Scracth Build Cessna 337

[Leo]

I made a quick modification for my Christine Eagle.

Wing Chord = 15CM
Added Balsa strip = 11mm (~7% of 15cm) at bottom (40% of chord).

Then covered with paper to create the wedge shape.

Tested today,

Flight was not satisfied, not sure was due to my weak battery (2s1p) or wind was strong. Landed just within 1~2 minutes.

She also needs some up trim. Will shift the battery backward and try again when is less windy.

[loyn]

may I know what is the problems you encounter ?

from the video , those with this wing fly very well in the slop glider version which wind is v strong.....

[dickeroo]

I made a quick modification for my Christine Eagle.

Wing Chord = 15CM
Added Balsa strip = 11mm (~7% of 15cm) at bottom (40% of chord).

Then covered with paper to create the wedge shape.

Tested today,

Flight was not satisfied, not sure was due to my weak battery (2s1p) or wind was strong. Landed just within 1~2 minutes.

She also needs some up trim. Will shift the battery backward and try again when is less windy.

Leo... here are two videos of a bi-plane, but they both have the step on top. Yours should work as well, but you will have to have the proper CG and enough power. Wind should not be all that much of a factor.

SIMPLE FOKKER:

http://www.rcgroups.com/forums/showthread.php?t=700845

http://www.youtube.com/watch?v=mzs7A3V1vLg

Let us know how you make out.

[loyn]

How the cg is being determine 40%? Constant for all wing shape ?

thanks

[dickeroo]

A suggestion...

How the cg is being determine 40%? Constant for all wing shape ?

thanks

The KF step seems to need the CG further back than a conventional airfoil. It has been reported by many different RC people that for the KF airfoil, although it has a very forgiving CG, about 40% is the sweet spot. Obviously, other factors with any particular aircraft must be taken into account. The best thing is to balance your individual aircraft accordingly. The reason for the CG needing to be further back on a KF airfoil may be due to the center of pressure shifting back as well. If you have the time, you should try to read the thread in the RC Groups, which is 80 pages long but has a wealth of information on it pertaining to the KF airfoil's characteristics. You might just want to pick out the posts by Tony65X55. That will give you a good basic understanding to start with. Here is the site:

http://www.rcgroups.com/forums/showthread.php?t=558321

So many people have had great success using the KF airfoil and the list of people seems to keep growing. To date, no negatives have shown up.
Thank you for your interest in this airfoil design. I hope you will share your experiences with us.

I wish you all the success with your adventure.

[Leo]

Tested early morning in no wind condition and with battery fully charged.
The plane didnt perform well, behaved as if the drag is too high. After a few unsuccessful flights, I removed the wedge (balsa strip and covering paper), then test flight again. To my surprise, the plane flew well, more lively and easier to gain height.
Well, not sure the 7% step that I added in should apply to just the step or the overall wing thickness? Perhaps, thinner wing could reduce the drag a little.
Oh, I did try to fly the plane inverted just to check if the step on top will be better but no luck.

[loyn]

The KF step seems to need the CG further back than a conventional airfoil. It has been reported by many different RC people that for the KF airfoil, although it has a very forgiving CG, about 40% is the sweet spot. Obviously, other factors with any particular aircraft must be taken into account. The best thing is to balance your individual aircraft accordingly. The reason for the CG needing to be further back on a KF airfoil may be due to the center of pressure shifting back as well. If you have the time, you should try to read the thread in the RC Groups, which is 80 pages long but has a wealth of information on it pertaining to the KF airfoil's characteristics. You might just want to pick out the posts by Tony65X55. That will give you a good basic understanding to start with. Here is the site:

http://www.rcgroups.com/forums/showthread.php?t=558321

So many people have had great success using the KF airfoil and the list of people seems to keep growing. To date, no negatives have shown up.
Thank you for your interest in this airfoil design. I hope you will share your experiences with us.

I wish you all the success with your adventure.

Thanks for all the information and links, will read through it to understand more.

Plan to build a basic airframe to test it out after completed my existing project

[renold]

The dynamic of KF airfoil is indeed complex and hard to understand. As was mentioned by Dickeroo that KF airfoil can perform in low and high speed, imply infinite speed range. Thats drew my interest in doing more literal research on fluid mechanics. I think it is worth invest some time to establish full understanding.

As mentioned, KF airfoil successful by build up vortex formation at about 40-50% after the leading edge where the step begin. The formation of such vortex layer will form part of it non-physical airfoil section that will guide the flow above. Such will reduce drag. The condition is this vortex formation must be steady and not turbulance. Ideally should be laminar flow. Otherwise, the vortex is not sustainable and turbulance and worst serparation in this airfoil section will happen, such results will stall.

We also know when air flow over a surface, the phenomenon called "boundary layer" will establish between flow and its surface. Air flow be laminar or turbulance partially depending on the boundary layer. Such is a function of airfoil chord length, flow velocity, temperature etc. Thanks to the great contribution of Reynolds Number (Re).

