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Discussion in 'Aircraft Design / Aerodynamics / New Technology' started by Eugene, May 29, 2017.
Wow! You guys so smart ! I wish that I can learn to talk like that someday !
I zoomed into photo on post #80.
With protractor measured 4° at root of flat bottom. The tip is less but hard to see from the tip shape. I don't know what speed.
At 5000 RPM you are getting 81HP and assuming prop efficiency of 70% this translates to 56HP. If you are flying at 80MPH straight and level this equates to a flat plate drag of 16 Sq. ft.
In other words the drag is approximately the same as dragging a 4 ft by 4 ft barn door through the air.... lot of drag. Now your wings contribute about 3 Sq. ft - half of which is induced drag. If you want to go faster the problem lies not in the wings ( ~18% of your drag) but the rest of the airframe.
Not once have the following questions, repeatedly asked by several here, been answered:
Is there washout, and how much?
Are the ailerons reflexed at neutral?
Are all gap seals in place and intact?
What is the attitude of the fuselage at cruise?
Further, the OP dropped the thrust line by 7 degrees. According to him, to no effect.
So from 4 degrees factory installed upthrust, to 3 degrees downthrust "blowing on tail", no difference in performance.
And at full power, lots of drag excess AOA on the wing, poor speed and incredibly sensitive controls. And a weight given of 900-1100 lbs, the variation all forward of the MAC, and the plane acting like an aft CG, phugating with single pilot.
No washout, no gap seals, no flex straight and level flight should be just like sitting on the ground with and angle of attack 4 1/2° to the bottom of the wing from level ground
Fuse in flight should be, or is?
Sorry I don't understand
Sorry, can you tell me what part of the aircraft you are measuring the angle from horizontal in the first photo?
Wing root, right next to the door. My airplane is sitting on perfectly level ground. Before I extended Front fork -this angle used to be a 4 1/2°. which supposed to be straight and level flight attitude for this airplane.
Misunderstanding. I thought you were providing a photo of the fuselage angle measuring point with the plane at rest.
Anyway, at this point, my take on this plane:
The stab on this plane is set to counteract an excessive forward CG with two pilots by design. It is producing a lot of drag, down force and positive pitching moment (counteracting the fwd CG) on the wing to do that. It is the opposite of a lifting stab. The wing is lifting the gross weight plus the stab load.
The factory may also have added upthrust compared to the original designer's intent for the lighter version in an attempt to reduce the nose down moment at higher speed. If that worked, it would only happen under power. On the other hand, the designer may have originally specified down thrust to reduce the tail's pitching moment reasoning that the prop wash on the tail produces the opposite effect to the lowered thrust line. But this is second guessing on my part.
Anyway, high rate of descent, power off is typical of a nose heavy design. In this case the nose-heaviness is a result of locating crew weight far forward, balancing that with the tail. And the variable number of crew requiring a big CG trim range.
A more typical tractor high wing cabin plane of a similar side by side 2 person seating would put crew under the center of lift to minimze trim changes with weight. The stab angle would be near zero, and drag minimized, as well as no additional load on the wing.
You can probably calculate the added load of the stab on the wing by taking its size and the airspeed and estimating it's negative AOA (you'll need to estimate the effect of the wing downwash and propwash). Add that to the AUW, and then do the numbers on the wing at 5 degrees.
Then work out the drag of both, add in the big engine out there with the small prop, and I think the plane's performance will come into line.
So, I was kind of going right direction by making angle of attack for horizontal stabilizer smaller. But clearly don't have enough knowledge to finish it up correctly.
Just wanted to make something clear for myself.
If I understand you correctly - you're thinking, what I am thinking. And what I was thinking all this time, looking at this airplane, that source of drag is coming from geometry and not from unprotected engine, struts and cables.
Did I get it right this time?
No, I think the geometry is probably necessary to handle the range of crew weight so that no particular combination will be beyond a safe controllable range. I think if you change the incidence of the tailplane you will only be compensating for that by changing the position of the stick to trim it back to the same flying position, depending on how much crew weight you have. And in a worst case scenario of alteration, get in trouble.
And also my take, is only my take on it. Not "right" necessarily. There may be more, and have been many takes on this one.
Let me be really clear: what I was suggesting was an explanation for the configuration and its performance, not a recommendation for a remedy.
And let me put this another way. If your stick is approximately centered at cruise, and your elevator stab and trim tab are all even, and your gross weight is at the middle of your permisable crew weight range, then the stab is set to the correct incidence.
Changing it will only change the position you hold your stick, deflect your elevator, .......and increase drag.
One final thing and I'm done with all I can think of on this plane. You mentioned you intended changing the forward sweep of the plane -- reducing it, if I remember correctly. The 3-view you posted earlier shows forward sweep. If what I wrote earlier about the crew weight being balanced by the H-stab is true, I would NOT try altering the wing sweep. That would make things worse.
And in everything I would consult the designer.
Also one other worry -- as I understand it the designer is Polish, you are Russian and I am American. It's really easy in speaking about things across 3 languages to make a mistake. When I say downthrust on a pusher, I mean that the propeller end of the engine is lower than the forward end. I don't know the term in Polish, or Russian, but please be very sure that when you changed the thrust line in agreement with the designer that you guys both were using the right words with each other. It seems very odd that the engine had a reversed thrust line out of the factory. A drawing, by both the designer and factory would be preferable to words. Just to make sure you guys are on the same page.
Yes, I think I understand now.
I should rephrase, what I was asking.
This is how I will explain this to my Financial advisor - I will tell her that my airplane does have all necessary puzzles. Size and shapes of those puzzles seems to be correct. But, how all this pieces got to put together for different Engine and wing - does not work very well to compete with all different LSA designs. There is no simple adjustments, no easy fixes. It is theoretically possible, but requires a lot of knowledge, that I don't have.
If I end up selling this airplane someday. I should sell it as Quicksilver competitor. This way new owner will keep he's expectations low, without assumption that 100hp will for sure give you 100mph.
This picture was send to me by original designer Jaroslav Dostal from Czech Republic.
3 degrees of downthrust, to me.
I do think you could improve your plane's performance, as others have said, by cowling the engine.
And apologies, Czech, not Polish.
edit: well, cowling the engine might push the CG aft. No easy answers.......
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