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Why USA 35B????

I guess we can call this demystifying the Riblett Airfoil.

So, what is the difference between all these airfoils anyways? Is there a magic Airfoil that can land you ever so slow, yet get you there on a dime?
Like all things in life, (in physics anyway) there is never something that comes for free, to gain something, you need to loose or expend something in return.

I have taken the liking of the Riblett GA 613.5, I heard so many great thing about it that I was all set to go out and build my wings on the airfoil, then it daunted on me, how does it really perform? So I decided at first that I would build a pair of wings and document that, and I might just do that (time, wife, health, and the old mighty dollar permitting) but I wanted to see a rough comparison how this works in computer models, so I went on and made sure that I had the best data possible to compare the Super Cub wing against Riblett or the USA 35B (unmodified) airfoils. I not going to make a full interpretation of the data, but I guess I was more surprised than most when it came to the results.

This is the data after 1000 iterations per variables, I am showing Re 2000000 and Re 8000000 for each of the foils, and I can send the data to anyone, if they chose to graph them themselves, but here is it. Showing Cl vs Alpha, then Cl/Cd then Cl/Cd vs Alpha:


35Bvs35BModvs613_5_Cl_Alpha_zps2ab8a71a.png



35Bvs35BModvs613_5_Cl_Cd_zps98094830.png



35Bvs35BModvs613_5_ClCd_Alpha_zps1c390870.png


Feedback welcomed,
 
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I have flown both airfoils and I prefer the GA 30-613.5
Stalls much softer and a few mph slower, and cruise speed is very close.
What airfoil program are you using?

Sent from my EVO using Tapatalk 2
 
Any twist of a wing more than 2 deg is an indicator of other design problems, in my humble opinion. Jerry Burr clued me into the excessive gap between the wing and aileron, causing stall initiation just outboard of the flaps. The twist in the wings adds a lot of profile drag, and could be eliminated with a few tweaks, resulting in higher airspeeds...I believe the Mountain Goat or Northstar uses an untwisted wing. Anybody know about their stall characteristics vs airspeed? I think you could improve a cub wing by: Putting on more aggressive Frise ailerons to fill the gap, be more effective, and less adverse yaw....(PA-12's?) a cuff on the outboard half of the leading edges...and a stall strip inboard leading edge...I have a design I used on R/C models that is very effective...

sigh, I really need to build another wind tunnel....but, no time, no time....
 
A buddy of mine, an aeronautical engineer and commercial pilot, did his PhD work in this area. He is not a fan of the 35B airfoil and drives 8GCBC. I can say that it is considerably faster than the cub and seems to get off plenty fast. He tows gliders with his as well. Although I didn't understand all he said about it, he seemed to think that the 35B was also not as safe with respect to stall characteristics. His 8GCBC has a CSU which may contribute to the airspeed but he wishes it was a fixed prop! He has his own wind tunnels and has spent a lot of time with this stuff.
 
This data is all coming form Xfoils. Yes laminar flow does continue much longer on the Riblett vs the 35B (Mod or not) and it might be a better airfoil for slots.
:Geureka:​
 
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I don't trust xfoil anymore, when I started looking at old naca tunnel data vs. xfoil, it was giving quite different answers on CL . Most other info was pretty close.
I have run the Riblett ,35B and various naca 4 digit airfoils in the CFD program at work, and the Riblett is a good choice.
Even ask Bob Breeden it flies nice.


Sent from my EVO using Tapatalk 2
 
What CFD is that? Do you have the polars to compare it? I agree that computer models are just that models, but Xfoil has dad a pretty good reputation for laminar flow, it does not give you data on turbulent hair, nor does it account for wing tip loss, but it has had some good results. I am hopping to run the same on FOAM to compare, but this is all I have for now.

I did not know Bob had a 613.5 Airfoil on hi plane. Is this on his current plane? Hi Bob, you out there?
 
SpainCub, I commend you for your investigation into the characteristics of the different airfoils. Have you taken into consideration the different shape which is midway between the ribs? Depending on how and who does the fabric application, that portion of the airfoil can have a drastically different shape. Thus, your calculations could yield a very difference performance once the airplane is flying. I suspect that the actual performance of the wing would be more in relation to the shape of the airfoil which is the average of those two shapes.

Look at some pictures of some early planes which only had a thin leading edge. The sag of the fabric between the ribs was large. The leading edge between the ribs had a fairly sharp entry producing abrupt stall characteristics. Compare those wings with the smooth plywood covered wings on a Bellanca or Mooney.
 
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Wasnt that why decades ago the alaskans experimented with full wrap leading edges to back of the front spar out to the end of the wings? But thats not a standard anymore either?
 
tempdoug said:
Wasnt that why decades ago the alaskans experimented with full wrap leading edges to back of the front spar out to the end of the wings? But thats not a standard anymore either?
Click to expand...
My Backcountry Cub has those full wrap leading edges, with no scallop between the ribs. Since I never flew it without, I can not comment directly on the difference. However, my Cub has very good performance at both ends of the speed range.
 
