# Roncz FW Airfoil as Pinch Hitter

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#### StarJar

##### Well-Known Member
In trying to design a flying wing ultralight I found that sometimes something that doesn't make sense can be the perfect answer in a different situation.
What I'm getting at is I have a situation where I want to run the main spar behind the pilot. That puts it at about 50% back on the 8ft. root-cord.
I know some of you are going to have a lot of ancillary questions and preliminary objections, so I'll answer those as they come, but here's the question I am really seeking.....Can I use the Roncz FW airfoil even thpugh I'm not using it on a laminar wing, but instead a fabric one.
Now I don't beleive any of you really know the answer for sure, but the more I toy with this airfoil, the more seems to be the lucky pick to complete this design.
Come back and I will have some pics here for your veiwing pleasure...Sorry couldn't find no coppyable pics of the Roncz Low Drag Flying Wing Airfoil.<<<<<link!!!----+

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#### ypsilon

##### Well-Known Member
Putting the spar at about 50% of the root chord, does that mean that you end up with 50% torsion nose, or do you want to have the spar carrying the torsional load, which would scare me.
I don't know which roncz section you are considering, but those laminar foils are picky about exact implementation and can be a disaster for fabric covered implementations. This ain't straight forward.
What makes you think it's a lucky pick?

#### StarJar

##### Well-Known Member
Putting the spar at about 50% of the root chord, does that mean that you end up with 50% torsion nose, or do you want to have the spar carrying the torsional load, which would scare me.
I don't know which roncz section you are considering, but those laminar foils are picky about exact implementation and can be a disaster for fabric covered implementations. This ain't straight forward.
What makes you think it's a lucky pick?
Oh so your saying that xfoil data should be not trusted on fabric. Doesn't add up.

The torsion of the non detachable stub is easilly transmitted to the fuselage. You follow? The spar enters the outer 10 ft panel, crosses the Local nuetral point and reaches the LE.
The result is the skin carries a bit of torsion but the spar carries hardly any in normal flight. In high G's the Cp shifts anywáy, it can all shift, so the spar you didnt once like now saves your a. So dont be scared by rumors from know it alls' scary but ignorant dissertions.

#### Himat

##### Well-Known Member
In trying to design a flying wing ultralight I found that sometimes something that doesn't make sense can be the perfect answer in a different situation.
What I'm getting at is I have a situation where I want to run the main spar behind the pilot. That puts it at about 50% back on the 8ft. root-cord.
I know some of you are going to have a lot of ancillary questions and preliminary objections, so I'll answer those as they come, but here's the question I am really seeking.....Can I use the Roncz FW airfoil even thpugh I'm not using it on a laminar wing, but instead a fabric one.
Now I don't beleive any of you really know the answer for sure, but the more I toy with this airfoil, the more seems to be the lucky pick to complete this design.
Come back and I will have some pics here for your veiwing pleasure...Sorry couldn't find no coppyable pics of the Roncz Low Drag Flying Wing Airfoil.<<<<<link!!!----+

The result of using a ”laminar” airfoil built to standards that make it ”non laminar”?

That is how I interpret your question about using a Robcz airfoil when building a fabric covered wing. My guess is that this frequently happen on light aircraft and I know it’s practiced on radio control models. Depending on the chosen airfoil this may or may not work, and it’s question on how well. At the best the performance will be as good as “turbulent” airfoil. If it is one of those “laminar” airfoils that do not work in rain or when bugged down by insects it will probably not work well. But then, that would be a bad choice anyway. Notice “laminar flow” airfoils are most so to a distance from the leading edge. Usually the flow transit to turbulent somewhere aft of max thickness on the top surface and even further aft on the bottom surface.

Common practice is to put the wing spar at the thickest point of the wing section. A lot of older airfoils had their thickest point 25% to 35% from the leading edge and that did set the position of the spar. If the thickest point is at mid chord the spar might go there. If strength, flutter and aerolastic effects are ok the wing should be ok with the spar at 50% from the leading edge. If there is an objection to a wing with a more aft placed spar it is that it may come out slightly less structural efficient. That is heavier. But this is a trade of, the airplane may come out at the same weight anyway.

