I know I've heard that negative rigging is a "free" and legal means of picking up 2-3 mph in cruise. But I was wondering what the theory is as well as some specific points.
1. How does negative rigging increase cruise?
2. If -2% is good, is -4% better? (i.e. what are the trade offs)
3. How does negative rigging interact with wing root vortex generators?
4. Can negative rigging negate/reduce drag from gurney (wickerbill) flaps at cruise? (the thinking at the -2 degree setting they are even further into the (lower) boundary layer)
Thanks!
Jeff
Edited 1 time(s). Last edit at 12/25/2007 03:41PM by JeffKing.
By reducing the nosedown pitching moment, which reduces the required taildownload, which reduces the induced drag of the tail and also the loading on the wings, which reduces the induced drag on the wings.
2. If -2% is good, is -4% better? (i.e. what are the trade offs)
It is measured in degrees, not %. Too much is not good. it decreases your stability and makes you work harder to control the airplane in pitch. It may also shift your cruise AOA outside the drag bucket.
3. How does negative rigging interact with wing root vortex generators?
Same as any other aft camberline change interacts with them. Not much difference.
4. Can negative rigging negate/reduce drag from gurney (wickerbill) flaps at cruise? (the thinking at the -2 degree setting they are even further into the (lower) boundary layer)
?? Yes, negative rigging will slightly reduce the gurney drag. What do you mean that the -2 degree setting places them even further in do the lower boundary layer? A negative setting is trailing edge UP, not down. So, with negative reflex the gurney's protrude less into the lower flow. It's a minor difference as far as gurney drag goes.
JimC
P.S. The website software seems to be seriously screwed up. When attempting to reply to a post, it can take up to 20 minutes for the spam prevention code to appear, and even more time to process a reply. On my machine, it seems as though this delay may be related to downloading an image called PIM.gif, whatever that is.
Yes, I meant degrees. So 2 degrees is about the optimal then?
As to the gurney flaps, just as there is a boundary layer on the top, there is one on the bottom (I assume). What I was saying is raising the flap up, (-2 degree), moves the gurney even further into the lower boundary layer, hence slightly reducing the drag at cruise. I've actually read NASA reports that claim the gurney's are not factor and even increase cruise, but even in NASA reports, authors tend to differ on the findings.
So, the tool here would be found on Art's page?, To determine the -2 degree up flap, you'd take the chord of the flap, multiply it by the sin of 2 degrees, and that should be the height offset between the tool and the end of the flap? (i.e with a 12 inch flap chord, you'd multiply 12 by (sin 2degress = 0.035) = 0.42"
That isn't what I said. -2 degrees is the maximum that Piper allows on a Cherokee.
>As to the gurney flaps, just as there is a boundary layer on the top, there is one on the bottom (I assume).<
There is.
>What I was saying is raising the flap up, (-2 degree), moves the gurney even further into the lower boundary layer,<
I don't follow what you are saying. The lower gurney is never outside the boundary layer.
> hence slightly reducing the drag at cruise.<
Drag inside the boundary layer isn't all that great and reflexing won't affect gurney contribution to drag all that much.
> I've actually read NASA reports that claim the gurney's are not factor and even increase cruise,<
What they increase is circulation. And they move the stagnation line clockwise if you are at the wingroot looking out toward the tip. If you are in a condition where increased circulation will reduce drag, then they will do so. If you aren't in such a condition, they will increase drag.
>but even in NASA reports, authors tend to differ on the findings.<
To some extent, that is the nature of science (and engineering) :-)
>So, the tool here would be found on Art's page?,<
Yes. And also described in the Cherokee Service manual. Note that you may not be able to get the full 2 degree flap deflection before the rivets in the flap contact the back of the wing. If that happens, the notch at the base of the johnson bar on the flap latching mechanism may not engage. That can be a limiting factor on the amount of deflection and may be the reason that Piper established -2 degrees as their tolerence limit. Also, it is normal for one flap to 'bottom out' before the other -- there is a lot of variation in individual Cherokee wings.
