Knife edge tucking
 

So I've noticed on my planes, they all want to tuck toward the belly on when knife edging. Right or left rudder, doesn't matter. Had a hangar 9 advanced 40, low wing, straight LE, slight dihedral. Some programming on the radio fixed this. My King Kobra does it a touch, but it's not as severe so I don't even bother with mix programming on that. Funtana S90 and Edge 540, they tuck BIGTIME. All are well balanced. And the tuck DOESNOTget any more severe as the planes consume fuel. I've tried different balance points, but its always the same result. I really don't want to have to program mixes all the time, ESPECIALLY because when I land, Iwant to land, don't really feel like having to remember to flip a switch to deactivate the mix. When Iland, Idon't want my rudder telling my elevators to go up. Besides, Iwant to fly my planes, not have the plane fly itself or a program do it for me.

I guess my question is why do planes do that? I don't want to just know fixes for the issue. If I understand why they do it, I can most likely set my birds up correctly to overcome the problem. I've heard people say its because of drag from the gear. Well, the King Kobra does it and that has retracts. I know what people will say, "that's why its not as noticeable." AH, well HERE'S THECATCH

When I just bank the airplanes on their sides, they don't tuck. If it was drag from the gear, they would tuck regardless. They don't. The only time they tuck is when Iinput rudder. Whether Isnap into it or slowly input rudder, same result, so it can't be that it's because one wing gets more airspeedI think that would make it roll anyway. I've tried this on every single plane, plus other planes of club buddies. Same result. Bank onto the sides, all good until gravity naturally tells the plane to start nosing down. Input rudder, and toward the belly they go.

Anyone have any advice on this? Anywhere I can look to try to learn why they do that and any setup solutions?

Thanks in advance!

RE: Knife edge tucking
 

Well it looks like this was adressed in this forum some time ago, and actually found some great comments.  One that made the most sense is the change in pressure on the horiz stab the rudder induces. Also, forwarding the CG seemed to help alot of people.

Anyone?

RE: Knife edge tucking
 

99% of the time - the cg location causes this to happen. the reason is that the wing is not working against gravity in knife edge - so the "trims" which cause a perfectly balanced hands off level flight, now become apparant.
\ If the model is "tailheavy", it will tuck to belly- if "noseheavy" it will pull to canopy.
It is ALL a COMPROMISE
a little mix causes no noticable adverse affects in most cases - so mix it out.
Some will have you believe all mixes can be trimmed out

Unless you can overcome gravity - this gets tricky-
Another thing typically happens in Knife edge when th e rudder is applied.
the lower stab is working in "clean" air -the upper side is somewhat blanked out- this can cause a loss in total stabilizer efficiency
The stab -being the balancing force ,will respond differenty depending on shape - and location- when the model is slipping . flying with the fuselage at a positive angle in knife edge is one of this example of this.
Flying level -then applying rudder , is another
There is no magic setup- you can get close in some cg/trim/etc., arrangements -but it's all compromise
choose which you prefer

RE: Knife edge tucking
 

Completely agree with rmh.

If the elevator has anything but neutral trim for horizontal flight, how can it act as a neutrally trimmed rudder when the roles get switched during knife edge flight.

CG location and wing AOA are key factors in the degree of trim that the elevator requires for horizontal flight.

Can those be adjusted to allow a perfect hands and trim free KE?, probably yes.

Would those adjustments generate undesired reactions for non-KE flight?, probably yes.

Experimentation is the key here;)

RE: Knife edge tucking
 

Agreed. Except for the part about not being able to trim out all mixes. It's possible with the right airframe.

RE: Knife edge tucking
 

Havent seen it yet. The closest I have owned was a 38% Columbo Extra. With the correct CG and 1/2 degree positive in the wing I had a pitch mix at low throttle and pitch mix with rudder as it did a very slight tuck in knife. In 30+ years of flying I have never had an airplane that would trrim 100% true in all flight attitudes and 100% true controls. Not even a helicopter.

RE: Knife edge tucking
 

Which is exactly my understanding in reading the posts. Yours actually nailed the "making sense" coffin for me, thanks! And Ifound today that yes its all about compromise. I shifted weight forward, and the tuck became less severe, but now she didn't flat spin as flat or like to hang as much. So I'm just going to do what I did today, practice and get better. As severe as the tuck was, I found that, as always, a little practice goes a long way in training your fingers so the maneuver becomes easier to make look good without program mixing. Man, now if I could only program the mix to turn off when I'm landinglike a turn-the-mix-off-below-25%-throttle kinda thing... lol

RE: Knife edge tucking
 

There really is no need to turn the mix on and off. The aerodynamic forces at work here wil be the same no matter the airplanes attitude. I found that if an airplane tucks in knife edge then it will tuck in horizontal flight too. Right now on my pattern airplane, I have 5 mixes working. All 5 are on all the time.

RE: Knife edge tucking
 

Exactly speed. My mixes stay on. Took me a while to convince some of my cofliers, but it does work that way.