#### Marc Bourget

##### Well-Known Member
Himat said: "The result of using a ”laminar” airfoil built to standards that make it ”non laminar”?"

By this did you (also) mean that the dimensions of most laminar airfoils are so "rigid" that adequate compliance sufficient to maintain "laminar" performance is not achievable with fabric construction methods?

mjb

#### Retiree

##### Well-Known Member
Himat said: "The result of using a ”laminar” airfoil built to standards that make it ”non laminar”?"

By this did you (also) mean that the dimensions of most laminar airfoils are so "rigid" that adequate compliance sufficient to maintain "laminar" performance is not achievable with fabric construction methods?

mjb

Yes, laminar airfoils need to be constructed to a very precise tolerance. This is the beauty of fiberglass wings constructed from female molds that are machined to very accurate precision.
If you want to play it safe, trip the airfoil to turbulent flow about 5% from the leading edge. This will be the worst performance you are likely to see from the airfoil in practice.
Doug

#### wsimpso1

##### Super Moderator
Staff member
I am a bit confused by your intent here.

You are building an airplane that needs the spar through centerline at about 0.5 C. It is fabric covered and so will be a turbulent flow.

You have found a laminar flow foil at is 12% thick, and has max thickness at 43% C, and you want to know if it is OK for this... Hmm, first off, do not get all hot over optimizing little details. If it does not make a better airplane, it is not an improvement. I suspect that grabbing this particular foil because it is thick aft is missing the goal of a better airplane.

I will not try to tell you either way if it is OK aerodynamically, but I will tell you that it will be turbulent flow and its stall behaviour may be "interesting".

Structurally, you will have to do all of your own homework. I will issue a set of serious cautions:

The foil forward of the spar, regardless of how far back the spar is, will impose lift and moments upon the other structures. With the main spar so far aft, these loads will be much larger than we usually see, and may require some extra effort to work out a structure that does the job at reasonable weight. You will do well to investigate conventional ribs and D-section, a forward spar that is pinned to the cockpit area, and aft spar that is pinned, etc. If this is a strut supported wing, the spar EI required as you approach centerline drops, but general beefiness and the GJ required rises because the you have to support all of that wing forward of the spar;

If the airplane is a swept wing design, remember that there will be pitching moments (Center of lift of the 3 D wing is aft of the wing cross section near the centerline), and figure out how your structure will handle them. With the spar so far aft, these pitching moments may result in significant nose-up twist of the tips under positive g, exposing you to tip stall... Unless I were dealing with structure that is already going to be very stiff torsionally, I would run the spars at more conventional positions until close to the centerline.

No matter what thickness the spar, you have to design it to be strong enough and stiff enough in spite of the depth the airfoil at the spar position.

SO, why not pick a turbulent flow foil that is thick enough that you can have a a reasonably light spar system anyway. The easiest way to do this is to go with a low cambered higher thickness foil. As an example, the 0015 foil is 13.3% thick at 0.5C, and so is deeper than the Roncz foil cited at the main spar near centerline, and can be much more efficient structurally (deeper and thus lighter) once you are outboard a bit and can run the spar at 0.3C.

Billski

#### StarJar

##### Well-Known Member
There are many cumulative things involved. I started working with another airfoil.

#### StarJar

##### Well-Known Member
I am a bit confused by your intent here.

You are building an airplane that needs the spar through centerline at about 0.5 C. It is fabric covered and so will be a turbulent flow.

You have found a laminar flow foil at is 12% thick, and has max thickness at 43% C, and you want to know if it is OK for this... Hmm, first off, do not get all hot over optimizing little details. If it does not make a better airplane, it is not an improvement. I suspect that grabbing this particular foil because it is thick aft is missing the goal of a better airplane.

