mstull
R.I.P.
I noticed a few unusual flight characteristics on my biplane, that I finally came up with an explanation for... if any of you are interested in U/L aerodynamics.
1. My biplane's climb rate is not too impressive. It won't climb at all with heavy pilots. Nor will it climb to high altitudes.
2. There is a very wide range of angles of attack that the plane climbs about the same. I would expect a more definitive sweet spot.
3. When I installed my little, winter fairing, to keep the wind off my legs, the plane climbs much better and cruises much faster.
4. With the winter fairing installed, when I reach cruise altitude, throttle way back, and lower the nose, it keeps climbing almost as well as it did with full throttle. (But in cruise like that, there is a very narrow sweet spot, of angle of attack, where it will climb.)
Seeing the tremendous difference between the 3D polars (airfoil analysis of a wing that has tips) and 2D polars (infinite length wing) forced me to explain that difference. And that led to the explanation for the former flight characteristics.
On a slow flying U/L, wing tip losses affect way more of the wing surface than just near the tip. Picture the lower pressure air on top of the wing like a vacuum chamber. If you open a valve to that chamber, it will almost instantly fill with air. The wing tip is that open valve.
Air is sucked in from the wing tip, causing the air on the upper surface to angle in toward the root. The same, but opposite thing happens on the lower surface as pressure is lost to the wing tip, and the air angles out toward the tip.
This angling air near the tip, effects the air next to it, closer to the root, angling it too, to a lesser degree. With a super low aspect ratio, like my biplane, almost the entire wing area is effected, multiplying this effect. Tip losses are tremendous.
The air blowing out the wing tip from the lower surface, and sucking in on the upper surface, causes a circulation around the tip, increasing the effect of both. In other words, the air sucking in towards the upper surface, helps suck more air from the bottom surface. And the bottom air helps blow more air to the top.
This effect is reduced at lower angles of attack. With high camber airfoils at low angles of attack, the lower surface of the wing has no angle of attack. So there's no pressure under the wing, reducing this effect. And at higher air speeds, the angling air gets swept more straight aft by the faster breeze. So the air angles less and effects much less of the wing area.
My little, winter fairing reduces drag just enough to allow the wing to go into this more efficient cruise mode, where it can climb at low throttle settings.
I'm making little, droop, wing tip fairings to reduce tip losses. I'm optimistic that they will make a significant difference at high angles of attack. I should get those installed this week. I'll attach a picture when I finish them.
Then the question becomes... If my biplane climbs to high altitudes with the new tip fairings, do I really want to take it all apart to try the Lexan sheeted wing? The biplane is soooooo much fun, and so reliable. There'll be no reason to try the new wing. I could just keep the Lexan wing as a spare.
1. My biplane's climb rate is not too impressive. It won't climb at all with heavy pilots. Nor will it climb to high altitudes.
2. There is a very wide range of angles of attack that the plane climbs about the same. I would expect a more definitive sweet spot.
3. When I installed my little, winter fairing, to keep the wind off my legs, the plane climbs much better and cruises much faster.
4. With the winter fairing installed, when I reach cruise altitude, throttle way back, and lower the nose, it keeps climbing almost as well as it did with full throttle. (But in cruise like that, there is a very narrow sweet spot, of angle of attack, where it will climb.)
Seeing the tremendous difference between the 3D polars (airfoil analysis of a wing that has tips) and 2D polars (infinite length wing) forced me to explain that difference. And that led to the explanation for the former flight characteristics.
On a slow flying U/L, wing tip losses affect way more of the wing surface than just near the tip. Picture the lower pressure air on top of the wing like a vacuum chamber. If you open a valve to that chamber, it will almost instantly fill with air. The wing tip is that open valve.
Air is sucked in from the wing tip, causing the air on the upper surface to angle in toward the root. The same, but opposite thing happens on the lower surface as pressure is lost to the wing tip, and the air angles out toward the tip.
This angling air near the tip, effects the air next to it, closer to the root, angling it too, to a lesser degree. With a super low aspect ratio, like my biplane, almost the entire wing area is effected, multiplying this effect. Tip losses are tremendous.
The air blowing out the wing tip from the lower surface, and sucking in on the upper surface, causes a circulation around the tip, increasing the effect of both. In other words, the air sucking in towards the upper surface, helps suck more air from the bottom surface. And the bottom air helps blow more air to the top.
This effect is reduced at lower angles of attack. With high camber airfoils at low angles of attack, the lower surface of the wing has no angle of attack. So there's no pressure under the wing, reducing this effect. And at higher air speeds, the angling air gets swept more straight aft by the faster breeze. So the air angles less and effects much less of the wing area.
My little, winter fairing reduces drag just enough to allow the wing to go into this more efficient cruise mode, where it can climb at low throttle settings.
I'm making little, droop, wing tip fairings to reduce tip losses. I'm optimistic that they will make a significant difference at high angles of attack. I should get those installed this week. I'll attach a picture when I finish them.
Then the question becomes... If my biplane climbs to high altitudes with the new tip fairings, do I really want to take it all apart to try the Lexan sheeted wing? The biplane is soooooo much fun, and so reliable. There'll be no reason to try the new wing. I could just keep the Lexan wing as a spare.
