Originally Posted by
JIMHI
I recently bought a used K61 and am pondering which of these two airframes would be best suited to this engine. I bought it off ebay and it is well used but as parts are readily available that does not bother ma a lot. It did come with Perry pump and carb so I am guessing the way it is set up it has seen this kind of use before. Also one other question, does the pump perform enough similar to a YS that tank placement can be farther away from the engine?
As Flywilly said the K & B .61 is a fine engine but not one of the super power house for a ballistic pattern plane like the Curare. The Curare is at the heart of the ballistic pattern style flying. You cruise around on both ends of the flight box, then you come screaming into the middle and perform your maneuvers and get out of center stage. Loops were getting really large so you need a good amount of power from the engine.
That said, nothing is impossible. If the engine power is limited your alternative course of action is to keep the plane as light as possible. If you can keep a Curare to under 7 pounds you may have a chance the K & B 61 will fly it fine.
Other point is that the Curare design has not move the fuel tank so far back that you need a pump to draw fuel. A fuel pump is really a double edge sword. The idea behind a fuel pump is 2 fold. 1) it is to draw fuel from a tank that is farther away from the firewall as you mentioned. 2) It is to force more fuel into the carburetor to increase power. To get more fuel into the carb to get power you need a corresponding amount of air for the fuel to burn. Otherwise you will have a super rich mixture. So in order to take advantage of the fuel pump you will need what Perry called a "pumper" carb. The pumper carb has a larger venturi. By it self the large venturi reduce the vacuum at the throat of the spray bar. So the carburetor cannot draw enough fuel. So you need the fuel pump to assist in fuel draw.
With all that there is a fine balance between everything that is going on. If one thing is not working properly your engine will not run properly. The other side effect is a poor idle. Landing a Curare with a poor idling engine is going to be a hand full. That's why Hanno put flaps on his Curare. If you watch videos of many of the top notch fliers during competition they shut off the engines right before the touch down.
One issue with the fuel pump is that it uses a rubber diaphragm as a pump. The diaphragm is moved by a tubing attached to a crank case pulse nipple. If you have an old engine chances is that you may also have a wrinkled and hardened diaphragm that will not work properly to supply the correct amount of fuel. As mentioned it is a very delicate dance between all the working parts.
So my suggestion is that you tested the engine on a test stand and get a feel on the behavior of the engine before deciding if you want to use the engine on your hard build plane. It is very disappointing when you spend a lot of time and money to get a nice airframe built only to find out the engine does not run right when you go out to maiden the plane.
Another issue that come up is to figure out if you are going to use a muffler or tuned pipe on the engine. Typically for ballistic style pattern flying you use an engine timed for tuned pipe use and tune a pipe for the specific combination of propeller, glow plug, nitromethane percentage and RPM of the engine.
FWIW, I personally like the Tiporare better than the Curare simply because of the "looks" of the chin line. The Tiporare has a much better chin line in my opinion but every one has different taste in what they like to look.