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-   -   Knife edge pull (http://www.rcuniverse.com/forum/aerodynamics-76/11308327-knife-edge-pull.html)

mvallyman 11-24-2012 08:08 PM

Knife edge pull

Im flying a 50cc EG Aircraft MX2.
On knife edge using right rudder it fly's strait but on left rudder knife edge it pulls to the gear quite a bit. Even though I have applied a mix with the Tx it will only work for one speed and still pulls to the gear at faster speed or pulls to canopy at slower speed.

What else can I inspect to find why it pulls on knife edge using left rudder only?

speedracerntrixie 11-24-2012 08:32 PM

RE: Knife edge pull
All aerobatic models I have owned have pulled to the gear with application of rudder. Moving the CG forward a little will help but usually not eliminate this. Adding a touch of positive wing and stab incedence will help too. What I think is happening on your setup is that you are over compensating with the mix at max rudder deflection. The control cross couple is not liniar but the mix that you set up is. If your TX has a multi point mix use it and decrease the amount of mix % you get at high rudder travel.

The first step however is to measure the airplane carefully and correct any mis-alignments.

mvallyman 11-24-2012 09:33 PM

RE: Knife edge pull

Yes you are correct that my mix is overcompensating at full rudder, but before I go into mixing it on a curve I was interested in finding out why it does this only going knifedge with left rudder (canopy toward you going left to right) and not right rudder.

Any idea's?

speedracerntrixie 11-24-2012 09:42 PM

RE: Knife edge pull
I don't have an overly technical answer for you other then to say it has something to do with the rotational mass of the prop. Some will say it's P factor, some will say it's spiral slipstream. Whatever you choose to call it, you are on the right track with the mix curve and the mix not being the same for both directions.

wingster 11-24-2012 09:52 PM

RE: Knife edge pull
As you move the cg back it will pull to the belly with left rudder. If you continue to move the cg back it will begin to pull to the belly with right rudder as well. I believe the difference betwen left and right rudder is due to the spiraling prop wash.

If you have adjustable stab halves you can reduce the pull with left rudder by decreasing the right stab incidence. In left rudder knife edge the right stab dominates since it is not shielded by the fuselage.

mvallyman 11-24-2012 10:01 PM

RE: Knife edge pull
Thanks to both of you for your input.
Both factors make sense with the prop factor but didnt know about the left stab not fully working with left rudder knife edge. I will play with both the mix curve and the incidence adjustment and see if there are any adverse effects with unequal stab incidence.

Foxman 11-26-2012 12:05 AM

RE: Knife edge pull
You could mix a bit of elevator in with the rudder to compensate. Just mix it one way.
Not an answer to the airframe problem I know but it might get around it.

opjose 11-27-2012 01:25 PM

RE: Knife edge pull
Gyroscopic precession also contributes a great deal.

In one direction precession is ameliorating any pull to the belly, while in the other it exacerbates it.

flyncajun 12-07-2012 12:06 PM

RE: Knife edge pull
To understand this problem and fix it go here hebertcompetitiondesigns.com
Search for Triangulation trimming

I offer a detailed 4page Heavy Laminated trim guide, with step by step details to solve these issues.

jetmech43 12-08-2012 06:29 AM

RE: Knife edge pull
Use Bryans chart it is by far the best info you can get for trimming. Read it, love it, live it

mithrandir 01-02-2013 11:46 PM

RE: Knife edge pull
imagine your plane with no propellor.... in KE it would push to the belly slightly both left and right rudder....
now.. superimpose on this tendency the effect of "P" factor... this is the tendency of a prop, with its axis at some significant AOA
having more "Bite" on one side than the other....

With right rudder.... the "P" factor is towards the canopy... nulling the belly pitch inherent in the design...

with left rudder.. the two effects combine to create a pitch toward the landing gear...

try the following test with no mixing.....

get the plane really high, pull the power to idle, push it straight down and apply rudder... see what happens....
repeat again applying rudder the other direction.... I bet there is a very slight trendency to push towards the gear both ways....
(This test is designed to reduce the influence of the motor/prop)

jnhiller 01-08-2013 05:20 PM

RE: Knife edge pull
If you haven't checked the wing and stab incidence, they should be the same. Next equalize right and left elevator deflection. I'd also lateral balance to center aileron neutral, adjusting the wing until no roll is noticed inverted or upright. Re-evaluate KE pitch after checking and correcting airframe deficiencies if there are any.

Foxman 01-08-2013 05:59 PM

RE: Knife edge pull
One thing that can make an airframe pull one way in knife edge is the tailplane height.
Changing this can make a big difference.
You can use anhedral or dihedral on the tailplane to cheat a bit as this has the effect of lowering or raising the effective height.
The old Prettner Curare had anhedral to compensate for a tailplane that was originally too high.
The full size Hawk/T45 has anhedral on the stab and it knife edges very well as a model.
Moving the cg forwards or back can help the knife edge trim but that will ruin some other part of the flight envelope. There is only one correct place for the cg and that is in the correct place.
Using too much rudder can also cause problems.

jnhiller 01-08-2013 06:36 PM

RE: Knife edge pull
Tail plane height, can be changed by changing the wing incidence, causing the tail to fly higher or lower, however this changes the flying angle of the fuselage and effective down thrust. Before considering changing anything the vertical up pitch condition should be evaluated also.

rmh 01-08-2013 07:08 PM

RE: Knife edge pull

ORIGINAL: opjose

Gyroscopic precession also contributes a great deal.

In one direction precession is ameliorating any pull to the belly, while in the other it exacerbates it.

The precession only occurs whilst the gyroscope is changing directions
While holding a steady attitude/line - there is no precession- It is basic physics
The prop running "out of square " that is slipping at some angle will cause it to try and move the craft of line
Just as a helicopter tilts the rotor slightly and whilst tilted the machine moves fore/aft/ left/right, due to the angle.
As the tilt is taking place (transitioning ) there is precession. Same on a fixed wing

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