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Extended upper leading edge skins

flywhatever

Registered User
I'm looking for folks wisdom or reason, why cub builders are extending the upper surface leading edge skins...I've heard why but so far am not buying it. But...well known, experienced builders are doing it, but so far I've not heard a credible answer....
 
It is my understanding that they don’t provide any difference. Other than adding weight. According to John Roncz, probably the world’s premier airfoil engineer and designer, they don’t work. According to a conversation I had with him there is no added benefit, aerodynamically, whatsoever.
There are quite a few “old wives tales” in the bush flying community.
Sorting fact from fiction can be a challenge.



Bill
 
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My understanding is it was started when guys were concerned with scalloping, of fabric, between the ribs. The extended metal is supposed to keep the airfoil true as opposed to the belief fabric would somehow would not in all conditions. Improperly applied and shrank fabric can cause ballooning between the ribs reducing the efficency of the airfoil. Properly applied the fabric pops level with the wing rib as soon as the wing gets some air moving over the top. One does need to be careful while sweeping snow off the wings between the ribs because denting the trailing edge will show. Some will brake an edge on the trailing side for strength
 
Have extended leading edges. I think they increased my cruiser speed 10kts! Decreased stall speed the same!
Believe it or not!not!
 
I was indifferent about extending the LE back until I saw a picture of the fabric puffing up above the ribs of a familiar plane on short final nearing touch down. The contour of the top of the wing changed at the most critical moment of flight. I don't see any reason to let that occur if I can prevent it.
 
I have no opinion on extending the leading edges, but cleaning up the dents and working over the leading edge on my ag plane made a huge difference in handling and performance. If I was determined to add 7 lbs to the wings, I’d look at thicker leading edges vs extending them.
 
I was indifferent about extending the LE back until I saw a picture of the fabric puffing up above the ribs of a familiar plane on short final nearing touch down. The contour of the top of the wing changed at the most critical moment of flight. I don't see any reason to let that occur if I can prevent it.

is there a way to find this picture?
Hi. I finally found the picture that says it all about extended leading edges. You have to d/click and blow it up to see what I mean. It is posted on S.C. Galore, Page 19, Item #1. or picture #163. A picture is worth 3,000 words. Jerry Burr
 
I remember the pic Jerry referenced. It was a Cub in normal flight and had reverse scalloping. The photo I'm referring to was from an overhead video of a slat winged Cub. The fabric is unremarkable until the moment ahead of touch down when it clearly lifts. Would it affect lift? Can't say, but it can't help. Would it affect durability? It must. How many cycles of that pulling on the rib lacing can it take? I saw this vid after Mike and I extended my metal. Where this other plane's fabric changed is exactly where my extended metal is.

I've got friends who've used fabric bands and carbon fiber bands to combat the scalloping. I couldn't care less about the scalloping. I'm focused on the fabric lifting. I have a big, heavy Cub hanging under very high lift wings. What Piper did isn't necessarily the right thing for evolved Cubs.
 
Fabric covering moves up and down in flight. Under normal load or climb the top and bottom wing coverings move up; in a negative gust they move down. In a stall the lower wing cover is pushed up while the upper cover "probably" un-scallops from rear wing to front as airflow separates and lift is lost. The horizontal tail top cover is usually pushed down in flight to offset aircraft weight forward of the center of wing lift. Look at the pics below. There's more scalloping at the wing root than tip which reflects normal span wise lift distribution.

We've all seen some of that (hopefully). The tighter the fabric shrink or the warmer the air the less the movement in my experience. The lower covering moves lots in real cold. Unless one surface moves more than the other lift should remain relatively equal.

Speeds may change if the airfoil changes basic shape. I'll not argue for or against any benefit in cruise by reducing scalloping unless there's added drag to reduce or extra lift for the tail to correct for. In a pre-stall high AOA the leading edge is carrying most of the load. I'd not want to create sudden unexpected flow separation by radically changing upper wing shape.

Gary
 

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My understanding is it was started when guys were concerned with scalloping, of fabric, between the ribs. The extended metal is supposed to keep the airfoil true as opposed to the belief fabric would somehow would not in all conditions. Improperly applied and shrank fabric can cause ballooning between the ribs reducing the efficiency of the airfoil. Properly applied the fabric pops level with the wing rib as soon as the wing gets some air moving over the top.
All fabric will rise between the ribs whether applied properly or poorly. The question should be to what amount does it rise? If you ever flew in a fabric covered low wing airplane or closely along side a high wing airplane this would be obvious.

It is my understanding that they don’t provide any difference. Other than adding weight. According to John Roncz, probably the world’s premier airfoil engineer and designer, they don’t work. According to a conversation I had with him there is no added benefit, aerodynamically, whatsoever.
There are quite a few “old wives tales” in the bush flying community.
Sorting fact from fiction can be a challenge.

Bill
I have a great deal of respect for John Roncz and his work with aerodynamics, particularly airfoils. I envy Bill for having had an opportunity to have a discussion with him. The very early airplanes only had a small straight edge for their leading edges which resulted in drawn in scalloping over the entire wing. Along the way this evolved into what we know as a leading edge. We all know that the shape of the leading edge dictates what the airflow does as it moves aft. What determines at what point this leading edge ceases to be effective to the flow of the air as it moves aft? This would be different with different airfoil shapes. On those early wings, at what point did the inward scalloping transition to an outward pulling scallop? Where is this transition on a stock Cub wing? Anyone, as I have no clue? As a general rule of thumb it is my impression that the aft edge of the leading edge skin should be located just aft of the thickest point of the airfoil shape. This would ensure a constant airfoil shape over the entire wing. Any less would mean a loss of available efficiency.

