Decalage angle

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Doggzilla

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And even though the other side is being partially supported in this pic, it appears to be doing the same thing on the other side as well.

 

Eugene

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Aircraft on this pictures in more or less in level flight position. Measuring to the bottom of the wing AOA = 2.5°. Electrical conduit is level. Does it look like angle is way too steep?

CCz67KleR+66zvHldvDfOQ.jpg Lbri8AW4TO2hXXjSiGQmrA.jpg kkNOizoWQxWXTiFhEz+%Jg.jpg UPmrXPu2RFS%djEOgLD6BQ.jpg 04L21jtiTB2%KxWzaT2qGA.jpg
 

Doggzilla

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Oh okay, just wanted to double check.

If you are requiring ballast while solo it's probably a bit high, but it also might just mean that it's set for slow flight at 5000 rpm. Since most people will just cruise around at that speed.

Also, the temperature at the time you set the prop pitch can also have an effect on top speed. If set during colder weather it may be too high pitch for warm weather. Since it grabs so much better in cold weather.
 

Doggzilla

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And when I say temperatures affect pitch performance, It's not insignificant.

In summer the Cessnas with longer pitch would have a significant speed advantage of 5-15 knots.
 

Eugene

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I did some home work. Hope this makes sense. Picture below is my designer's aircraft with same tail boom, horizontal and vertical tail.

IMG_5695.jpg

This is about 120 MPH airplane with 80 HP engine. On elevator you can see brackets without counter weights. Picture below shows rear cowling, that is much longer then first prototype during development.

Screen Shot 2019-05-21 at 17.09.31.png

Original cowling was shorter and sometimes during test flights pilots feel strange elevator vibrations on controls. After some tests they found some separation on the backside, like on picture below

37127038_2134298959931942_5707361643407605760_n.jpg

Longer cowling solved all problems and they removed balancing weights at some point

Screen Shot 2019-05-21 at 17.08.05.png

On Skyboy about 7 years earlier after beginning of 80-100 hp engines installations they was having same problems, but didn't know what to about it. Best they came up with, is to use heavier elevator weights with longer brackets. You can see difference on picture below.

IMG_5635.JPG
22815537_1504792169612859_8872927261447836_n.jpg

So, maybe I should build cowling like that. Not sure how to go around propeller. Here I found another picture of original short cowling.

Screen Shot 2019-05-21 at 17.10.29.png

So, if there separation on cowling like that? What do I have on my airplane????!!!!
 
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poormansairforce

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The curve on his plane has a much larger radius, that is what I was suggesting that your plane needs. Don't know if that is the problem but it looks questionable. You've tried everything else so thats why I pointed it out.
 

poormansairforce

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Look at it this way, he had a problem with the fuse contracting away from the wing as well as contracting down towards the tail. This is happening at the same time as the wing is contracting down to the the trailing edge. Not good for pressure recovery so this is not a good design.

He obviously doesn't understand this stuff because he stuck a molded fuse bottom on your plane with a sharp curve for the air to follow. Air has viscosity so depending on speed it can't easily fill in behind the sharp contour creating drag. A row of VGs would probably give some answers as to if its a problem area or not. There may well be other areas that have issues but this would be cheap and easy to check.

P.S. he gets away with it because he has large wing root fairings.
 

Doggzilla

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Well, with a rear prop it's going to ruin the flow anyways before the elevator. Some buffeting will always be expected.
 

poormansairforce

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Well, with a rear prop it's going to ruin the flow anyways before the elevator. Some buffeting will always be expected.
True, but as Eugene pointed out, the designer's tractor setup had issues with it as well. Bad design. And there are other pushers out there where this doesn't seem to be a problem.
 

BBerson

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A high wing has less wing/fuselage intersection separation issues than a low wing. The prop generally evacuates the area when under power.
The problem I see here is a lack of any engine cowl and the disturbed upper wing surface which is a bad place. Almost worse than a dive brake. I don't know of any aircraft that fly uncowled, above 80 mph.
 

Doggzilla

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Ya, if vortexes are rolling off any components into the prop or tail it's going to create buffeting. Especially since the prop has such a strong moment arm being mounted so high.

If the flow into the prop is disturbed it going to cause a noticeable rocking behavior on the pitch axis.
 

poormansairforce

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A high wing has less wing/fuselage intersection separation issues than a low wing. The prop generally evacuates the area when under power.
The problem I see here is a lack of any engine cowl and the disturbed upper wing surface which is a bad place. Almost worse than a dive brake. I don't know of any aircraft that fly uncowled, above 80 mph.
Titan Tornado and Kolb MKiii are two for starters.
 

poormansairforce

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Right. Looks like some Tornados are cowled, some not. This one says cruise at 75kts. http://lightsportaircraftpilot.com/titan_tornado_experimental_aircraft/pictures.html

They are all tandem, I think. A big advantage.
That is a 50 hp Tornado with an uncowled engine performing that well. The MKiii is side by side. Here is an Rans S-12 (side by side) with a uncowled Viking 90 and at the 4:50 mark he talks about cruising 80 on a draggy frame.
The thing that all these designs have in common is the lack of a "butt bubble" that the Skyboy has been endowed with. So there is no excuse for the lack of performance.
 
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Doggzilla

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I think I may have been wrong about the new rear cowling making it better. It may actually be the problem.

Turn the aircraft sideways and look at the fuselage. Its a very thick and stubby double sided airfoil. But what is even more important is that the wing acts as a "fence" on one side, and the other side has the airfoil shaped belly and landing gear fairings on the other end.

So the rear of the fuselage is like a 3 sided airfoil with a fence on one end. There is no way for the low pressure to be relieved.

This usually isnt a problem because a tapered rear usually has open space above and below on most aircraft that use it, which allows air to flow in and reduce the low pressure field on each side. But in this design the wing and the bubble belly are stopping the low pressure from getting relief air from above and below.

The question now becomes how to dissipate it. Because if its dissipated off center this creates lateral flow and a vortex. In fact, with the landing gear mount sticking into the low pressure area and disturbing it, it very well may be the source of the unusually high control buffeting.

I would really love to see some smoke generators mounted just behind the cockpit doors and belly, and see what that flow is doing. In fact, it can probably just be done from the window by a T shaped tube. That would give us some definitive answers to what the flow is actually doing, instead of guessing.
 

poormansairforce

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I think I may have been wrong about the new rear cowling making it better. It may actually be the problem.

Turn the aircraft sideways and look at the fuselage. Its a very thick and stubby double sided airfoil. But what is even more important is that the wing acts as a "fence" on one side, and the other side has the airfoil shaped belly and landing gear fairings on the other end.

So the rear of the fuselage is like a 3 sided airfoil with a fence on one end. There is no way for the low pressure to be relieved.

This usually isnt a problem because a tapered rear usually has open space above and below on most aircraft that use it, which allows air to flow in and reduce the low pressure field on each side. But in this design the wing and the bubble belly are stopping the low pressure from getting relief air from above and below.

The question now becomes how to dissipate it. Because if its dissipated off center this creates lateral flow and a vortex. In fact, with the landing gear mount sticking into the low pressure area and disturbing it, it very well may be the source of the unusually high control buffeting.

I would really love to see some smoke generators mounted just behind the cockpit doors and belly, and see what that flow is doing. In fact, it can probably just be done from the window by a T shaped tube. That would give us some definitive answers to what the flow is actually doing, instead of guessing.
Thats why I mentioned VGs on the belly as a simple test to confirm where the problem area is located.
 
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