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Question On Folded Airfoil Designs

BATTLEAXE

Well-known member
#1
Something I have noticed with the folded airfoil design is that most FT models and forum beta builds go with a folded airfoil that folds the top wing panel over the spar and connects at the training edge with the remaining UPPER wing panel creating the control surface for the ailerons/elevons/flaperons. This design lends itself to making the control surface to have a high AOA given the angle of the top panel of the wing sloping down to the TE. I can see how this would greater increase lift by 1. further slows down the air traveling under the wing in the pocket created in the connection of where the upper and lower wing panels meet, (even more when a spacer is used between the joint), 2. the increased AOA gives more inherent lift from the rush of air pushing against the surface. Both points work off both the Newtonian and Bernoulli principles of how an airfoil works. Pic shows what I am talking about:
20190810_171123.jpg
Now I have flown both this wing design and the under camber wing design you will find with the Mini Scout, Mini Speedster, SE5, and on all these planes whether it is the folded airfoil or the under camber style I have noticed that when you increase throttle from level flight that the plane will increase its lift respectively, causing you to re-trim or manually use down elevator input to maintain said level flight on the pitch axis, some more then others.

In building the Baby Blender I realized the wing TE is made from the LOWER wing panel to create the ailerons/elevons/flaperons style of control surfaces:
20190815_101036.jpg
As you may have guessed here lies my question. Does the latter airfoil style lend itself to a more consistent level flight in various throttle settings as compared to the first pic or does it even matter? Either by theory or practical experience all comments and responses are welcome, and if there is an airfoil design that is more consistent that hasn't been mentioned here please let me know. This one has me scratching my brain box.

Thanks for reading
 

Brett_N

Well-known member
#2
using the bottom panel or top for the control surfaces doesn't really matter since you are still using the same type of airfoil. FYI, this is called a "Clark Y". Single layer under-cambered, as you mentioned, have different characteristics altogether. But, that top wing will favor slower speeds with the trailing edge extending down and below the flat plane of the wing, acting like a great big flap.

Here's some fun reading.

While these are kFm styles, it's still some great reading how different airfoils react in different flight conditions. these are "fold over" or layered type builds instead of using a spar (in most cases) but the shapes are relatively similar.

https://www.rcgroups.com/forums/sho...man-(KFm)-Airfoils-Advanced-Theory-Science-**

Here's some more on the Clark Y type.

http://aeromodelbasic.blogspot.com/2011/10/clark-y-airfoil-is-ideal-for-rc-model.html
 

BATTLEAXE

Well-known member
#3
using the bottom panel or top for the control surfaces doesn't really matter since you are still using the same type of airfoil. FYI, this is called a "Clark Y". Single layer under-cambered, as you mentioned, have different characteristics altogether. But, that top wing will favor slower speeds with the trailing edge extending down and below the flat plane of the wing, acting like a great big flap.

Here's some fun reading.

While these are kFm styles, it's still some great reading how different airfoils react in different flight conditions. these are "fold over" or layered type builds instead of using a spar (in most cases) but the shapes are relatively similar.

https://www.rcgroups.com/forums/showthread.php?1296458-**-Kline-Fogleman-(KFm)-Airfoils-Advanced-Theory-Science-**

Here's some more on the Clark Y type.

http://aeromodelbasic.blogspot.com/2011/10/clark-y-airfoil-is-ideal-for-rc-model.html
Thx @Brett_N, I appreciate your feedback and I do understand the Clarke Y design. Actually the design that uses the upper panel for the control surfaces is more akin to the Clarke Y design given the negative space under the wing creates a small under camber from behind the spar back to the TE. The other example would be more like just an asymmetrical flat bottom wing profile, (I don't know if there is a name for it), but i was wondering if there is a performance difference related to the speed versus lift consistency.

I am just asking because the first plane I flew well was the Mini Scout which uses the full under cambered wing, then I flew a folded Clarke Y design in a different plane and i didn't notice a difference in the speed/lift relation. I add more throttle from level flight and it wants to porpoise and stall unless you add down trim or down elevator to maintain a relatively level flight. Maybe that's just a common result of fixed wing flight and it's something everyone has to compensate for, not sure. Thx for the links and i will make a read out of it
 

Hai-Lee

Old and Bold RC PILOT
#6
The wing profile is the important thing and not what the wing is made of, or whether the control surfaces are part of the bottom or top of the wing. It is the airflow over the shape that determines the lift.

As for the tendency to nose up under throttle application it is simply that the higher airspeed over the wing profile/shape generates greater lift! Normally on high lift profiles or profiles that are not symmetrical the extra airflow generates considerable additional lift and so most of such designs have significant down thrust angle to compensate for such an issue.

Some persons have been known to mix a little throttle into the elevator so that increased throttle adds a little nose down trim to keep the plane from climbing on the application of throttle.

Please note that you can also have the nose pitch upwards with throttle where the thrust line is way below the wing and the drag from the wing acts like a minor brake causing the plane to seem to pivot at a point just below the wing and for the rotation to cause the plane to climb markedly under throttle. Yet again the use of a down thrust angle can negate the problem.