Many here have installed VGs on the leading edges of their wings. Do they know Why? VGs effect the airflow as it moves aft across the wing. What happens when the trialing edge of the leading edge skin is moved aft a little bit? A little more? Will this effect your VG function?

We all know that a stall begins with the airflow separation at the wing trailing edge and then progresses forward. This separation is determined by the shape of the flow of the air starting at the leading edge. Somewhere as the air moves aft the shape of the upper camber of the airfoil ceases to make much difference. It seems to me that this is where the trailing edge of the leading edge ought to be.

To be able to accurately answer the question of where should the trailing edge of the leading edge be located, we would have to have access to a computer program which would act as a visual wind tunnel with streamlines.

The wings on my Cub came with the extended leading edge skin. I'd have to measure it to tell you how much. I covered the entire leading edge with a thin layer of felt to make the rivets almost disappear producing a smooth leading edge over the entire wing. I don't know whether the felt helped or not. It does look nice. I can say that the stall characteristics and the ability to fly at a very low speed are excellent. This wing is clean with no VGs or other devices.
 
Somebody should take some drone footage at a big stol contest. Get a bigger sample group in a standard setting.
 
I am curious about the scalloping on the latest versions of fabric when the wings are covered with the blanket method. The last 2 revisions of fabric seem to tighten up long ways more the previous versions with not much if any scalloping. Wll have to get some air to air photos to see.
 
I am curious about the scalloping on the latest versions of fabric when the wings are covered with the blanket method. The last 2 revisions of fabric seem to tighten up long ways more the previous versions with not much if any scalloping. Will have to get some air to air photos to see.
I don't know what sequence you use when you shrink the Dacron. I've found that when you do your initial shrinking over the ribs alone, this shrinks the fabric span-wise without being able to pull down between the ribs. When you shrink in the center of the bays first the fabric tightens between the leading edge and the trailing edge first pulling the fabric down. The first method maintains as straight a surface as possible. The second method make scallops which can never be raised.
 

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I was not able to make the NACA study link work. Here it is again.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081179.pdf

View attachment 45442."

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"The results of these tests indicate that the usual sagging of the wing covering between ribs has a very small effect on the aerodynamic characteristics of an airfoil"


Bill




???
Bill, i think the concern is with the fabric being able to rise above the top surface of the rib more than staying below the surface. or the wing being thicker than the rib profile, how will it effect things, so if the fabric is down between the ribs will it be able to be sucked up that much over the top of the ribs in between bays?? a video camera would tell if thats happening. ive never looked or thought about it where some of these guys have. i remember Darrell Starr showing how his engine cowling puffed out in flight, and air getting by his engine baffles. always thought that was weird.
 
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I don't think it will have any affect at low AOA like cruise, at high AOA the air is only attached at the very leading edge so even if it billowed a little it is in dead air and will have no affect. I'm sure you can find a picture of someone with very loose fabric, or zero rib stitching, with a huge billow and interpret that to mean whatever you want. The reality is this is just not an issue.....IN MY OPINION, based on the research I did before I removed the extended LE that came on my Smithcub.

Best regards

Bill
 
Any one know if Piper changed the LE on the PA-25 Pawnee? It might be interesting to find out as they would have had a choice if there was a chance for better performance. Plus they could watch the upper fabric when bored.

Gary
 
I was not able to make the NACA study link work. Here it is again.

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930081179.pdf

"The results of these tests indicate that the usual sagging of the wing covering between ribs has a very small effect on the aerodynamic characteristics of an airfoil"


Bill
Bill, I read this report and did note the conclusion as you stated. The original question was asking about extending the leading edge aft of Piper's original location. This NACA report does not discuss that question at all, only the sag and lift between the ribs. Moving the leading edge aft will effect the shape of that sag. If that NACA report had discussed the differences between a leading edge skin of an unknown dimension and no leading edge skin at all, then we may have received an answer to the original question.
 
I put this post in the wrong thread - the one about rib stitching extended leading edges. I'm sorry; it really only belongs here, but it's duplicated.

My -12 today, in steady-state level flight. The sun is low, so it's easy to see the contours on the top of the wing when zoomed in. The fabric is very slightly convex. Maybe 1/4" Maybe? I would argue that the fabric scallops we see on the ground are a non-issue. I did a hard turn (2 g?) to the left and Andrew, the photographer in his -12, saw very little if any change in the fabric contour from that in level flight.
Nov 4, 2019 over Longview, Wa.   Photo by Andrew C..jpeg
 

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You guys can go ahead and tell yourselves that scallop doesn't matter all you want. My experience and farmer sense tells me otherwise. The issue to me is reduced airfoil profile between the ribs that trends towards a cruise profile rather than lift. Compare a Cessna airfoil to a Beaver for high lift vs. cruise. Ballooning fabric at 40-95 is not the same as having the full airfoil at 25 when you're trying to get airborne in the shortest distance. MHO.
 
I think we are debating the difference between shades of gray here. Seems to me that the airfoil between the ribs on fabric covered single engine airplanes is going to maintain the same airfoil in proportion whether it sags or not. And the amount of sag or expansion is truly minimal. I’ll wager that any differences in lift (between ribs as opposed to at the rib itself) are hardly worth measuring, if they’re even measurable.

Having said that, it sure was fun working the field experiment with Gordon! We have found a new excuse for our wives: “Hon I’m going flying today—Science demands it!”
 
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