We use CD-Adapco and a 200 core HPC to run it and it still takes 8 hours for our car model to run.
I will have to dig for the old wing data it was a few years back, that is how I chose my VG configuration and position.
http://www.cd-adapco.com/

Sent from my EVO using Tapatalk 2
 
Also, Breeden's planes have/had a stock airfoil, mine had the Riblett.

Sent from my EVO using Tapatalk 2
 
So here's a thought to start some wild controversy: If I understand correctly, the Maule airfoil is the same as the cub, only the bottom is exactly flat? Why not take that one step further and add about 3/8" convex or "belly" to the bottom of the airfoil? I read in kitplanes awhile back that that is what Randy Schlitter did to the airfoil on the S-7 as he felt he could get better cruise. As I look at the stock USA35B airfoil, this would be easy to do without changing anything until at least 2" aft of the front spar, so my thinking is that the critical part of the airfoil would be left alone.
 
From a previous thread....




Extended Leading Edge skin(ELES)


I spoke with John Roncz, probably the leading airfoil engineer in the US. He is the one who has designed all of Rutans airfoils and others like john Sharp's Nemesis (dominated Formula 1 for many years). The guy is no yahoo, he knows airfoils and aerodynamics. I asked him about the Extended LE skin (ELES). After a long dissertation on the 35B airfoil origins etc his response was that he did not know of any wind tunnel tests, so take it with a grain of salt, but based on what he knew about airfoil performance, he did not think it would have any positive affect. At high AOA the air is only attached at the LE edge of the wing forward of the front spar. Thus any airfoil deformation aft of the forward spar is in dirty air and basically has no affect.
All that said, the ELES, remains an unknown because we have no wind tunnel testing, and furthermore we must add the VG's to the equation.
It does add rigidity, and strength, to the wing. Makes it easies to clean the frost off, and minimizes the scalloping. But it adds weight, 4 to 6 pounds.
Steve Tubbs, Jerry Burr and John Roncz all have expressed reservations as to its effectiveness at high AOA. Others swear by it.


Bill
 
Bill Rusk said:
....... his response was that he did not know of any wind tunnel tests, so take it with a grain of salt, but based on what he knew about airfoil performance, he did not think it would have any positive affect. At high AOA the air is only attached at the LE edge of the wing forward of the front spar. Thus any airfoil deformation aft of the forward spar is in dirty air and basically has no affect.
All that said, the ELES, remains an unknown because we have no wind tunnel testing, and furthermore we must add the VG's to the equation.
It does add rigidity, and strength, to the wing. Makes it easies to clean the frost off, and minimizes the scalloping. But it adds weight, 4 to 6 pounds.
Steve Tubbs, Jerry Burr and John Roncz all have expressed reservations as to its effectiveness at high AOA. Others swear by it.


Bill
Click to expand...
Bill, I highly respect the views of John Roncz and you. His expertise is well known.

If we were to take your information literally, then we would assume that if an airplane had been on it's back, flattening the tops of all the ribs, it would have no effect on the high angle of attack characteristics? Am I misunderstanding something? What happens with the low angle of attack characteristics? The implication is that the shape of the top of the airfoil is irrelevant. What happens to span wise flow with the irregular shaped upper camber between the scallops? Perhaps the flow will remain more completely attached to the upper surface at a higher AOA if the scallop is eliminated?

We need to have some of our GoPro gurus tuff the top of a wing and mount their camera in a position to show us all what is actually happening up there. Then show us the full speed/angle of attack range of characteristics.
 
Fabric sag between ribs documentation...

John Scott
 

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Thanks John, here is the result of the paper...


As far as sag is concerned, the aerodynamic effect, con-sidered as being caused by a reduction in thickness, is negli-gible. In the worst case measured, that of the USA.–35B, theeffect due to a reduction of 0.3 per cent of the maximum thick-ness produces a drop of only ,07 in the maximum value of itslift coefficient,but no appreciable difference in the drag.
*
*


/


It should be noted also that this aerodynamic discrepancytakes place only at the highest angles of at attack, and that forordinary flying attitude the sag effect,even for the abovecase, is of no appreciable consequence.
These considerations lead to the conclusion that the aero-dynamic effect due to sag in airfoils of wood-fabric construc-tion does not warrant the incorporation of sag in a model wing, such error being in most cases within the limits of experimentalaccuracy.
Click to expand...
 