I will not try to tell you either way if it is OK aerodynamically, but I will tell you that it will be turbulent flow and its stall behaviour may be "interesting".

Structurally, you will have to do all of your own homework. I will issue a set of serious cautions:

The foil forward of the spar, regardless of how far back the spar is, will impose lift and moments upon the other structures. With the main spar so far aft, these loads will be much larger than we usually see, and may require some extra effort to work out a structure that does the job at reasonable weight. You will do well to investigate conventional ribs and D-section, a forward spar that is pinned to the cockpit area, and aft spar that is pinned, etc. If this is a strut supported wing, the spar EI required as you approach centerline drops, but general beefiness and the GJ required rises because the you have to support all of that wing forward of the spar;

If the airplane is a swept wing design, remember that there will be pitching moments (Center of lift of the 3 D wing is aft of the wing cross section near the centerline), and figure out how your structure will handle them. With the spar so far aft, these pitching moments may result in significant nose-up twist of the tips under positive g, exposing you to tip stall... Unless I were dealing with structure that is already going to be very stiff torsionally, I would run the spars at more conventional positions until close to the centerline.

No matter what thickness the spar, you have to design it to be strong enough and stiff enough in spite of the depth the airfoil at the spar position.

SO, why not pick a turbulent flow foil that is thick enough that you can have a a reasonably light spar system anyway. The easiest way to do this is to go with a low cambered higher thickness foil. As an example, the 0015 foil is 13.3% thick at 0.5C, and so is deeper than the Roncz foil cited at the main spar near centerline, and can be much more efficient structurally (deeper and thus lighter) once you are outboard a bit and can run the spar at 0.3C.

Billski

#### Himat

##### Well-Known Member
Himat said: "The result of using a ”laminar” airfoil built to standards that make it ”non laminar”?"

By this did you (also) mean that the dimensions of most laminar airfoils are so "rigid" that adequate compliance sufficient to maintain "laminar" performance is not achievable with fabric construction methods?

mjb

Yes, I presume that at the Reynolds number light aircraft operate at a fabric covered wing do not get laminar flow due to the construction method.

#### ypsilon

##### Well-Known Member
Oh so your saying that xfoil data should be not trusted on fabric.
Depending on the section: Yes

So dont be scared by rumors from know it alls' scary but ignorant dissertions.
Assuming this is part of the reply to my comment: Don't worry. I am not scared of other people's design, as long as I am not forced to take a ride.
If, however, this is just some can of self-reassurence: Go ahead, and don't worry either.

Read again what wsimpso1 had to say, and come back if you want more details

#### Topaz

##### Super Moderator
Staff member
Just to add a little to what Billski had to say, the biggest concern you should have in an airfoil right now, based on the sketches you've shown earlier, is whether it has a pitching moment compatible with your twist distribution, so that the airplane trims at the design speed with no elevator/elevon deflection. If you don't match up these parameters, the airfoil will be "fighting" the twist distribution and you'll be carrying drag-producing elevator/elevon deflection when you don't want it.

This is an efficiency-of-flight issue, not a safety-of-flight issue, but you should be able to determine the necessary pitching moment for your "ideal" airfoil when you do a trim plot of the aircraft across the design speed range. In most cases, it's not going to be terribly critical, but it's quite easy to pick an airfoil that does cause problems if you don't at least have a desirable Cm range in mind. It's not a matter so simple as picking a "flying wing" airfoil, because there are different kinds of "flying wings" and airfoils are optimized differently for the different types. An airfoil set up for a flying plank would not be the same as one set up for a swept-wing tailless aircraft, for example.

Beyond this concern, just follow what Billski said.

As an aside, if you're going to use a laminar section with a build method that does not produce laminar-flow tolerances, be sure that the airfoil data you're using is for that 'foil in dirty conditions ("standard roughness") and not the data for the airfoil in laminar conditions. Even modern laminar flow airfoils suffer performance degradation in all parameters - including maximum lift coefficient - when operating in turbulent flow.