Forgetting to set or determine the required thrust angles is a common new builder issue but experience will soon guide you towards that sweet performing and balanced design!

Have fun!
 

BATTLEAXE

Well-known member
#7
The wing profile is the important thing and not what the wing is made of, or whether the control surfaces are part of the bottom or top of the wing. It is the airflow over the shape that determines the lift.

As for the tendency to nose up under throttle application it is simply that the higher airspeed over the wing profile/shape generates greater lift! Normally on high lift profiles or profiles that are not symmetrical the extra airflow generates considerable additional lift and so most of such designs have significant down thrust angle to compensate for such an issue.

Some persons have been known to mix a little throttle into the elevator so that increased throttle adds a little nose down trim to keep the plane from climbing on the application of throttle.

Please note that you can also have the nose pitch upwards with throttle where the thrust line is way below the wing and the drag from the wing acts like a minor brake causing the plane to seem to pivot at a point just below the wing and for the rotation to cause the plane to climb markedly under throttle. Yet again the use of a down thrust angle can negate the problem.

Forgetting to set or determine the required thrust angles is a common new builder issue but experience will soon guide you towards that sweet performing and balanced design!

Have fun!
I knew you would chime in at some point in time here, it's always good to have your Yoda-like feedback in the universe of RC flight.

Talking about thrust angle and designing planes, there is a theory that I am developing in my own head based on observation of other designs of extreme differences in prop/wing placement/CG balance on different designs. In a tractor style plane with a high wing design there tends to be down thrust. The same tractor style with a low wing design there tends to be no down thrust. In a pusher style where the prop tends to be over the wing, (explorer or the sea otter) the down thrust seems to point at the wing CG, etc. In theory, in designing does it make sense to point the thrust angle at the wing because of drag the wing creates a significant amount of drag and a said pivot point?
 

Sero

Well-known member
#8
The same tractor style with a low wing design there tends to be no down thrust. In a pusher style where the prop tends to be over the wing, (explorer or the sea otter) the down thrust seems to point at the wing CG, etc.

The climb on throttle is annoying and varies on the design of the plane, but can be altered with thrust angle, CG, trim and mixes. I try to avoid mixes when possible as I feel it is cheating to a certain degree.
My Mustang which has a NN speed wing pitches up hard under power, moving the CG a bit forward and adding a bit of down thrust reduced that. It also has to do with the fact that there's a 700 watt motor in it too, the more power you have the more it will pitch up.

To touch base on the FT Sea Otter it doesn't have any thrust angle as it is a t-tail for that reason. In one of the videos Peter said he designed it that way to avoid pitch under throttle. Basically there's enough prop wash over the horizontal stab to keep the plane straight under increased power.
 

BATTLEAXE

Well-known member
#9
The climb on throttle is annoying and varies on the design of the plane, but can be altered with thrust angle, CG, trim and mixes. I try to avoid mixes when possible as I feel it is cheating to a certain degree.
My Mustang which has a NN speed wing pitches up hard under power, moving the CG a bit forward and adding a bit of down thrust reduced that. It also has to do with the fact that there's a 700 watt motor in it too, the more power you have the more it will pitch up.

To touch base on the FT Sea Otter it doesn't have any thrust angle as it is a t-tail for that reason. In one of the videos Peter said he designed it that way to avoid pitch under throttle. Basically there's enough prop wash over the horizontal stab to keep the plane straight under increased power.
Good to know, something I hadn't thought of for the sea otter. So many variables.

I run Spektrum DXe Tx which needs a laptop to program. Do you know how to do mixing on this model, I just haven't found it yet? I figured out how to change to elevons and such but haven't found the general mixing options yet. Any help would be appreciated thx
 

Sero

Well-known member
#10
Good to know, something I hadn't thought of for the sea otter. So many variables.

I run Spektrum DXe Tx which needs a laptop to program. Do you know how to do mixing on this model, I just haven't found it yet? I figured out how to change to elevons and such but haven't found the general mixing options yet. Any help would be appreciated thx
I have very little experience with that Tx, but I believe you need a cord and it can be done from the app in your phone too. That being said I don't believe it will do custom mixes, but dont quote me on that.
 

BATTLEAXE

Well-known member
#11
I have very little experience with that Tx, but I believe you need a cord and it can be done from the app in your phone too. That being said I don't believe it will do custom mixes, but dont quote me on that.
I have the cord and I use my laptop. even tho I haven't tried the app for the phone I do hear it is more limited then the laptop. just haven't found the mixing option yet.
 
#12
I think overall the CLARK Y type airfoils (full (not the Clark Y) and semi symetrical) have way better performance for speed and duration. The under-cambered airfoils are good for going slow on a park flyer. Of course if make a very thick CLARK Y airfoil it too will fly similar to an under-cambered wing. This IMO based on flying various FT ans Experimental Aircraft designs over the years. Nothing really wrong with either one. Not very fond of any of the KFM airfoils--I will let someone else make a comment on them--they are just OK --mostly for sloping stuff!!
 

