So how much do interior aerodynamics play in performance and top speed on a jet? For instance in an open install, there's bulkheads, formers, and install components that create interior drag.. I'm sure a lot of the air gets buffered and routed as pressures build up in various areas, but its drag nonetheless .. (on a SM Gripen i used to have, adding ducting through the intake alone made quite a world of a difference vs the completely open install )

but how about an fully bypassed/ducted install ?? for example if there are ducts (naca ducts , vents etc .. ) does it make sense to clean up the area where the air gets blown in ( such as cooling inlets in the read that have formers only few inched behind .. )? how much interior drag will things like formers create ? is this at all substantial ? I'm debating if i should bother with foam or balsa interior fairings around formers to clean airflow a bit .. or would the difference be negligible ?

~V~

Have a look at the Ducted Fan section of RCU they go to extrodinary lengths to clean up the insides of the jets with supprisingly good results.

Mike

Wotjek,

Look at the performance of the 'stock' EDF planes that are offered. The vast majority of them have bare minimum power systems, and the ducting is quite 'dirty'. Just by cleaning up the ducting, I've been able to avoid purchasing upgraded power systems. And, for the most part, the stock EDF planes have great difficulty lifting off grass; they are hard ground or asphalt/concrete only runways. Clearly there are exceptions; e.g., the Habu.

Think about this! The Racing boy spend a lot of money to polishing the carbs, heads( input and output ports) and the exhaust system to have ever last ounce of HP for the drive system instead of loosing it to pushing air and exhaust!

interior ducting is very important. BUT. for those of us that have the Boomerangs. I have a boomer XL and it is truly amazing how the turbine gets much air at all, with the bulheads, fuel tanks and other junk in it's way, the air is very dirty by the time it gets to the turbine. but with a P-120, she still goes. but I guess it would go a bit fast and preform better if it had clean air, or better ducting.



Mark

Its huge. Internal drag is just as important as external. Example. Friend and I have Eurosports. same power engine. Difference is that he has full length intake ducts connected to a full bypass. I have short cut off intakes ( to fit the larger tanks) and no bypass at all. He is easily 25-30 mph faster than me. From the same speed pull up to vertical we have the same vertical performance. Scotty

this is what i'm trying to clean up some more ... Basically i completely sealed off the bypass now, and cleaned up the ducting ... so all there is left are a few naca cooling inlets that could cause resistance (around 2" sq total area) , and only two protruding aileron push rods ... I think the venturi suction through the outer pipe should make up for any drag from the inlets ... 36lbs thrust will do for now ... however with slight mods, a 50lbs big block would fit [8D] .. maybe on the next one , hehe ..


~V~

As most of the post have stated cleaning up the inside of your jet is very important. We have gotten away from this becuase of the large power to weight ratios that we all experience. In comparison to full scale aircraft we are very spoiled by having that excess amount of power. Taking a look back at the ducted fan era, we had to do what ever we could to sqeeze every bit of power out of those units. Ihave flown flown my Euro Fighter with and with out fullbypass and there is a very big diference in performance, but Ido have to say if you have a large engine P160 or bigger you really cant tell. So its all about how much excess thrust is present to actually make a difference.

Henry is right on the money. I used to fly internal combustion D/Fs that had 8lbs thrust on a good day. Manufactures used to spend a lot of R&D on internal ducting and airflow as these fans systems had a finite amount of thrust they could give, and any increase in efficiency resulted in better performance. The very top end Ducted fan jets were going over 230 mph with a D/F system that gave about 2/3 the thrust of a P60.

Nowadays if you want to go faster, the trend is to ignore airflow efficiencies, and just bolt in a big engine. Not really good engineering in my opinion.

Roger