Author Topic: Tee Dee questions (Read 1930 times)

kenneth cook · « **on:** July 05, 2009, 02:24:33 PM »

I recently completed a tee dee powered speed plane. This plane does resemble a full bodied plane so I guess it would be considered a proto plane. Today I flew it with pretty good success. I've had nothing but problems making the tank due to the bellcrank halfway into it. The next difficulty was piping the supply line through the body with little room. I thought I may have kinked the supply a bit but apparently not enough to cut off fuel supply. There isn't enough room to insert your finger into the venturi to choke it so you have to insert the syringe and dribble in the prime. The only prop on hand was one I was using on a mouse racer which was a APC 4.75x4. I tried to run a full tank on the ground but this wasn't successful. I have a few guesses here. One, I think due to the cylindrical shape tank the fuel isn't getting centrifigully forced to the pick up or the prop I'm using is loading the engine and heating it up. The entire engine is cowled in. Once confident I was getting a fair run I set up the lines and launched pretty rich. It was moving along pretty fair so I had the pilot raise the plane high overhead to lean it which worked. I believe the total capacity for the tank is 1/2 oz. Regardless if this enough, its all that I can put in the plane as there isn't room for enlargement. Starting inverted was causing fuel to siphon out if the nose wasn't pitched high during flipping. I could've just capped one of the filler pipes, but I didn't try it. The plane took off nicely but could of used a bit more speed. I'm sure I could've dialed in the needle a bit. I was using the standard plug as well. Would it benefit me to install a pressure tapped backplate? I ask this because I would therefore have a somewhat sealed fuel system which wouldn't siphon during the flipping over and starting procedure. I also wanted to know if this would encourage fuel draw because priming is a pain in the neck. In addition, would a APC 5x3 be a better choice of prop? It just didn't seem to sound like the recognizable note that I'm used to. I was very impressed with the results today and I'll be looking for improvements. I was using Sig 35% nitro for fuel and will more than likely stick to this fuel. It has been brought to my attention that Xenalook has started production once again for the high compression plugs so I will definetly have to install one of those. I would think that the venturi could be opened up but I'm not sure how much. In the event I get a pressure fitting does anyone know the max size of what this hole should be. Never drilling one of these out myself I was concerned about how much material is left betweeen the fuel jet holes and the inside of the ventui. If anyone has had similar experiences with one of these types of set ups, I'd gladly like to hear your input. Ken Cook

Robert McHam · « **Reply #1 on:** July 05, 2009, 05:07:36 PM »

Ken, #80 drill is about right. Remember it is about pressure not volume, the hole does not need to be large. the tubing should also be small in dia, again, so as not to add volume.
A one way valve in the pressure line would be a good idea too.

Robert

kenneth cook · « **Reply #2 on:** July 05, 2009, 08:12:19 PM »

Robert, I should of been more clear on my question. I wanted to open the venturi hole . Currently, I still have the screen in place. I'm not sure that when you weigh the balance of removing the screen vs catching debris is worth taking the chance. I was hoping to remove the screen drill the hole larger and try to reinstall it. I have a few older tee dee's that never came with a screen. Ken

minnesotamodeler · « **Reply #3 on:** July 05, 2009, 08:20:06 PM »

A completely cowled-in engine is going to overheat and sag out lean pretty quick, especially sitting on the ground. I had to cut the front of the cowling off my "Frankenstone" custom 1/2A Fierce Arrow, couldn't get it in the air quick enough to keep it cool (flying off a stooge).

Here's before and after.

Bill Barber · « **Reply #4 on:** July 05, 2009, 09:27:02 PM »

I have to qualify that I am not an expert on TD's , but I have seen them screaming on friends combat ships . Just using pressure from a bladder seem to be all they needed to turn up the RPM's .

kenneth cook · « **Reply #5 on:** July 05, 2009, 09:31:48 PM »

Ray, my plane is cowled in just like yours. Once the plane was started inverted, we quickly removed the clip flipped it over and the pilot ran for the handle. I did set it off rich in an attempt to compensate for the heat but this didn't result in adequate speed which I was searching for. It wasn't bad and the plane did run the tank out on the 2nd attempt. The first attempt launched hard but the setting was too lean and the engine quit. At least this is what I thought happened due to the cowled engine. I cut holes in the sides of the fuse so exhaust could exit. In trying to adjust the needle, the heat being displaced through the holes was amazing. I believe there is a lot of room here for improvement and further experimentation is only going to lead to better flights. Ken

kenneth cook · « **Reply #6 on:** July 05, 2009, 09:45:17 PM »

