Airfoil selection for an aircraft out there?

[WARPilot]

I’m 5’10”, 200# and fit in the cockpit. It is small and could use a 1/2” here and there.
We do limit the number of buyer pilots with a cockpit that doesn’t have a lot of room. I hope to gain some tiny amounts.
Upscaling adds issues also. Materials and weight being two.
The current WAR aircraft are actually around 57-58% scale.

 

[BJC]

The originals are not terribly large airplanes, and linear scaling much smaller than 80% to 85% results in an unusable cockpit volume.

The current WAR aircraft are actually around 57-58% scale.

Not a WAR, but a 60% scale P-47 under construction here fits the Verner well, and has a roomy cockpit.


BJC

 

[BigL]

Billski has a good a point about the looks only having to be reasonably close. The 'make-up' will cover the imperfections. Now, given the choice of a true-to-scale plane with all the bad behaviors of the original, or one that I could fly and have fun in - I'm going to choose that which is easier to fly. Being realistic, I'm not young and brave, and I don't have to fly this thing because my country gave it to me to dominate the world in.

Consider our progress to date. We have gone from a 230XX with high drag and a stall that has 'attitude', to a NACA with less drag but still a stall that could bite, especially if the tip section is thinner than the root. Then the Riblett is this modern airfoil that fixed the problems of the the NACA stall. Billski has made a suggestion to even increase the Mean Line (Camber percentage) of the tip. Say progressing from a 37A-315 to a 37A-415. What is happening at the tip rib is that the behavior past the 'stall' (if you could even call it that) is becoming more benign - the fall-off of Lift Coefficient (Cl) starts at a greater Angle of Attack (AoA) and is 'softer' still, which makes for a well behaved wing. An interesting cosmetic point about the -415 is that its underside is flatter than the -315. Maybe that would satisfy the 'looks' aspect.

Chris did ask about reducing the tip thickness which has some Aero Engineering basis when considering that the thinner airfoil produces slightly more lift and less drag. Yes, shock and horror - well at least down to a certain point according to NACA (see Theory of Wing sections; referred to here as TOWS). Considering now the NACA 6 series, the down-side is that the thinner airfoil is stalling at a much lower AoA. That equals Tip stalling first. So although one or two aspects might look good, the 'sum of the parts' adds up to a less appealing aircraft.

I'll throw in a bit of controversy now. With the 'soft stall' you get using the Riblett, stalling isn't a 'break' any longer (if not aggravated) - more of a 'mush' making flying on the 'backside' a lot safer, and with all that power out front, do you really need flaps? OK we do have a 14-degree limiting 'three-point' landing stance, so this consideration would better fit a 'bush plane'. The LSA Bearhawk is capable of being flown like this ("look mom, no flaps").

In short - a good aircraft is going to be a compromise between all that we want and a bunch of things we have to live with.

 

[Heliano]

One more way of getting the root to stall first: slotted ailerons like frise-type retard
stall along the span of the ailerons.

 

[Heliano]

Sorry, folks. Pressed the wrong key here and the image was repeated.

 

[BJC]

Sorry, folks. Pressed the wrong key here and the image was repeated.

Still have time to edit them out?


BJC

 

[PiperCruisin]

I would expect more people to say "Holy Carp! BigFoil is awesome!" Anyhoo, for fun, attached is a comparative analysis of the GA(U) 37A316 vs the NACA 64A316 and the S8036 (I like it but now we just need a volunteer to try it.). The data created comes from JavaFoil which may not be the best, but produces some smooth plots and I'm less concerned with quantitative accuracy vs. comparative analysis.

 

[wsimpso1]

Can't argue with your logic or priority placement. I'm only thinking that airfoil shape can have a big visual impact. Only to purest perhaps . . . ?

The P-51A through D models is the 45-100, and its coordinates are hard to come by, but it has a leading edge radius that looks like it is about 1%c. That is pretty small. No one on here is attempting to persuade you to use the 45-100.

All of this is your choice, I am just trying to make sure you have all of the pieces to the puzzle. I am concerned that a bunch of other stuff will not scale quite right and that this one topic is distracting you from the rest of them. Span to fuselage length will likely not scale, nor will wing to tail proportions. Cockpit area is likely to be big, and so on.

