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Aug 17, 2019, 05:03 PM
aerodynamics is just a theory
If it is helpful for anyone, I can post dimensions/drawings of the wing, I have a fully parametric 3d model I created for easily making different design changes etc so I have all the relevant dimensions on hand
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Aug 17, 2019, 05:47 PM
If it flies - I want one!
Petem's Avatar
Corsair,
Before you make major mods or scrap the airframe, suggest try flying the wing inverted for an extended period.
If the nodding goes away, the cause is likely to be the wing shape/section.
I had a small, conventional layout model years ago with that nodding behaviour as soon as you applied positive 'G' - went away when flying inverted (symmetrical section).
Eventually figured the cause was the tailplane going into the wing wake when angle of attack increased.
In your case, it could be the rear of the wing, including elevons, getting disturbed flow from the high KF step?
Cheers, have fun playing,
PeteM
Aug 18, 2019, 03:04 AM
Registered User
davereap's Avatar
If it comes in fast and you cant slow it..its most likely too nose heavy.. or sometimes the elevons are much too small, less likely..
If it is very pitchy then its tail heavy..
I tend to use bigger elevons with less travel.
With most V wings KFm types you can get them to slow up to a crawl and just plop in for a landing..they also can be super stalled so they parachute down whilst still having control.
Last edited by davereap; Aug 18, 2019 at 03:15 AM.
Aug 18, 2019, 11:53 AM
I Look, Listen, and Learn
Timbo383's Avatar
Quote:
Originally Posted by Petem
In your case, it could be the rear of the wing, including elevons, getting disturbed flow from the high KF step?
Which is what I've been thinking since he laminated the two steps together....the kfm step is literally twice the thickness of the flat wing. And if the elevons are not adhered tightly top and bottom I think that would cause some additional fluttering giving it that bobbing action he is seeing.
Aug 18, 2019, 05:55 PM
looking up, down under
scruffy1's Avatar
if you can't make the step smaller, can you add a further small "step" behind the big step to perhaps turbulate the flow more effectively ?

sort of a bastardised kfm3
Aug 18, 2019, 06:03 PM
Registered User
Gregco10's Avatar
I had a 1/2 insulation foam as the top step with dollar tree as the bottom and elevons. It lacked pitch up control. I added a 1/4 inch strip to lengthen the elevons and it flew great. It didn't help that it was a stretched delta either (was very stable on pitch) The step seemed to block the elevons for sure in my case.
Aug 19, 2019, 10:08 AM
aerodynamics is just a theory
Quote:
Originally Posted by Petem
Corsair,
Before you make major mods or scrap the airframe, suggest try flying the wing inverted for an extended period.
If the nodding goes away, the cause is likely to be the wing shape/section.
I had a small, conventional layout model years ago with that nodding behaviour as soon as you applied positive 'G' - went away when flying inverted (symmetrical section).
Eventually figured the cause was the tailplane going into the wing wake when angle of attack increased.
In your case, it could be the rear of the wing, including elevons, getting disturbed flow from the high KF step?
Cheers, have fun playing,
PeteM
I agree with this and the other recent comments, I'm convinced it is a blanking of the elevons at moderate angles of attack from the 'trapped' vortex shedding behind the step

Other commenters are right, it is a fairly large step for the size of the wing

I will try flying inverted some more (tried a bit but it was squirrely, no bobbing though) and then lengthen the elevons and see what happens. May not have time to fly until later this week

Thank you all for your input so far! Looking forward to solving this mystery
Aug 19, 2019, 11:04 AM
Registered User
racermark's Avatar
Hi there all
Can someone explain why the flying inverted should change anything. I was not aware the airflow could tell the difference.
Maybe the angle of attack could make a difference. This however should be evident by flying at a bit of down angle.

Not saying there is not something I am missing but when we put wings in the wind tunnel and run them upside down (and yes we have done that)they do show lift equal to the negative lift we see when they are run in the as raced down force direction. (as long as they are at the equal angle of attack)

Not sure I understand but maybe there is something to be learned.
Mark
Aug 19, 2019, 03:21 PM
aerodynamics is just a theory
Quote:
Originally Posted by racermark
Hi there all
Can someone explain why the flying inverted should change anything. I was not aware the airflow could tell the difference.
Maybe the angle of attack could make a difference. This however should be evident by flying at a bit of down angle.

