ductedfan

Good Day all you jet fans. I am sure that this thread already exists, but I will give it a try anyway. I am trying to gather all of my materials and start scratch building a few jets with glow engines (Rossi .81, OS .77, .91) and I need your help. Does anyone have or does there exist a formula or rule of thumb concerning the dimensions of the thrust tube??? I need all of the dimensions, such a length, inlet diameter, exit diameter, suggested materials, etc. I need it all!!! Your help in this matter will be greatly appreciated. Thanks...

Dave Wilshere

Length will be down to the aircraft choice, but the ideal is probably no shorter than 20" if it's a fast machine.
work out the fan swept area and that will give you a circle diameter, I'd then reduce the diameter 10% on faster flying models! leave it the same on slower. There are so many variables, inlet area and length has a huge influence on overall performance.

You need to give a whole load more info if you want things pinned down more.

Kevin_W

My Feedback: (1)

I agree! It depends on which fan you plan to use. The front end of the thrust tube should fit tight around the back end of the fan unit. Tractor fans (like Dynamax, Ramtec, and Viojet) used a molded fiberglass thrust tube with a cap that enclosed the engine (to direct airflow over the head), then the tube could be constricted gradually by 10-15% back to the outlet. In the mid to late 90's most manufacturers tried to "area rule" their thrust tubes to maintain a constant area in the tube allowing for the area that the tuned pipe occupied. Pusher fans (like Byron) just used a sheet of thin mylar because it was light weight and the heat from the pipe was not an issue. I don't remember how it was attached to the fan, but it still needed to fit snugly to the back of the fan, and there was a limit to how much the diameter could be reduced (I never flew pusher fans, so I'm not sure what that limit is). As Dave mentioned, the inlet design is much more critical than the thrust tube. As I recall, with tractor fans the general rule was a minimum inlet area of 1 1/2 to 2 times the fan swept area was needed. The inlets also have to be as smooth as possible inside, with gentle reduction of the area as they reach the fan, and no sharp turns at all. Large radius inlet lips are crucial as well. Pusher fans usually have no inlet ducting due to the engine and pipe taking up that area, so they just need as much inlet area as they can get. This is usually achieved by cutting a humongous cheater hole in the bottom of the plane just in front of the fan unit. It is a huge challenge to design an efficient glow powered ducted fan airplane. Good luck with your project!

ductedfan

Cool Dave, thanks for the info. I'll get to the research and see what I can come up with. Thanks again...

FalconWings

My Feedback: (57)

All the above info is good. i'll give some additional pointers from a purist standpoint.

DFs have very little power. The fan has very little pressure drop allowance, therefore even though it is important to compress the airflow to increase exhaust velocity, you have to take care not to increase the back pressure too much as it will quickly degrade performance. Ideally you want to capture as much air from the fan as possible (hence why a fully ducted typically performs better) and then accelerate it using a smooth throat. Not much you can do regarding exhaust tube length, as it will be determined by the design of the model/location of the engine.

I've made a small sketch for you. Don't pay attention to the numbers, but to the concept. In reality the only way to size an exhaust duct is to analyze it. But if you follow the idea you should get "good enough" results. Hope it helps.