23015 leading edge radius is 2.48%, 23012 is 1.58% and the 63-x15 64-x15, and Riblett 37A315 all have 1.58%. At the tip, there is no difference, and at the root, yeah it would be small, but let's look at the rest of the wing. Tank put on a speed strake, mostly to fit the main landing gear wheels, and its proportions seem to have been chosen for that purpose. You probably will face that challenge too. You can certainly put on any leading edge radius on the strake you want at the fuselage and blend it to the airfoil to make things look right.

If you go ahead with 23015/23012, be prepared for tip stalls and other bad behaviour. Find that in your tests and you get to choose between a clean airfoil that looks right but behaves badly, or the same foil with some seriously non-period correct stall strips at the root, VG's on the outboard part of the wing, and who knows what else to make it behave. I doubt that either result will be desireable. Maybe a clean wing that behaves well is worth what I believe is a subtle looks issue.

Alternatives are still the Riblett laminar flow foils (35A315 and 37A315) and Riblett turbulent foils (30A315). Compare them to the 23015 and ask yourself if this distinction really matters. The leading edges are not that much different. The strake at the fuselage can further help with the feel of the real thing. Then get on with how to make the planform look right when you have the fuselage big enough for people, the wing big enough for your desired stall speed, and the tail big enough to control the whole thing. Then comes structures. Have fun.

Billski

 

[wsimpso1]

I would expect more people to say "Holy Carp! BigFoil is awesome!" Anyhoo, for fun, attached is a comparative analysis of the GA(U) 37A316 vs the NACA 64A316 and the S8036 (I like it but now we just need a volunteer to try it.). The data created comes from JavaFoil which may not be the best, but produces some smooth plots and I'm less concerned with quantitative accuracy vs. comparative analysis.

Cool outputs. Mach number for Raymond is about 0.15, probably won't make much difference.

 

[raymondbird]

The P-51A through D models is the 45-100, and its coordinates are hard to come by, but it has a leading edge radius that looks like it is about 1%c. That is pretty small. No one on here is attempting to persuade you to use the 45-100.

All of this is your choice, I am just trying to make sure you have all of the pieces to the puzzle. I am concerned that a bunch of other stuff will not scale quite right and that this one topic is distracting you from the rest of them. Span to fuselage length will likely not scale, nor will wing to tail proportions. Cockpit area is likely to be big, and so on.

23015 leading edge radius is 2.48%, 23012 is 1.58% and the 63-x15 64-x15, and Riblett 37A315 all have 1.58%. At the tip, there is no difference, and at the root, yeah it would be small, but let's look at the rest of the wing. Tank put on a speed strake, mostly to fit the main landing gear wheels, and its proportions seem to have been chosen for that purpose. You probably will face that challenge too. You can certainly put on any leading edge radius on the strake you want at the fuselage and blend it to the airfoil to make things look right.

If you go ahead with 23015/23012, be prepared for tip stalls and other bad behaviour. Find that in your tests and you get to choose between a clean airfoil that looks right but behaves badly, or the same foil with some seriously non-period correct stall strips at the root, VG's on the outboard part of the wing, and who knows what else to make it behave. I doubt that either result will be desireable. Maybe a clean wing that behaves well is worth what I believe is a subtle looks issue.

Alternatives are still the Riblett laminar flow foils (35A315 and 37A315) and Riblett turbulent foils (30A315). Compare them to the 23015 and ask yourself if this distinction really matters. The leading edges are not that much different. The strake at the fuselage can further help with the feel of the real thing. Then get on with how to make the planform look right when you have the fuselage big enough for people, the wing big enough for your desired stall speed, and the tail big enough to control the whole thing. Then comes structures. Have fun.

Billski

Wonderfully insightful info and thanks again Sir. You are the absolute best! The OP must be very grateful as well. Would like to see some front angle pictures of your wing or will look for others. I think the SAL Mustang uses same laminar Riblett airfoil so will be good to have a look at that.

Bird

 

[BigL]

Looking at FW190 footage I found reference to a 'D' model which used a V12 engine, with increased engine cowl length and increased rear fuselage length.

The W.A.R. FW190 three-view posted in this discussion suggests only 2 foot of cowling length and a horizontal tail area of less than 20%. Would going to the 'D' model better suit the needs for stability and cooling of the UL engine with a prop extension?
I do note that the fuselage width of the W.A.R. definitely doesn't look true to scale.

My apologies for not giving my thumbs-up to many of the valuable posts found in this thread. Some were really spot on and some good ones were good because they brought considerations to light that have encouraged more discussion. Thank you every one.

