[Hughes500E]

So best not to worry about it until I reach a higher level of skill and understanding.

Thanks guys!

[Helijet]

In the early days of turbine modeling some airframe manufacturers such as BVM took what was learned in ducted fan designs ( ie flow through, area rule, etc ) and incorporated these elements in turbine installations. This was more seen in American designs than european who prefered open engine / double walled tube engineering. There was quite the debate for a number of years as to what was better and in the end there was no clear winner. . The arguments ranged from one was safer than the other, one more efficient, quieter etc. It was generally agreed that static thrust - ie the aircraft taking off from the ground to " getting on step " the open engine design seemed to have an advantage ( although this may be more anecdotal ). In all out speed the clean flow design of all ducting works better. This I would agree to be true as there is less turbulance and cleaner air flow / drag through a flow through design ( ie a 25 lb Bandit with a P70 hauls a** ) As for safety - neither system was proven to be safer despite what manufacturers advertised. The full bypass requires more planning when installing components and access to internal bits and pieces where as the open design is quicker to build and allow more flexibility to component placement. There used to be an argument that a full bypass prevented stuff from getting sucked into the engine. My counter is you should use a FOD screen ALL THE TIME FOR ALL INSTALLATIONS. So, long story short...if you want all out speed and precision flying go with a clean install - the full bypass. If you have a small cramped fuse then an open engine design may make the build easier. My Aviation Design F16, CARF Tutor, PCM L1011, PST Panther, Reaction 54, Skygate Hawk all have open engine designs and fly fine. My Carf Ultra Flash, Skymaster F5, PST F84, ML F86 and Carf Eurofighter all have full bypasses. They fly fine too. The one major point though is .....if you want to go fast then the plane externally and internally has to be slick. The full bypass would then have the advantage. Otherwise go with what the manufacturer designed and have fun!

Hope this helps some.

Dean W.

[Hughes500E]

Thanks Dean, I'm intrigued! It looks like the engine fit will have the ducting up to or possibly over the front of the engines FOD screen! The ducting is very thin, I now have concerns re ingesting fiberglass bits etc. A better understanding will help me decide on how much effort I will put into the ducting or possibly constructing a bypass!

Question: WRT the bypass ratio and its relation to fuel economy. You mentioned efficiency, with respect to cruise speed (scale flying), could there be/ have you experienced any noticeable flight time changes with a like model that utilized a full bypass?

Would it be enough to have any influence on my decision?


Thanks

[Helijet]

The engine only requires 1.5 times the compressor face area. The engine will take in air wherever it can so no worries about a collapsed inlet as surrounding air will also be taken in. As for efficiency, once the jet is on step or in cruise you will be able to throttle back and fly around the patch due more to the reduced drag of the airframe. There really is no efficiency gained by the engine in a full bypass design. In other words the turbine does not ( despite some claims ) breathe easier / suck less fuel etc. However most jet pilots like to turn, crank and bank and " poor on the coals " as flying around in circles gets boring after a while. Here is my humble opinion....in a scale design you are concerned with the outward appearance of the model. That is where I put my efforts. A scale model often needs lots of stuff ( ie valves, lines, actuators etc ). The easier time I have at accessing the internal mechanisms for maintenance, checks and so on the easier my life is and the more enjoyment I get out of the model knowing everything is working properly. Keep the internal workmanship simple, clean and accessable. Scale planes also gain weight very quickly so you need to consider what is needed or not and the weigh penalty of ducts / bypasses etc. I generally like the approach of the Europeans. Keep the underwear skimpy but go all out on the dress!

Dean W.

[reo]

Originally Posted by Helijet

The engine only requires 1.5 times the compressor face area. The engine will take in air wherever it can so no worries about a collapsed inlet as surrounding air will also be taken in. As for efficiency, once the jet is on step or in cruise you will be able to throttle back and fly around the patch due more to the reduced drag of the airframe. There really is no efficiency gained by the engine in a full bypass design. In other words the turbine does not ( despite some claims ) breathe easier / suck less fuel etc. However most jet pilots like to turn, crank and bank and " poor on the coals " as flying around in circles gets boring after a while. Here is my humble opinion....in a scale design you are concerned with the outward appearance of the model. That is where I put my efforts. A scale model often needs lots of stuff ( ie valves, lines, actuators etc ). The easier time I have at accessing the internal mechanisms for maintenance, checks and so on the easier my life is and the more enjoyment I get out of the model knowing everything is working properly. Keep the internal workmanship simple, clean and accessable. Scale planes also gain weight very quickly so you need to consider what is needed or not and the weigh penalty of ducts / bypasses etc. I generally like the approach of the Europeans. Keep the underwear skimpy but go all out on the dress!

Dean W.

Dean, a lot of good information for all in your posts in this thread.....I see why the call you 'the perfesser'!! .....Ron

[Hughes500E]

Thanks Dean! After my reply I said to myself there is probably more concern regarding gear doors, drag etc.

Dean, when you see my new Hawk please don't look too close, I have already decided to not install gear doors etc (yet)

Ron, I agree!

[kev-o]

I didn't do gear doors on my lil jl tutor iether so no worries there just show us the empty fuel jugs deans jl toot didn't have em iether too much of a pain in the a$&

[Hughes500E]

Phewww, I almost started working gear doors into my schedule!

[kev-o]

Cheers 🍷

[Pepperpete]

Can you guys please link exactly what you use for fuel tubing from the UAT to the pump? I've always used tygon and now I'm concerned that might be the wrong stuff. Thanks.