Not saying there is not something I am missing but when we put wings in the wind tunnel and run them upside down (and yes we have done that)they do show lift equal to the negative lift we see when they are run in the as raced down force direction. (as long as they are at the equal angle of attack)

Not sure I understand but maybe there is something to be learned.
Mark
If flying inverted, the elevons would be moving towards the flat bottom of the wing to hold the nose up. KF Airfoils 'trap' a vortex behind the step but in reality that vortex is constantly forming and shedding

At higher angles of attack (pulling up or in a turn) that vortex shedding as well as other separation from the wing can 'blank out' the elevons, essentially having them 'see' stagnate air which would explain why the plane seems to temporarily lose all elevon authority, then quickly regains it and loses it again, making it 'bob'



I just glued on some elevon extensions, and now it is clear that the original elevons were probably easily 'blanked' by the wing section because looking down the leading edge of the wing, you cant even see the tips of the elevons move until they are at about 90% throw. Now you can see them much better. I'll post a video that shows this phenomenon after I test the plane again, but feeling optimistic
Aug 19, 2019, 06:15 PM
aerodynamics is just a theory

Time for a KFm-4


Okay I think I have exhausted all other possiblities

Did a round of testing this afternoon and here is what I learned:

1) Not blanking of the control surfaces, as I extended the elevons and the problem got WORSE

2) 99.99% positive it is NOT elevon flex or slop, as
- I moved the control horns to the top to reduce slop in the slot they rested in
- I improved elevon hinge slop

3) moved CG back again, had more control but fluttered just as bad

4) tried flying inverted.

And guess what, it DOES NOT BOB (or at least exhibits much less bobbing) when flying inverted. Further, when flying straight and level under power randomly the plane will take a sharp nose dive for the ground...


I don't know why, but my KFm-2 wants to become a KFm-1 (step on bottom) and I think this is why there is such weird behavior under hard turns/maneuvers (anytime there is a reduction in stability already, it wants to turn into a KFm-1)

I thought this could be my motor mount being loose, but I think this would actually have the opposite effect, making the plane want to climb... haven't ruled it out 100%, but based on my observations from the flights today it seems that there is just some 'unlucky' combination of geometry at work here. I remember KFm-2 wings on other planes I have built (traditional, wing and tail) being very pitch sensitive so perhapse something is awry... being an aerospace engineer I know there is a specific reason this is happening, and not knowing why is killing me

The wind today exacerbated the problem. It is very obvious the plane will randomly create negative lift where has flying inverted doesn't have this problem... very strange indeed.

Well, I will keep this thread updated as I move on to try this same design but as a KFm-4 (original intent for this plane)



On a side note, should any of you want to follow the test process of this (and future) plane, you can follow my Instagram where I am keeping a 'design log' of sorts: https://www.instagram.com/highlydistracted/
Aug 24, 2019, 04:54 PM
aerodynamics is just a theory
So, Mk.2 is just about ready for maiden. Mk. 2 is a KFm-4 airfoil, everything else is the same as Mk.1 (except for slightly different winglets but that is somewhat irrelevant for now).

Before that though, how do you guys suggest I balance the CG on this wing? Use a traditional Wing CG calc like I did for the last wing, or move it further back since (from my understanding) KF airfoils like the CG slightly further back?
Aug 25, 2019, 06:59 AM
Registered User
davereap's Avatar
http://wingcgcalc.bruder.com.br/en_US/?
dont forget you will have to use two panels, one for the slot. and one for the the main wing.
A projected drawing will show you have the correct shape..
COG at 20% is a good start... 15% is too nose heavy...

on a standard V wing its sometimes difficult to get the COG forward enough if you use smaller lipos.. try a nose extension..
Last edited by davereap; Aug 25, 2019 at 07:31 AM.
Aug 25, 2019, 10:54 AM
Reap the wild wind
headlessagain's Avatar
Good luck with Mk 2 corsair2014. My KFm-4 Mini Wing was rock solid and actually flew better than the similarly sized and equipped EPP Bonzai from HobbyKing
https://www.rcgroups.com/forums/show...9&postcount=35
Depron Mini Wing KFm-4 on 4.75 x 4.75.#16 PK 12Feb12 (3 min 43 sec)

Andy
Aug 25, 2019, 05:15 PM
It seemed like a good idea!!!!
Hookit's Avatar
+1 to both Davereap and Headless.

My version of Dave's DeltaVee flys amazingly well, 6mm Depron KFm4.

Revitalised a flying buddies Mini Wing built by Headless with a new motor/Prop combo and it was awesome just like when I saw it fly four years ago.

KFm thanks Dick it's the unbeatable gift that keeps on giving.
Aug 25, 2019, 05:26 PM
Registered User
Dickeroo's Avatar
Quote:
Originally Posted by Hookit
+1 to both Davereap and Headless.

My version of Dave's DeltaVee flys amazingly well, 6mm Depron KFm4.

Revitalised a flying buddies Mini Wing built by Headless with a new motor/Prop combo and it was awesome just like when I saw it fly four years ago.

KFm thanks Dick it's the unbeatable gift that keeps on giving.
Hookit... Thank you so much. It put a big smile on my face. The KF airfoil concept seems to have invited many more people to build and fly all different types of aircraft without spending a fortune or their time building RC aircraft. Also having the ability to quickly get their creative designs into the air and safety back on the ground. This is the greatest group of people to hang with. It has been my pleasure all the way.

~ Dick


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