 

[raymondbird]

Looking at FW190 footage I found reference to a 'D' model which used a V12 engine, with increased engine cowl length and increased rear fuselage length.

The W.A.R. FW190 three-view posted in this discussion suggests only 2 foot of cowling length and a horizontal tail area of less than 20%. Would going to the 'D' model better suit the needs for stability and cooling of the UL engine with a prop extension?
I do note that the fuselage width of the W.A.R. definitely doesn't look true to scale.

My apologies for not giving my thumbs-up to many of the valuable posts found in this thread. Some were really spot on and some good ones were good because they brought considerations to light that have encouraged more discussion. Thank you every one.

Looks better too! Especially that round leading edge -gig

 

[raymondbird]

One more way of getting the root to stall first: slotted ailerons like frise-type retardView attachment 118837
stall along the span of the ailerons.

Hey, never heard of this benefit before. All you hear about is the less adverse yaw benefit. Any idea how much of an effect it is? I want to build my wing without wash-out, so looking for all the help I can get. Thanks for sharing!

 

[WARPilot]

Looks better too! Especially that round leading edge -gig

That would be an easy change. Engines are available and it would only require modest changes.

 

[3V152]

An interesting example of how using computer generated airfoil data can vary from Wind Tunnel results was kindly supplied in the form of the BigFoil.com generated analysis for the Riblett airfoil with 35% of Chord flap. I tried to find one for the NACA 63-215 from BigFoil.com for 20% of chord to compare with the NACA Wind Tunnel data for 60-degrees of Flap, but the program was already having trouble with the 25% of Chord and just 25-degrees of Flap. I will try to attach a photo from the Theory of Wing Sections for the NACA airfoil used as an example in an earlier post. From it we can compare the computer generated characteristic with that of the test results. Both examples are for a Reynolds Number of 6-Million. The 63-215 is similar to the Riblett but none of the computer generated results come close to the NACA tested Flap Stall Angle of Attack of 10 degrees at 60 degrees of Flap.

My special thanks to PiperCruisin for bringing this to light.

Another example of wild results generated by computer analysis such as the JavaFoil presented in BigFoil.com is for the USA 35B. If we used this program to choose an airfoil for the WAR FW190 we would choose the USA 35B airfoil with a claimed Coefficient of Lift (Cl) of about 1.8. In real Life it returned a Cl of only 1.2. We could have become very misled by the program. That being stated, sometimes the program responds well to the airfoil shape and gives a close result. I am referring to the Cl vs Alpha diagram for the Riblett GA 37A215.

BigFoil is commended for gathering so many results together in one place and especially where presentation of the Wind Tunnel results was included along with the computer generated data. Although the wind tunnel Cl vs Alpha for the NACA 63215 is not labeled correctly.

Computer generated data can be useful for comparison purposes. If we compare the Riblett to the NACA the difficulty the program had with supplying a full set of returns for the NACA with a 25% of Chord flap shows that the two airfoils behave differently. That carries some merit.

Keep in mind that the NACA-TR-824 presented wind tunnel data for NACA63-215 is for a split flap, not a plain flap as calculated by JavaFoil and presented at https://www.bigfoil.com
I decided not to digitize the split flap data because I can't generate reliable computer results that will match them, and they aren't particularly popular compared to other styles of flap. Any computational method, be it JavaFoil, XFoil, or full-blown Navier-Stokes CFD, will struggle with the large amount of separation inherent to flapped configurations and near stall.

From the JavaFoil website: JavaFoil
" As with all engineering computer codes, it is up to the user to judge and to decide how far he wants to trust a program. Because JavaFoil does not model laminar separation bubbles and turbulent flow separation, the results will be incorrect if larger areas of flow separation are present. Massive separation, as it occurs at stall, is modeled to some extent by empirical corrections, so that maximum lift can be predicted approximately for "conventional" airfoils. If you analyze an airfoil beyond stall, the results will be quite inaccurate. It is questionable, whether any two dimensional analysis method can be used at all in this regime, as the flow field beyond stall is fully three dimensional with spanwise flow and possibly strong three dimensional vortices."

Here are some early results from bigfoil.com validation study:

New airfoil website

Hello everyone, I was trying to find a nice airfoil my homebuilt plane, then things got a little out of hand and I ended up making a website with all of the airfoils I could find, all of their JavaFoil-computed data for varying Re, Mach, roughness, flap setting, etc... and where I could find...

www.homebuiltairplanes.com