Equipped with the above, the operation of KF airfoil with no exception needs to comply with Reynolds Number. In mathematical expressions: f (Re) = {length, velocity, viscosity}. We have the geometry KF airfoil specified. Air viscosity is known. The next is to find the velocity range and the conditions for sustaining the vortex formation.

I do not have the luxury of wind tunnel test. If any one can help, please advise.

[dickeroo]

Here is a site I highly recommend visiting with many pictures.

The dynamic of KF airfoil is indeed complex and hard to understand. As was mentioned by Dickeroo that KF airfoil can perform in low and high speed, imply infinite speed range. Thats drew my interest in doing more literal research on fluid mechanics. I think it is worth invest some time to establish full understanding.

As mentioned, KF airfoil successful by build up vortex formation at about 40-50% after the leading edge where the step begin. The formation of such vortex layer will form part of it non-physical airfoil section that will guide the flow above. Such will reduce drag. The condition is this vortex formation must be steady and not turbulance. Ideally should be laminar flow. Otherwise, the vortex is not sustainable and turbulance and worst serparation in this airfoil section will happen, such results will stall.

We also know when air flow over a surface, the phenomenon called "boundary layer" will establish between flow and its surface. Air flow be laminar or turbulance partially depending on the boundary layer. Such is a function of airfoil chord length, flow velocity, temperature etc. Thanks to the great contribution of Reynolds Number (Re).

Equipped with the above, the operation of KF airfoil with no exception needs to comply with Reynolds Number. In mathematical expressions: f (Re) = {length, velocity, viscosity}. We have the geometry KF airfoil specified. Air viscosity is known. The next is to find the velocity range and the conditions for sustaining the vortex formation.

I do not have the luxury of wind tunnel test. If any one can help, please advise.

Renold... check this site out: http://www.stenulson.net/rcflight/slimbeag.htm

It has a lot of information about the KF airfoil regarding its performance and the building of the wing itself. There is also a video link embedded with all the text. The builder has had great success with this airfoil and continues to experiment with a double step as another variation of the KF concept.

[Leo]

Tested early morning in no wind condition and with battery fully charged.
The plane didnt perform well, behaved as if the drag is too high. After a few unsuccessful flights, I removed the wedge (balsa strip and covering paper), then test flight again. To my surprise, the plane flew well, more lively and easier to gain height.
Well, not sure the 7% step that I added in should apply to just the step or the overall wing thickness? Perhaps, thinner wing could reduce the drag a little.
Oh, I did try to fly the plane inverted just to check if the step on top will be better but no luck.

I tested the plane again with step reduced from 11 to 5~6mm. Thus the total wing thickness = 5 + 5( Original foam thickness) =11 = ~7% of 150mm chord.
The step is at the bottom 40% chord.
Well, I could fly around and also able to do inverted with some down trim (up trim in normal flight orientation).
The plane won't gain height as good as the original foam plank wing without step, but seems to help when it rolled, felt more aerodynamic. Other than that I dont see the big difference as compared to plank wing.
However, the structure will enhance the wing stiffness, and yet as simple to build as the plank, so my interest is still there for future application.

[dickeroo]

KF airfoil should give you a wider speed envelope.

I tested the plane again with step reduced from 11 to 5~6mm. Thus the total wing thickness = 5 + 5( Original foam thickness) =11 = ~7% of 150mm chord.
The step is at the bottom 40% chord.
Well, I could fly around and also able to do inverted with some down trim (up trim in normal flight orientation).
The plane won't gain height as good as the original foam plank wing without step, but seems to help when it rolled, felt more aerodynamic. Other than that I dont see the big difference as compared to plank wing.
However, the structure will enhance the wing stiffness, and yet as simple to build as the plank, so my interest is still there for future application.

Leo... take a look at this video which shows a very wide speed range.

http://www.youtube.com/watch?v=frdjhmgnmgo&mode=related&search=

When the step is on top at 50% you will get more lift. When it is on the bottom at 40% you will get very good stability. Popular Science is doing an article of the KF airfoil concept and it may be in the January issue. Did you notice if you had better air penetration with the KF airfoil? Make sure your center of gravity is back at about 40% for best results.

[dickeroo]

KF Hellcat maiden flight is a wind-beater.

Dave Reap created the KF Hellcat, which was based on Kaos2's design of the XRegal. The reason it is called the XRegal is because it has a KF step on the fuselage as well as on the wings. In his maiden flight he faced some pretty stiff wind which he was able to handle well because he had full control authority during the entire flight.

Here is the video:

http://www.youtube.com/watch?v=shQLKiEZ3Lk

If you want to see the original Regal fly, go to this link:

http://www.youtube.com/user/Regal49

Just thought that you might find these videos of some interest.