moves air backwards. I think the math gets pretty complicated...