>To determine the -2 degree up flap, you'd take the chord of the flap, multiply it by the sin of 2 degrees, and that should be the height offset between the tool and the end of the flap?<
Umh, No. These flaps are single-slotted Fowlers. The pivot point is several inches below the bottom surface of the wing, so the trailing edge of the flap starts out going more aft than down and more forward than up when moved. The appropriate reference line is the vector between the radius point of the hinge and the trailing edge of the flap, and if you do it by trig, the offset is approximately perpendicular to that line and the distance used is not 12 inches. One easy way to compute the appropriate offset is to draw the wing airfoil section, flap & flap hinge mechanism, and Art's referencing tool, then reflex the flap -2 degrees in the drawing and measure the difference in Art's tool offset. As an interesting aside, Piper slightly modified the 65(2)-415 airfoil section, removing some of the undercamber from the aft lower surface of the wing. I don't know that they ever documented this, but if you place a full-scale drawing of a 65(2)-415 airfoil up against the end of a Cherokee wing the difference in that area is pronounced. It is possible that they did this to reduce the nose-down pitching moment of that airfoil in order to reduce the stabilator download. But, that would also reduce the stability of the aircraft, and it is sheer speculation on my part.
> (i.e with a 12 inch flap chord, you'd multiply 12 by (sin 2degress = 0.035) = 0.42"<
P.S. My description of flap reflexing above does not apply to the ailerons. They are not fowlers, their hinge line is at their upper forward edge, and their reference line can be set using the top surface of the aileron for the reference. The chord is constant in that case, and if they are adjusted first, the flaps can then be set even with them. Unless of course, one of the flap noses bottoms out against the wing before you get a match (a common occurance). If that happens, you will get to do the adjustments twice, interspersed with some entertaining language. Ask me how I know this...... :-)
Keep in mind that when you get through with all this, the yokes should still be level (tops aligned) after you get through. Since you have to make the changes in increments of 1/2 rotation of the push rod ends, the quantization may not allow for an entirely perfect fit. As an aside, this is also a good opportunity to replace your pushrod ends with new ones (and the bolts, too). They aren't terribly expensive, and can really free up the aileron travel. Also, as the pushrod ends go bad and stiffen up, they cause the bolts to elongate the mounting holes on the aileron fittings, and there is no simple fix for that -- don't let it happen to you.
JimC Wrote:
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> >So 2 degrees is about the optimal then?<
>
> That isn't what I said. -2 degrees is the maximum
> that Piper allows on a Cherokee.
Well, that makes it easy then!
> >What I was saying is raising the flap up, (-2
> degree), moves the gurney even further into the
> lower boundary layer,<
>
> I don't follow what you are saying. The lower
> gurney is never outside the boundary layer.
OK, bare with me here. First we know that the gurney is typically 1-2% of chord. On a Cherokee, that would make it in the 0.65 to 1.3" range.
And my thought here was empirical. Doing tuft testing, at a give airspeed, I've observed the flow at the end of the flap is slightly turbulent/less energized at 0 degrees flap. As the flap is extended to +10 degrees, the tip flow starts to detach (although the region of energized flow begins to move closer to it, the detachment is more pronounced at the tip.)
But that is the top side. I have not observed the flow on the bottom side, but one would think, with a negative flap angle (-2 degrees) the same mechanism might be taking effect there, abet reduced to some degree. Hence, the boundary layer thickens at the bottom flap tip, and the portion the gurney lives in is less energized (less speed) hence less drag.
Not sure if that is right, but hopefully you now understand what I am saying.
> > hence slightly reducing the drag at cruise.<
>
> Drag inside the boundary layer isn't all that
> great and reflexing won't affect gurney
> contribution to drag all that much.
I mentioned this to my AI during annual today and he's all for it (since he sees what it does on a Maule). He can't do it without seeing the Piper documentation that this 2 deg neg setting is allowed. I don't see it in the service manual. What I do see is a template for an adjusting tool. Do I understand that the template dimensions imply a 2 DEG neg? 1967 PA28-180C
Kevin
Edited 1 time(s). Last edit at 02/04/2008 07:51PM by KevinS.
> Do I understand that the template dimensions imply a 2 DEG neg? 1967 PA28-180C <
No. The template dimensions imply a zero setting. However, no measurement is perfect, and if you wind up with settings for control travel that average two degrees up from book, you are still within tolerence. As an aside, both mine and Art Mattson's Cherokees do average 2 degrees reflex. Both are within limits and both have gentle accelerated stalls. Caution -- your mileage may vary.
My hunch is that the choice of 2 degrees as a tolerence limit may have been selected because on a lot of Cherokees, the flap nose bumps the rear spar at about 2 degrees reflex.
Which brings up another question. Piper removed some of the aft undercamber from the underside of the wing -- but, they did not remove it from the wingtip. The result is that the two airfoil sections don't match. Anyone know why?