RE: Knife edge tucking
 

In agreement here about trimming the plane so the elevator and stab are neutral so that an apparent trim does not exist or is reduced when rolling 90 deg.

I've owned (still own) only one plane that will knife edge across the whole pattern, hand off right stick (with no trim mixes). It has an adjustable stab that was dialed in neutral. It also balances laterally both upright and rolled 90 deg to ensure no roll couple. And... probably has some luck involved that the drag of the rudder produces no roll.

On most planes, the goal would be to choose between eliminating the pitch or roll component of knife edge. Dealing with one with the stick is fairly easy. Dealing with two at a time is much harder. It might take some effort to figure which of the two can be neutralized.

RE: Knife edge tucking
 

Tho it seems improbable- I have been able to to trim out some little 16 ounce foam models to knife very well
I also tried tricks such as mounting different sized batteries above or below the wing - to change vertical cg.
Learned a LOT playing with these things - simply because changes can be made on the spot and results easily compared .
Everything I fly now is electric-power.

The power available is far different thanthe powe r available with IC engines - and the planes are easier to setup as power is instant at any speed /attitude.
My pattern flying friends are also switching over - they found the same results I did.

RE: Knife edge tucking
 

Robert, it's more probible then you may think. For a while I was bitten by the foamie bug as well. A buddy of mine was manufacturing kits and I had let him borrow a couple of my designs. The ones of mine that flew the best was a 39" Yak 54 and a 28" Jungmeister. Both of them did beautiful knife edge. The Jungmeister did just about everything very well, I wish I could find my drawings as I would like to build another.

RE: Knife edge tucking
 

I still have one of the 1990 TOC Jungmanns I designed .
They are similar to the Jungmeister
We did these absolutely to scale -with wings clipped 9.5 %
they were very good aerobatic models -In my book far better than other bipe designs - which Ultimately required all sorts of stretching and respacing of wing panels, to fly well.

RE: Knife edge tucking
 

I couldn't disagree more. If I do a rudder turn, the planes DONOT dive, it's only when the plane is on its side and rudder is input. AND, when you're talking aerodynamics, they can't possibly be the same with different plane attitudes because the plane is flying thru the air differently. In level flight, the plane flies a certain way. That's the design. Every surface on the plane is meant to do a certain thing in certain situations. Case in pointwing loading. Wings are designed to carry the plane a certain way given certain circumstances (level flight primarily). When you turn the plane on its side, the wings are no longer the primary cause of lift. Lift comes from a combination of the fin, the rudder, fuselage side, engine/prop, and any FGs that may be on the plane.

Given that, again, the aerodynamic forces change dramatically.

NOW. Barring that, the need, that I have anyway, to turn the mixes on and off, is simple. In the case that I do decide to use the mix crutch instead of practicing, there's a problem. In the event I'm landing, and I forget to turn off the mix, proper landing techniques go out the window because if I input rudder, now the plane pitches up. How do I overcome this? By practicing. Ok, so if I'm practicing, might as well practice perfecting the maneuvers with a properly set up bird than practicing shortcuts, flipping switches, etc. Which is precisely why I don't use the expo crap. Don't get me wrong, I do believe it can be a very useful tool, but once I learn a plane, I take out the expo. And so far the system has worked.

Am I splitting hairs here? Perhaps. But I guess that's the only way I'll be able to make sure the blade makes the cut (pun intended). Plus, you'd be surprised how many different moves or variations of moves I've learned just in the past week by not using mixes or expo. My point exactly. Again, I'm just trying to understand the forces at work here so I can augment my building/flying skills. I really do appreciate all the responses and advice here.

LOVEthis site!!!

RE: Knife edge tucking
 

Here's the thing, which, I think you're onto something. When the plane is on it's side, like rmh said earlier, the bottome stab is in "clean" air, now the top stab is in the mix of a vortex created by the rudder deflecting air up across the upper stab's deflection (let's not forget that most stunt/aerobatic/3D rudders are longer on the bottom than top). So now the upper stab is in a lower pressure zone which pulls the upper stab "up", and the plane wants to roll toward that up. Meaning the bottom stab, in "clean" air wants to stay put, so the whole plane rolls with the tendancy. Any thoughts on this?

RE: Knife edge tucking
 

The pressure differential you noted always exists when there is a angle to the direction of flight.
The upper halve does ,however ,see disturbed as well as lower pressure air - so it will not be as effective as the halve opering in a higher ,cleaner flow environment
Possibly- the difference in efficiency could be 50% ( who knows as it varies with angle/position/speed etc..)
The trick in getting best performance is to have the SAME drag in knife edge as in level flight
Possible - yes but probable- no.
Models which can do this are really strange looking ducks -
Typically stubby winged bipes with huge interplane plates and a tall fuselage and cabane.
The reason for this is to provide the SAME lift without respect to position of wings relative to the horizon
These are fun to fly but really require a different approach to some typically ordinary maneuvering
For example -if you bank them - they simply bank- with no change in flight path!
The reason of course is that lift is about the same in ANY attitude ( in level flight)
The current crop of "pattern models" lean to this setup - just not so extreme.
a few pieces of depron and some thought - will let you proove out most of this .
The flight characteristic you want to research can be duplicated quickly , for pennies .

