I am turning about 21,500 on the pipe with my K&B 7.5cc DF turning and 8x8 prop. The problem is that the motor is blubbery rich, then as i lean it, it jumps on the pipe. Richening the motor will not get it back off the pipe. I am thinking the pipe is too long? The needle is pretty broad until it jumps on the pipe, then I can't get it off until I richen it way up... The piston and head are perfectly caramel color and my plugs last 20+ flights.

You have the pipe a bit too short. Lengthen it about 1/2 inch and try it. The needle should be more stable. Anytime you have trouble needling a healthy engine with a tuned pipe it means your pipe is too short or your prop is too big. If you can't lengthen your pipe at least 1/2 inch try an 8x7 prop instead.

...but with an 8x8 prop the engine is going to do quite a bit of unloading in the air. a half inch too short on the ground might even be too long in the air.


dave

Chuck and Zag both have it right...... this is the nature of pipes.

If you want it user friendly......easy throttle response...... it has to be a bit longer.

If you want all out performance in the air, you actually want to set it so it's short on the ground, which makes the engine a bear to needle (which is what you are seeing). The engine will surge and may only jump on the pipe after one of those surges. Then it will want to say there and not trickle off with throttle movement. In the air as it unloads, it will go to full song, and will tend to be well behaved now that it is within the tuned range of the pipe.

On a ducted fan setup (expecially the 7.5 engines) the later is typically the way they were set up for fan use to get every last bit out of them. There was little you would notice of a 'midrange' throttle setting.

Unless you absolutely need those last 500-800 rpm, lengthen the pipe up a bit. Let us know your results.

The engine does not surge on the ground at all. When I bring the throttle back, the engine responds normally, and actually has an idle that is nearly perfect. The midrange is very good considering it is a DF engine. (i have flown fans before) The wierd part is that when I throttle back on the ground then go back to WOT, it will not get back on the pipe, unless I either pinch the line, or lean the needle. It unloads a HUGE amount in the air. I have to launch(its on a whiplash) with the engine well below peak, if not it will go lean in the air. It is just difficult to find the right needle setting to launch to get max performance without going lean. This past sunday I launched too rich, and the motor actually ran out of air to feed the fuel, and started 'missing' at high throttle. It would quit from flooding, kinda wierd for our engines.

i've also had this problem with bladder fed engines and the best solution is inflight mixture control. this allows a quicker launch because you don't need to be so rich when on the ground.


dave

Without going into this a great deal, what you are seeing is the difference between a cold pipe and a hot pipe.

Trust your first reading/pinch/needle setting where it jumps on the pipe.

On the racing engines (fan engines are very similar), we usually get one shot at seeing the peak RPM as the engine fully stages on the pipe. After that first approach, it will not get up there again (on the ground) with the hot muffler. If you try too hard to re-adjust, you will either overheat the engine, or end up with it too rich. Its tricky sometimes.

When the engine gets up on the pipe, it will increase in power, and require more fuel. You will normally have to launch rich to allow for that increased fuel flow. On most of my performance engines, I typically launch 600-800 rpm down from peak rpm. on the QM40 engines, they launch 1000 rpm down.

Sounds like what you have there is set up fairly well. As long as it throttles and transitions in the air, you have it set up very well. Do not let the transition on the ground 'fool you'. Keep in mind, the mixture will not change much from flight to flight. Once you have it set where you are happy with flight performance, remember that needle position. You will rarely have to move the needle more than 1/8 turn either way on any given day.

The hot pipe deal sounds plausible, I have heard of that before come to think of it. You may think I am lying, or exaggerating, but I have to set the needle about 2000 rpm rich to cover the unloading in the air. I launch at around 19,500, with the engine then climbing to the full rpm once unloaded. I can peak it to almost 23k on the ground, but it will not make the climb out before it starts to sag. I guess the whiplash's airframe allows the engine to really unload. I have never had a flame out, and it is very very easy on plugs.
Thanks

Inflight mixture control and a longer pipe sounds good.

I had a Jett incowl muffler on a OS 160FX that ran great on the ground but would not complete a flight. Tried to richen it on the ground but had no luck. In flight mixture control would have helped.