In addition to looking at your model Ray, I noticed the engine bolts down into the body leaving the top block to be removed. This is where I had some problems. My top block has the blind nuts inserted into it so the engine can mount inverted into it. The problem I encountered was the tank is in the lower portion of the fuse. Trying to install the fuel tubing onto the tank and then onto the fuel nipple keeping it the desired length without kinking was truly frustrating. Your model has the tubing exiting out of the fuse then back in. This is where lessons are learned and I'll never do this again. I wanted to keep it clean on the outside so I elected the inside. I just came across a 1980's magazine article showing a 1/2 a plane that is almost identical to mine. They clearly show the tubing exiting on the outside of the fuse then back into it. The difference is that they're using a pinch off due to the use of a bladder tank as Bill described. I wasn't sure how to install a bladder tank unless there was a way to install it in a smooth compartment. I've tried to make bladder compartments for .15 size combat planes without good results. It restricts the expansion too much placing a lot of pressure on the bladder. I felt this would've happened here as well due to the restriction of space inside this plane. The plane in the article was built on a Cox mag pan and a fiberglass top shell. This had more space than mine as well. I surely want to try and build another after some tinkering with this one. Ken

minnesotamodeler · « **Reply #7 on:** July 06, 2009, 12:40:05 AM »

Hm, so in effect your engine mounts to the cowling which is then bolted on? I've not seen that approach before. I can see how with an internal fuel hookup it would present problems if the tank wasn't mounted in the cowling as well. The external fuel line is certainly simpler--here's another sample, my LittleAxe.

Concerning the bladder pressure tank, you want to use a container tube that is slightly bigger diameter than the inflated bladder; otherwise you get "false pressure" from the sides that diminishes as the bladder deflates, resulting in a very lean run. I put a 1" dia. tube, plastic or fuelproofed cardboard, in the wing at the high point in combat wings that have no fuselage--pretty unobtrusive there. A similar tube running fore-&-aft in a fuselage would work as well, you'd just have to have exterior access to it, and fuel line outside too for pinching off. Not as elegant as an all-interior arrangement but oh-so-much more practical.

George · « **Reply #8 on:** July 06, 2009, 07:33:31 AM »

Quote from: minnesotamodeler on July 05, 2009, 08:20:06 PM

A completely cowled-in engine is going to overheat and sag out lean pretty quick, especially sitting on the ground. I had to cut the front of the cowling off my "Frankenstone" custom 1/2A Fierce Arrow, couldn't get it in the air quick enough to keep it cool (flying off a stooge).

Here's before and after.

Ray, perhaps I'm missing something but I don't see ANY exhaust exit on your "before" picture. Generally, an enclosed cowl should have an inlet, a flow path, and an exit that is larger than the inlet (to account for expansion). Check out the flow path in some speed planes, etc.

Nice plane.

George

john e. holliday · « **Reply #9 on:** July 06, 2009, 07:42:51 AM »

I went back and looked at the before picture. The air inlet looks way large with no way for the air to escape. For the incoming air all that is needed is just a slot/slit to let air in. Then behind/or above the engine should an air outlet two to three times bigger. Will help pull air over the engine while flying also. The speed planes I have seen with engine completely enclosed usually had pilot on handle ready to go as soon as needle was set. No fiddling with setting. Also the head would be above the cowl or flush. DOC Holliday

minnesotamodeler · « **Reply #10 on:** July 06, 2009, 03:48:10 PM »

Cooling air Exit:

I thought I had it all figured out...found I know not nearly enough about air flow thru cowls; don't do 'em any more.

kenneth cook · « **Reply #11 on:** July 06, 2009, 03:51:44 PM »

John, my model is as you've just described. I was concerned on how to exit the exhaust out of the model initially so I made two exit cutouts on each side of the fuse. The front has a slit approx 1/8x 3/8 just in front of the venturi and the glow plug is sticking out one fin above the belly. I was concerned that this wasn't enough but during my second flight the engine didn't sag. It did sag trying to run a tank on the ground though. I was also trying to run it too lean. Had I run it rich it probably would've run the tank. I can't wait for future flights it does show a lot of promise. Generally, I don't usually go this route alone. My fellow club members weren't available this past weekend and I do try to gain knowledge by accident so to speak. Ken

Jim Thomerson · « **Reply #12 on:** July 06, 2009, 07:05:51 PM »

You can drill the TD venturi out to 1/8 inch. And, of course, you will not leave any metal shavings where they can do harm. Then you will need pressure to make it run right. I have mostly used backplate pressure on TD's of various sizes. I do use a one way valve in the pressure line. TD's need a head prime to start. Supposedly they run best with the exhaust slots front and back.

Robert McHam · « **Reply #13 on:** July 06, 2009, 07:57:55 PM »

Quote " Would it benefit me to install a pressure tapped backplate?"

Sorry Ken I was focusing on the above question.
Larry Renger would most likely have good answers for you regarding enlarging the venturi.

I don't know lots but from what I recall is that if you do enlarge the venturi, this will cause a vacuum drop which will make fuel draw more difficult. Seems if you enlarge the venturi for more air flow, you must also provide more fuel for it to burn else you will go too lean. Too lean is not good for a high RPM engine. This is especially true for an engine which relies on the fuel as its sole source of lubrication.
By lowering the vacuum for fuel draw it is almost necessary to use pressure for the fuel system which leads us back to a pressure or pressure bladder fuel system.

If you do this you will have headaches trying to tune the beast without a fine pitch needle valve assembly like those available from Texas Timers. See pic.