#### StarJar

##### Well-Known Member
Just to add a little to what Billski had to say, the biggest concern you should have in an airfoil right now, based on the sketches you've shown earlier, is whether it has a pitching moment compatible with your twist distribution, so that the airplane trims at the design speed with no elevator/elevon deflection. If you don't match up these parameters, the airfoil will be "fighting" the twist distribution and you'll be carrying drag-producing elevator/elevon deflection when you don't want it.

This is an efficiency-of-flight issue, not a safety-of-flight issue, but you should be able to determine the necessary pitching moment for your "ideal" airfoil when you do a trim plot of the aircraft across the design speed range. In most cases, it's not going to be terribly critical, but it's quite easy to pick an airfoil that does cause problems if you don't at least have a desirable Cm range in mind. It's not a matter so simple as picking a "flying wing" airfoil, because there are different kinds of "flying wings" and airfoils are optimized differently for the different types. An airfoil set up for a flying plank would not be the same as one set up for a swept-wing tailless aircraft, for example.

Beyond this concern, just follow what Billski said.

As an aside, if you're going to use a laminar section with a build method that does not produce laminar-flow tolerances, be sure that the airfoil data you're using is for that 'foil in dirty conditions ("standard roughness") and not the data for the airfoil in laminar conditions. Even modern laminar flow airfoils suffer performance degradation in all parameters - including maximum lift coefficient - when operating in turbulent flow.
Good points. Efficiency, also is obtained by utilizing the sweep. And also the diminished penaltly of highly twisted (7°) washout. It appears that 6° works well on this planform with airfoils with 0 to +.02 Cm.
There appears to be small oportunity to make the root foil a little 'lifty' and Cm around 0.0, while making the tip foil so it's not designed for lift so much as easier to have a stronger positive pitching moment, and a shape which is more efficient at around 0 and -1 aoa (not flat bottom).
Theres a pretty familiar feel to it now after trying everything under the sun on it in xflr5 and prudent use of xplane.

My next goal will be to put the candidate foils in dirty conditions as you say--good advice-- I was reacing that conclusion myself. lol.

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#### StarJar

##### Well-Known Member
So working late into the night I came up with what seemed the best combination of root and tip foils eaking out all others.
As I sit here now I can remember profili grapghs and values but the actual airfoils have alluded me. But I wrote them down on scratch sheet. They are.....Root----Roncz Low Drag FW airfoil thickened to 13% and camber increased to 4.5%. Tip------ Boeing Vertol vr-12 modified. The mod being more reflex and a new rear camber mod. I play with and modify airfoils, which has taken a few years to get the knack of.
I need to test the combining of these two (for midspan) and as Topaz suggested, in 'dirty' condotions.

#### StarJar

##### Well-Known Member
Any body know what a good N crit is to simulate flow over a fabric covered 'laminar' section? I know this was covered somewhere, I believe Topaz's design thread.
Edit; a-pay-rent-ly Ncrit 6 gets rid of laminar flow.
Testing around, turbulator at .05 c is not what you want for this.

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#### StarJar

##### Well-Known Member
If it does not make a better airplane, it is not an improvement. I suspect that grabbing this particular foil because it is thick aft is missing the goal of a better airplane.

I will not try to tell you either way if it is OK aerodynamically, but I will tell you that it will be turbulent flow and its stall behaviour may be "interesting".
If the stall is an abrupt one the fact that the tips are at 6° less angle, there appears to be always a straightforward recovery. In this case an abrupt stall can mean a quicker recovery actually.

I tested this airfoil on xfoil with Ncrit 6, the 'dirtiest' 'windtunnel' setting and it appeared to have normal lift drag and stall patterns. Of cpurse we don't know for sure what it will do but that is less of a concern with 6-7° of washout and a parachute for the testing of it initially.