RE: Knife edge tucking
 

Here's the thing... most planes, even when they are stable in pitch (i.e. CG adequately forward) tend to tuck toward the belly in KE.. Does that mean they are tail heavy?.. Nope!... if you were to remove the tuck by moving the CG you would end up ridiculously nose heavy.

The reason they usually tuck toward the belly is that the tail is trimmed in level flight to operate in the downwash of the wing.. This means that the tail is set to a more positive angle (down elevator if you like) than it would be if no downwash were present. Now you roll the plane into KE.. In KE the wings don't make any lift so there is no downwash. Now the angle of the tail without downwash acting on itworks like down elevator and tucks the plane toward the belly.

Pretty difficult thing to eliminate.. Longer tail moment should help as it moves the tail away from the worst downwash. Low aspect ratio wings may suffer worse than high aspect as downwash angle will be greater.

Steve

RE: Knife edge tucking
 

Years ago before all this fancy computer mixing we used to dial out unwanted tendencies by changing the incidence of the wing and stab or adding or removing dihederal. I would fly all my pattern airplanes in primer to determine what needed to be done. If I needed to cut the wing in half to change dihederal or shim the wing or stab I would still be in primer, no need to mess up a nice finish. I still try and get my airplanes as close as possible mechanicaly before I mess with the mixes, expo, ATV, and dual rates.

RE: Knife edge tucking
 

pkoury,

So without mixes, what would be done to the geometryto stop the plane tucking toward the belly in KE?... Anhedral stab maybe?

Steve

RE: Knife edge tucking
 

To eliminate it yes, but to reduce it, forward CG does help. If you doubt this, turn off your mix and move your CG 1/2" back and see how much worse it gets.

RE: Knife edge tucking
 

Yes, I've no doubt moving the CG forward helps reduce tuck to the belly (because it means more up trim on elevator). But like i said in my fisrt post; you would (on most models) end up very nose heavy if you were to move the CG far enough to eliminate it altogether, which would compromise other areas of flight.... For instance.. i like to have a CG that's far enough back to requireminimal, or even zero, down elevator for inverted, and one that will allow the model to dive vertically with negligeable pull out... If i were to move the CG forward i'd compromise those aspects.

Steve

RE: Knife edge tucking
 

Would there be any difference between a flat and foiled stab? The plane pictured has a flat 1/2" stab and will KE beautifully hands off the right stick and I'm talking from one side of the pattern to the other. The plane is an Akrobat II, kitted by local designer Gerry Graham back in the eighties. Powered by a Saito 125. It is a testament that KE doesn't necessarily suffer any couples.

btw, the plane has a bolt on stab, which was tweaked with a very slight shim to fully neutralize the stab/elevator.

http://pages.suddenlink.net/arlyn/P0002765.JPG

RE: Knife edge tucking
 

Another thought is that knife edge presents a contrasting lifting body section. The lifting body structure upright is symmetrical whereas the lifting body in KE is not. In another life I'm a sailing enthusiast and am very aware of the changes that take place when a sail boat heels. The immersed hull form goes from symmetrical to asymmetrical and depending on the asymmetrical shape (and its center of lift) can produce what some sailors call monster yaw. In water, the forces are stronger but the speeds are slower.

It seems to me that it is very possible that some yaw and pitch couples in KE are the product of the KE lifting body asymmetrical shape.

RE: Knife edge tucking
 

Yes - short answer
also the relative weight (vertical CG) will change knife edge flight
I have flown flat foamies which fly extremely well and changed roll couple by placing battery pack from lower fuselage location to upper area.
Also, you don't need wing in central fuselage location for great knife edge flight.
A very scale ZLIN 526 will -properly setup, do perfect knife edged flight as well as slow or multiple rolls -on a string wiuth no visible deviation.
You can setup just about any model to do this - if you tweak dihedral properly.
Just make the proper compromises to CG and dihedral - and sometimes thrust line- most models then become friendly in knife edge.

RE: Knife edge tucking
 

Interesting point, funny you mention that. My Funtana and Funworld and the l8 advanced 40 (all flat stabs) tuck worse than my King Kobra (foil stab). Not sure if its because of the stabs or CG. In the flat stab planes, the rudders are bigger in relation to the plane, and I have them balanced favoring tail, more responsive for 3D type stuff. On the Kobra however, balance favors nose, and the rudder is relatively smaller. Hmmm, think we're onto something here...

By the way, the other day I experienced exactly why Istarted this thread, to understand why planes do this to try to set them up so I won't have to use a mix. I gave in and set up the mix. It helped alot on KE. BUT, I forgot to turn it off, and upon landing almost stalled the plane in.

If it's a question of CG, incidence, etc, I'd rather just practice and learn the timing. OR, is there a such a thing as a mix for a mix? For example, having the mix only activate above a certain throttle point?