Wayne Mackey said:
Mr. Bill Fike built the ribs that were on the mudhen. I installed them on
cub spars while working for Dave Hendrick's. The wing were built for
another plane but never used. i bought them and used them on the mudhen. They had 80" cord and were made like a univair rib. The
leading edge shape and upsweep from under side was close to that of a
champ. Had about a one inch up camber in the bottom of the wing.
We used cub flaps and ailerons with two inch added to the trailing edge.
I first flew the plane without the slats and it flew good. It may have
been a little faster in cruise but also a little faster in stall. The stalls
were gentle but without much warning. This plane had 233+ square
foot of wing area. With that much wing and them numbers I was not
impressed. When the slats went on it became very gentle and a
great performer.
As far as the cub airfoil I don't have a clue why it was used. There
are many good airfoils out there. I have used a few different ones
and tried a lot of devices and at this point still prefer the cub for the
slow stuff.
I also few the cub with taylor craft wings That ag-pilot flew. It has
large flaps and performed good when lite. With two people and a
lode of fuel I wouldn't use it like a cub. Wayne
Click to expand...

Hello Mike, Hope you are still around here to see if the following is correct, is the wing you are talking about the Astro Wing from Fike?

View attachment The_ASTRO-WING_2.pdf
 

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Fabric Sag Between the Ribs

SpainCub said:
Thanks John, here is the result of the paper...
Click to expand...




The NACA ( NASA ) technical report shows a reduction of .07 from the maximum lift coefficient because of fabric sag between the ribs. The maximum lift coefficient of the US 35B is 1.39. A reduction of .07 brings the maximum lift down to 1.32.

That is a 9.5 % reduction in lift. 9.5 % is a huge reduction in performance. It varies with stall speed, temp and density, but it is still hundreds of pounds of lift. Adding even 10 pounds of sheet metal to the wings is well worth the extra weight.

An increase of 10 pounds at a landing weight of 1800 lbs is not even 1 % (.55 %)

Jonny
 
Jonny, would you have a link for that NACA report? It seems I've read other reports that said no difference from the scalloping. I'm curious!
 
Longwinglover said:
Fabric sag between ribs documentation...

John Scott
Click to expand...




Gordon,

The link is at post #78, incase I can't forward it here.



Screenshot_20200202-103138.jpg

Screenshot_20200202-104142.jpg




Sorry, I couldn't add the link, but managed a screen shot. Interesting how the "verbage" states that "only in the worst case is this reduction of any consequence". Sounds like straight opinion, which might not even have been written by the scientists themselves. Landing at the stall angle could easily be considered "worst case", which is a typical cub landing. The data, which is a fact; shows a drop of maximum lift of .07

1.39 - .07 = 1.32 That is 9.5 %

For people who don't like math: a 9.5 % increase in the weight of a 1100 pound cub is 104.5 pounds.

Jonny
 

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Jonnyo said:
The data, which is a fact; shows a drop of maximum lift of .07

1.39 - .07 = 1.32 That is 9.5 %

For people who don't like math: a 9.5 % increase in the weight of a 1100 pound cub is 104.5 pounds.

Jonny
Click to expand...
That alone is reason enough to do your best at minimizing any scalloping. In fact, it is easier than Bill Rusk's method of taking out 1,672 ounces. ;-)
 
gander said:
I wonder why piper chose usa 35b airfoil?? Clearly there are better foils out there for the application. …..
Click to expand...

They probably used something like this to determine what to use.
I think they did a bang-up job, considering!

20200204_114915.jpg
 

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So, before this discussion even started. some guy( who wound up) in Afton Wy. decided to "improve" the whole cub concept. and what airfoil did he decide to use?
Mind you, he had access to a lot of options (revealed here), and he wound up using what? The whole project was subject to full flight certification testing, so the re-certification testing cost thing is a moot point.
one thing no one can quantify amongst all this data (circles, arrows and a paragraph and a half on the back be damned), is pilotability/ feel for that ragged edge we seem to be drawn to. And,even then, some of that magic that Ms's Jameneau and Piper had found (perhaps stumbled onto) seems to have been lost, or some what compromised in that pursuit.
That airfoil, and the feel for flight that it provides is akin, to what da Vinci and the Mona Lisa brought to the world of art. not perfect, but extremely difficult to exceed.
 
That guy in Afton decided to use the USA 35B which is not the same as Piper's USA 35B "modified". Back in posts #4 & 5 it is indicated that Ms's Jameneau and Piper increased the dimensions of the upper coordinates by 4%. A little thicker. Likely this was for a little more lift at low speeds. Why? They only had 40 horsepower and needed all the lift they could get. Look at the size of the wing on the 2 place Aeronca C-3 with 37 hp. Over time the HP 55, 65, 100, 125, 135, 150 increased, as well as gross weight. A little at a time. Piper's objective was to keep costs down. It costs a lot less to increase the size of the engine than it does to change the design of the major airframe parts. Piper was so pleased with the characteristics of his E-2 Cub wing that he used the same airfoil on all of the various models up through the PA-23. Just look at all of those other models (except the PA-23). Notice how much alike, the design of their parts is.
 
mongo said:
I have flown both airfoils and I prefer the GA 30-613.5
Stalls much softer and a few mph slower, and cruise speed is very close.
What airfoil program are you using?

Sent from my EVO using Tapatalk 2
Click to expand...
On what airframe did you fly this airfoil? cub clone?
 
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