The foil forward of the spar, regardless of how far back the spar is, will impose lift and moments upon the other structures. With the main spar so far aft, these loads will be much larger than we usually see, and may require some extra effort to work out a structure that does the job at reasonable weight. You will do well to investigate conventional ribs and D-section, a forward spar that is pinned to the cockpit area, and aft spar that is pinned, etc. If this is a strut supported wing, the spar EI required as you approach centerline drops, but general beefiness and the GJ required rises because the you have to support all of that wing forward of the spar;

If the airplane is a swept wing design, remember that there will be pitching moments (Center of lift of the 3 D wing is aft of the wing cross section near the centerline), and figure out how your structure will handle them. With the spar so far aft, these pitching moments may result in significant nose-up twist of the tips under positive g, exposing you to tip stall... Unless I were dealing with structure that is already going to be very stiff torsionally, I would run the spars at more conventional positions until close to the centerline.

No matter what thickness the spar, you have to design it to be strong enough and stiff enough in spite of the depth the airfoil at the spar position.

SO, why not pick a turbulent flow foil that is thick enough that you can have a a reasonably light spar system anyway. The easiest way to do this is to go with a low cambered higher thickness foil. As an example, the 0015 foil is 13.3% thick at 0.5C, and so is deeper than the Roncz foil cited at the main spar near centerline, and can be much more efficient structurally (deeper and thus lighter) once you are outboard a bit and can run the spar at 0.3C.
Those are all good points Bilski, and I appreciate it.
One thing I was concerned about is the angles that all the attach fittings come together. Pending an actuall load test, I want to cnc the ribs to hold two sets of 'telescoping' tubes near the leading edge and trailing edge to take the torsion load and some local G load.
I'm counting on my 'thick' three tier lattice 'skin' to be very strong. It has great inter weaving with the rib edges and good bonding where it envelopes the sparcaps.
But back to the telescoping tubes (the main spar attaches similar to a KR), the telescoping tubes (with inner and outer epoxied sleeving) need a little web sytem of their own that I need to figire out and design.
One thing I considered after reading your post, os slanting the outer spars a certain amount. I mean I considered it before too, and never really abandoned it. But I need to do a little more testing to get to a more final design of it.
Thanks.

#### StarJar

##### Well-Known Member
Also, pinning the wing (stub) forward of the pilot has been recognized as a requirement by me from the first iteration. Of course with an eight foot root chord this is doable.

I also thought more aboout possible nose up twisting of the tip under load;
The main spar has no anti twist qualities of its own. Anti twist depend totally on the skin and forward and rear tubes. I will pay close attention to those elements when load testing happens (and in design).
Thanks for highlighting that possible problem.

#### karoliina.t.salminen

##### Well-Known Member
Reynolds number is the greatest problem. You will face low Reynolds number problems first because laminar flow airfoils are designed for medium-high reynolds number. The surface roughness probably fixes many of these problems at low Reynolds number, but a laminar flow airfoil is very inefficient at low Re. The L/D just sucks.

#### StarJar

##### Well-Known Member
110 l/d at 1M Re# at Ncrit 5 isn't bad.
My flying wing design has an 8 ft chord so the Re should be much higher than that.
I think you have to say "Compared to what?", for polar comparisons, and look at each laminar airfoil individually. Compared to themselves at rough Ncrit and low Re the Roncz foil will mostly look bad only compared to itself ( in smooth conditions). However the Cm actually imprpves a bit.

#### Retiree

##### Well-Known Member
I tested this airfoil on xfoil with Ncrit 6, the 'dirtiest' 'windtunnel' setting and it appeared to have normal lift drag and stall patterns. Of cpurse we don't know for sure what it will do but that is less of a concern with 6-7° of washout and a parachute for the testing of it initially.

Sorry, but this is not what is meant by testing the dirty airfoil. What you are simulating is a dirty wind tunnel, which means a wind tunnel with a lot of turbulence in the free stream.
A dirty airfoil in the wind tunnel is an airfoil that has the boundary layer tripped near the leading edge. You can't get away from the problem with some (most?) laminar airfoils, they do not perform well with a mostly turbulent boundary layer.
Doug