Question   I'm making a tank for my P/A 40 and notice that the needle valve is almost 3/4" of the beam were the other engines all have 1/2"  of the beam ( O S 40 & 46 V F ) the pick up frome tank should be about the same hight as the needle valve. has any one have this problem?   The P/A 40 is the older type not ultra lite. were should the tank hight be?   

I hope this isn't a thread hijack, but since Larry and I fly together I know he won't mind.

I can appreciate the sensitivity of an engine to tank height with a METAL tank having fixed-location metal tubes for pickup and uniflow. But how about plastic CLUNK tanks, where the clunk starts out well below tank or NVA centerline while on the ground and then moves around in maneuvers? Does the same basic "get the tank centered on the NVA or slightly above" rule still apply? Larry and I were having a hard time getting my Vector 40 (Magnum 36 new style, .250 delrin venturi, ST NVA, 4oz rectangular clunk plumbed for kinda-uniflow, 11x5 Zinger) to run consistently a few days ago. Big surge in rpm at launch followed by unpredictable run in the air, plus used up all 4oz in 4 mins flat. What is the accumulated wisdom on clunk tank positioning?

Randy,
I fly with these guys also, and would like to add that Mike's Magnum has been exceptionally hard to start from the beginning.  Could this indicate tighter than average machining tolerances?  What do you consider a reasonable bench run break-in time for the Magnum?

Thanks,
Kim Mortimore

Hi Randy:

A quick run down to the garage reveals the front of the tank is 4.5" behind the NVA, and (more relevant I would think) the end of the clunk line is 8" behind the NVA. I think this may well be the answer to the "surge on takeoff" (sometimes even followed by engine stopping)...I had not taken into account the length of the feed line. I have room to move the tank at least 1.5" closer to engine.

The engine is on 1/16th shims, needed to get the thrust line to line up with the cowl opening, but the tank is on the bearers, i.e. 1/16 above the NVA centerline when model is upright. Probably should get it even as a starting point.

Plumbing is: Feed line brass tube at 9:00 position (seen from front of model) with slight angle inside tank to point the flex tube at rear outside corner of tank. Clunk moves freely from top to bottom of rear outside corner. Copper ram air tube on outside of nose, connected with fuel tubing to vent line of tank at 4:00 position. Vent line inside tank is solid brass, curved to about 3/4 of the way to the outside rear corner (thus "kinda uniflow"). Overflow line from top of tank out opening in bottom of cowl. Tried running with overflow plugged (which I would expect to work best) and open (which did work best).

Perhaps should re-do the vent/uniflow line to tie it to the clunk so they move together, a setup that works great on the round 6oz clunker feeding the ST51 in my Score (with overflow blocked during flight).

I will try to post a photo.

OK, now this definitely IS a hijack. Sorry, Larry.

My flying buddy was having some issues with his motor run, using a clunk tank, and what he did was rotate the "lid" of the tank (with the feed and vent pipes), effectively moving the pivot point of the clunk.

Worked a treat on his model

Here's the plumbing on that Magnum.

Engine has two hours of bench time at 6 min runs, lots of thermal cycles. Kim must have been so engrossed with his cool All Australian on Sunday that he missed seeing that the Magnum starts easily by hand now, just a few flips. I was careful to tighten all the backplate and head screws before Sunday's flight tests. One thing to do is chage to different fuel line to eliminate the possibility of a pinhole somewhere. Actually I had a few decent flights, just not predictable enough for comfort in the pattern.

HI Mike

Looking at the pictures  the  tank  i s really far away from the  motor, you will do better  and  have  better  draw  moving the tank forward....try  the  things  suggested  here  and  let  us  know  how  it is  working

Regards

Randy

When I use clunk tanks, I often set them up with a center vent. Goes in and is angled to the inside,  center of the tank. You can adjust what is nominally tank height by changing the height of the vent. Just move the vent up and down in the tank and it will act like you moved the tank. Can't remember where I leaned that trick. Could have been Brett or someone. But whatever, it works.

Sorry it wasn't clear. What would be considered a "uniflow" vent (in other words, not the pick up and not the overflow), on a tank with the exit out the front and not a slant tank (though that doesn't really matter except for the explanation), the vent enters the tank and is bent at a 45 degree angle (or thereabouts) to the inboard side of the tank. This usually puts it outside the fuel in the tank once the plane is airborne. Or soon after anyway. The tube terminates about half way between the front and the back of the tank and in initial adjustment is about the height, measured from the motor mounts, as the needle valve of the engine. It can be twisted up or down to raise or lower the relative height of the termination of the tube relative to the motor mounts. This acts just like raising and lowering the tank with a hard tank.

What can I say? It works. I'm Brett or someone could post the reasons based on fluid dynamics, but like a lot of things, I don't have the math.

Sucess! Set up the in-tank brass tubes per Randy P's suggestion and got a good steady run. Launched at 10900 with a 11-4 Zinger Pro, 5.2 sec laps, consistent upright and inverted. Moved the tank 1.5" closer to engine, no more surge and flameout on takeoff. Also am getting 5.5 minutes on 4 oz, enough to do the full pattern without sweating the clover. I think the Magnum wants a little more prop, will try 11-4.5 Thunder Tiger next.

Another thing I learned is, Never post on a control line forum that your engine "starts easily by hand." Today it would start easily for everyone EXCEPT me, to the great amusement of the hecklers in the gallery. My moto mojo was not working for me today, no doubt due to my hubris in making such a portentious claim.