Robert

john e. holliday · « **Reply #14 on:** July 07, 2009, 08:11:19 AM »

Rar, I love the slats in the air outlet. But, even tho it looks large, how big is the area leading up to the vents? Looking at the pictures it doesn't look much bigger than the opening in the cowl. Something we did years ago for an experiment was to close down the intake opening and try to guide the air to the front of the cylinder fins. You probably wouldn't beleive how small an opening we could get by with. In some cases the plane went faster.

What got us onto this was remembering a local stunt grunt that had a new plane. Beautiful cowl with large intake opening and very little outlet. The engine was getting hot and the plane seemed to be struggling. The second flight after setting his needle a little richer on about the 6th lap the cowl blew apart. He did not beleive in reinforcing the wood, thank goodness as the engine cooled off and the plane gained speed. Later, DOC Holliday

Robert McHam · « **Reply #15 on:** July 07, 2009, 10:02:58 AM »

Although I have little experience in the ducting, I have read that it is most important to have the rear opening significantly larger than the cooling intake opening. When you look at the pictures of the speed planes from competition this seems to be true.

Quote by Doc: "Something we did years ago for an experiment was to close down the intake opening and try to guide the air to the front of the cylinder fins. You probably wouldn't beleive how small an opening we could get by with. In some cases the plane went faster."

I myself believe this to be correct in every sense.

Robert

minnesotamodeler · « **Reply #16 on:** July 07, 2009, 10:26:58 AM »

Whatever the reason, it didn't work...don't matter now since it's been modified (read: cut off entirely, see pic 'way up there) and works well now. Only "cowling" I do now is covered top, inverted mount, open bottom. I'll leave the sophisticated stuff to the speed guys.

kenneth cook · « **Reply #17 on:** July 07, 2009, 03:24:09 PM »

Reading what Jim posted above is what my son just recently suggested. This brings me to adding the timing shims under the cylinder. I do have some of these shims but they're not graduated and without checking them with a mic I'm not sure what I have. I've read this can add an additional 1000k. I can purchase shims of various thickness. How do you know what is correct though. It seems these shims should lower the compression though. Should I just find the correct shims that line the exhaust ports front to back? This brings me to the next problem, will this cause concerns for the tank which is directly behind the engine. I could possibly install a small aluminum baffle just in front of the tank. I did try and run one of my tee dee's with a shim under it and it lost rpm's on the stand. I guess its an educated guess or trial and error. Ken

Bill Barber · « **Reply #18 on:** July 07, 2009, 05:12:04 PM »

   Ken , if I can quote correctly the info I got years ago from kustom Kraftsmanship .

   " Because of an accumulation of production tolerances , the crown of the piston can extend above the inside shoulder of the cylinder when the piston is at TDC. Optimum design transfer and exhaust porting occurs when the crown of the piston is exactly level with the shoulder at TDC. One way to check this is with a depth micrometer "

   Then he goes on to other ways to check it and then using shims under the cylinder raise it to design height.

kenneth cook · « **Reply #19 on:** July 08, 2009, 03:38:10 AM »

Bill, I have noticed this by just looking at some engines. What happens when the ball socket joint wears? Is it best to compensate for this? I'm pretty good at resetting the joint actually. It took a lot of patience and ruined many to get it correct. I've also noted that some of the high compression plugs have a raised ridge around the perimeter of the plug. I figured that this was for sealing against the gasket and in the event the piston did raise above the landing it would have clearance. I've also discovered this when trying to use a Norvel button head plug on a Cox engine. I've had slight interference with the piston as it approaches tdc. The Norvel plug is completely flat on the bottom. This hasn't happened on all engines I've tried it on so I would say the tolerances are a bit different from one to the other. I've noticed that all pistons aren't completely flat on the top as well. I would imagine that due to the dome shape head this wouldn't be a problem though. Ken

Larry Renger · « **Reply #20 on:** July 08, 2009, 07:16:40 AM »

According to the Tee Dee instruction sheet, you can drill the venturi out to 5/32" when running pressure. I prefer to use the crankcase side port, but backplate pressure taps work OK if you have a check valve in the pressure line.

George · « **Reply #21 on:** July 08, 2009, 09:11:29 PM »

Quote from: kenneth cook on July 08, 2009, 03:38:10 AM

... I've also noted that some of the high compression plugs have a raised ridge around the perimeter of the plug. I figured that this was for sealing against the gasket and in the event the piston did raise above the landing it would have clearance... Ken

Ken, I think you are referring to the "squish band" that is designed to create a specific burn pattern.

George

Wayne Collier · « **Reply #22 on:** July 10, 2009, 07:22:27 PM »

I tend to think Cox engines just do well with the entire cylinder and head hanging out in the wind. I've never worried much about the orientation of the exhaust ports as long as the cylinder fit and height seemed correct. I use shims under the head to optimize engine run but have never used them under the cylinder as a means of causing the ports to aim any certain direction. I also believe in sub piston induction -- this is when the bottom edge of the piston skirt rises slightly above the bottom edge of the exhaust port when the piston is at TDC. Somehow it just seems to me that this lets engines breath better. You gotta breath good to run good.