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Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Eugene, May 29, 2017.
You can see the shape of the pod from this plan.
Rotax, did you ever have the engine cowled?
That Skyboy is about three times as wide. Hard to say if VG's can help at that wide.
So is Icon A5, but they do use VGs as well. They must make a difference.
Wondering if engine installation on Boorabee had any kind of angle like 1° or 2°?
That is a really nice aircraft.
The engine on the Boorabee had no offset. I tuffed the rear of the pod and noticed that the airflow was critical, that is it was so close to stall that a slight yaw caused the airflow on one side to become very turbulent, increasing the yaw until the fin straightened the aircraft causing the effect to be repeated in the other direction. Mind you the yaw was only slight but could easily be seen by looking at the wingtip against the horizon. The addition of VGs made the aircraft fly as straight as an arrow.
If you look at the Icon it has the cowling I was talking about. It's extended all the way up to the leading edge of the wing so that high pressure does not spoil the low pressure on top of the wing.
Yours has a half spine that acts similar but not exactly, so the partially exposed front of the engine can still produce high pressure that destroys some lift on the center of the wing.
This means a higher angle of attack to get level flight. and higher drag.
It's too early in the morning for me to understand all of it. I will look it over when I get home tonight. This Russian not very smart and need picture to understand things.
Looks like I need Russian translation for this one. Preferably with picture. Sorry for being so slow.
Here is large version and I can make them easy enough using 6061 aluminum. Is this look about right?
I was told that most important area on the propeller is anywhere from 75 to 80% of the diameter. So, at higher speeds that area is completely submerged in all kinds of turbulent rough air. Vortex generators should help to feed propeller with better airflow.
Re: Post # 1033 = WOW! Those drawings of the condition of the air,
that contacts the pusher prop of SKYBOY, tell a very undesirable story!
Icon A5 Intake Port
To clarify what I said before about the engine cowling, here is a motorcycle in simulation showing the same effect. Forgive the extremely poor quality drawings, its midnight here and Ive been up working since 5am.
Look at this wind tunnel data as an example. The chase motorcycle has almost 2 meter pressure wave in front that is being used to help relieve pressure on the cyclist. This shows how far forward the pressure wave can move even at very low speeds. Even with something as aerodynamic as a motorcycle. At high speeds on aircraft it is FAR worse.
Now imagine what is happening in front of the engine on the skyboy. See how the pressure on the front of the engine is centered over the wing? This cancels out some of the low pressure/lift of the wing and causes interference drag.
As speed increases and the pressure increases, the engine pressure moves forward and also outwards to the sides. It also starves the prop of airflow.
The Icon A5 simply extends the cowling forward so that the high pressure is in front of the wing instead of over it. So the engine drag does not interfere with lift or cause interference drag with the wing. The pressure wave is also forward enough that it does not interfere with the flow to the prop behind it.
So the cowling is not completely about making the outside of the engine "smooth", but relocating the pressure forward enough to make sure the pressure from engine drag is not cancelling out the lift on the wing. This means the lift to drag ratio is better and it requires a lower angle of attack.
And as a benefit the prop gets better airflow as well.
Hope that explains it better.
Youll actually notice the same effect on WWII fighters. Fighters with windscreens directly over the wings were significantly slower than those which had cockpits further back that did not interfere with the wing. Comparing aircraft with the same engines shows this very clearly.
The Hellcat was 60mph slower than the Corsair with the same engine, and the P-47 was almost 30mph slower despite having a better turbocharger and several hundred additional HP. The only advantage the Corsair had was that the cockpit was furthest back and did not interfere with lift or create interference drag with the wings.
This rule holds true for basically all aircraft from all nations during WWII. The further the cockpit was rear the faster it would be compared to other aircraft with the same engine. Yak-1, KI-44, KI-84, as well as FW190. All far faster than anything else with the same engine or power that had cockpit windscreen further forward.
The Mustang is the only counter example, but it was also tested with far higher fuel quality and manifold pressure. Its performance is close to the P-39/P-63 when using normal grade fuels instead of 150 octane.
The engine needs the upper half of a NACA cowl ring. With proper front lip shape.
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