Piotrsko

Well-known member
#14
I myself prefer the NACA 64 series at about 8%, but they have a nasty stall due to the sharp LE, BUT are wicked fast and dont reproduce in foam.

Most of the airfoils I have seen here are closer to Goettengen than Clark, the high point is too far forward and the LE radius is off, some even look like Schweitzer glider series.

Undercamber on a symmetric wings gives you a flat bottom with an upraised symme LE.
 

Hai-Lee

Old and Bold RC PILOT
#15
I knew you would chime in at some point in time here, it's always good to have your Yoda-like feedback in the universe of RC flight.

Talking about thrust angle and designing planes, there is a theory that I am developing in my own head based on observation of other designs of extreme differences in prop/wing placement/CG balance on different designs. In a tractor style plane with a high wing design there tends to be down thrust. The same tractor style with a low wing design there tends to be no down thrust. In a pusher style where the prop tends to be over the wing, (explorer or the sea otter) the down thrust seems to point at the wing CG, etc. In theory, in designing does it make sense to point the thrust angle at the wing because of drag the wing creates a significant amount of drag and a said pivot point?
You have a basic idea into the subject but there are a few other things to consider. Aircraft weight and wing incidence angles can cause higher drag and can give pitching effects even on low wing designs. Dihedral can effect where the mean centre of drag is and so can add or subtract from any pitching moments.When the pitching is due to increased lift it can often be overcome with lower KV motors or even SF props.

I have observed people increasing the motor size and KV trying to make their favorite bird go a lot faster only to have them discover that the bird is extremely unstable in pitch Vs throttle setting. There are a plethora of pages on the internet describing how to set thrust angles and the like but such information is often ignored. To many that fact that it flies at all is good enough!

Have fun!
 

BATTLEAXE

Well-known member
#16
The reason why I posed this question was because I noticed that on the Baby Blender build the wing is folded opposite to the FT norm of the top wing panel forms to the TE and the control surfaces. With the BB it is the lower panel that creates the TE and the control surfaces, and I was wondering if there was an advantage to either and /or why would they be different? If there was an advantage with the pitch vs throttle setting with the BB wing fold I would modify all the wings I build from now on to that same fold, or vise versa. As this thread has continued I am starting to see that it is just an inherent issue to flying the Clake Y wing profile.

Is there any mixing that can be done with the DXe spectrum Tx programming to compensate?
 

Sero

Well-known member
#17
The reason why I posed this question was because I noticed that on the Baby Blender build the wing is folded opposite to the FT norm of the top wing panel forms to the TE and the control surfaces. With the BB it is the lower panel that creates the TE and the control surfaces, and I was wondering if there was an advantage to either and /or why would they be different? If there was an advantage with the pitch vs throttle setting with the BB wing fold I would modify all the wings I build from now on to that same fold, or vise versa. As this thread has continued I am starting to see that it is just an inherent issue to flying the Clake Y wing profile.

Is there any mixing that can be done with the DXe spectrum Tx programming to compensate?
In the FT Baby blender video Josh mentions they used the Armin wing from Ed at Experimental Airlines you tube channel (seems his channel name changed to Tesla Details) I guess FT decided to use their usual wing design after that.
 

AkimboGlueGuns

Biplane Guy
Mentor
#18
No airfoil is immune from inherent pitch up with an increase in airspeed. The faster the foil moves the more lift it produces, which manifests in a nose up attitude. The under camber that most FT airfoils have will absolutely amplify this effect. The airfoils that reduce this tendency the most have reflex (the trailing edge of the airfoil has a pitch up) but the trade off is lower lift at low speeds. Airfoil optimization is going to come down to what you want the airplane to do. In the case of the Scout, it's a gentle slow flying airplane which doesn't really have the power to make enough lift to overcome the elevator force. The baby blender is also a slow flying plane, but the airfoil design is different because Josh took the inspiration for some design elements from Ed at Experimental Airlines who uses an airfoil design which has a more neutral trailing edge.
 

BATTLEAXE

Well-known member
#19
In the FT Baby blender video Josh mentions they used the Armin wing from Ed at Experimental Airlines you tube channel (seems his channel name changed to Tesla Details) I guess FT decided to use their usual wing design after that.
I do remember Bix sayin that they did get inspiration for the wing design from Ed @ E.A. and I do recall him saying Armen wing you are right sir. If I remember correctly as well that the Baby Blender version one was a very early design in the FT program. What I don't know for sure is whether they changed it to the design they use now because it is easier to build, or adapt to other models, or flies more consistent. I think the BB is the only design they use that style of wing on, unless there are some delta wings they do. I know I built the Mini Arrow and it uses the current design of the top panel creating the elevons, which come to think of it they have a gauge to set the resting elevon pitch to what looks like flat to the bottom parallel panel once set. I wonder why they didn't use the same fold design as the Baby blender in the delta wing designs?

Another thinker