Knife edge trimming question
 

I have finished my new Impulse and trying to trim it. I have started with +0.5 degrees of wing incidence and 0 degrees of stabilizer incidence. The engine is a 140RX and it has 2 degrees of right thrust and 1 degrees of down thrust. The CG is at the recommended position which is on the back of the wing tube. Nearly everything looks good so far, the uplines, downlines, straight flight are all good, just the way I am used to.

But I have a problem with the knife edge flight. While holding right rudder in knife edge the plane tracks good, flies straight. But when holding left rudder and flying knife edge it pulls to the belly. I have been asking around about this problem but so far, I have not received a satisfactory answer and a solution. Probably, I am missing something and doing something wrong but I can not figure out what. Can someone please advise a solution or an explanation to this situation?

Of course I can mix it out easily but I would like to know what causes this behavior.

Here a few photos of the new Impulse.

Thanks
Alp

RE: Knife edge trimming question
 

I have found this very helpful.very nice bird

http://www.hebertcompetitiondesigns.com/interest.htm

RE: Knife edge trimming question
 

That is a very helpful article. Thanks.

RE: Knife edge trimming question
 

Another usefull trimming chart, this one I can vouch for personnaly, well it worked for me: http://www.mini-iac.com/Portals/57ad...20trimming.pdf

Mike

RE: Knife edge trimming question
 

Hello Alp,
A certain amount of this may be unavoidable, because the swirling airflow from the propellor means that are airplanes are not truly symmetric, no matter how carefully made. The large difference between left and right knife edges means that the CG is just a small amount too far aft. Try moving it forward maybe 2mm at a time, and retrimming for the verticals. This will probably involve adding a tiny bit more poositive incidence to the wing. You may end up with close to 0.75 degrees incidence in the wing.

How many percent of radio coupling would it take to fix the left rudder knife edge. if it three percent or less you may prefer not to compromise the verticals, as can possibly happen when moving the CG forward. I have to say, that many airplanes are trimmed borderline tail-heavy, because of the good effect on the verticals.

Hosca,

Dean Pappas

RE: Knife edge trimming question
 

Alp
Dean is right
probably need a little more pos inc in the wing
use the snaps to determine if you are tail heavy,, I think your are not at this time.
if you were,, the right rudder knife edge would be going to the belly also.
When you get the wing inc more positive the say .75 you will also have straight downlines along with the perfect left rudder knife.

another way to fine tune you c/g
do a knife edge loop ,,, at the bottom of the loop with minimal throtle if you go to the belly ,,tail heavy and vise versa.
fine tune you snaps with the rudder input ,,always use at least 18 deg of ail. throw for upline snaps,
and very little elevator. ,,, this will need to be fine tuned once you have adjusted the c/g perfectly

re- read my article very carfully all the information you need is in there
Bryan

RE: Knife edge trimming question
 

Hi Bryan!
Back in the days when there wasn't a chance that our Pattern ships would actually pull out of a knife edge loop, we used to test by pushing a substantail amount of rudder into a power-off vertical downline, but most designs (even some very good sellers) curve to the belly in this condition. I don't even suggest this test anymore, as I would receive too many letters and e-mails from readers who want me to help fix the bad designs they spent a lot of money on!

take care, All

Dean

RE: Knife edge trimming question
 

Dean and Bryan,
Thank you for your explanations and suggestions. I have a clear view of everything now. Next time I go flying I'll do what you suggest and try to make it a perfectly trimmed aircraft (if there is such a thing ;))

I have read many articles about trimming but, Bryan, your article was really helpful, it helped to clear many question marks in my head.

Alp

RE: Knife edge trimming question
 

I agree with Dean. I suggest to keep the incidence an T/L in close range also. No more than 0.5 degree difference.

Regards,

Vicente "Vince" Bortone

RE: Knife edge trimming question
 

I increased the wing incidence to 0.75 degrees and moved the CG a little forward and the problem is nearly solved. I only needed 2% rudder-elevator mix. I'm sure I can take out the mix by working some more on the CG and incidence but I am happy at the moment, so I'll live with the mix. I have to fly a lot and get used to the plane and the P09 program, Turkish Nats is on the 31st of May and I must prepare myself for the big event. After all, the first 3 will go to European Championship in August. I hope to be able to finish in first 3.

Thank you all for your help.
Alp

RE: Knife edge trimming question
 

Dean,

It sounds like a business opportunity. LOL....

Seriously, you can't go wrong with the suggestions Dean and Bryan have made. Actually, these are more than suggestions...fixes might be a better term.

One other thing you can take a look at is relative incidence between the panels of both stab and wing. There should no difference in incidence between wing panels and stab panels at the start. You may add a bit negative stab incidence to the stab half that is pointing to ground during knife edge. Remember that the top surfaces are blanked a bit by the fuse angle making the bottom just a bit more effective. Don't overdo this because it will affect other characteristics

MattK

RE: Knife edge trimming question
 

Wow!

I've gotta admit I was a little hesitant to start messing with decalage again, but after using Bryans method I can quite happily fly all day on knife edge.

It works wonderfully.

RE: Knife edge trimming question
 

Do you mean to add negative stap incidence only to one half of the stab and leave the other one the same? I can try this but wouldn't that cause some other major problems?

RE: Knife edge trimming question
 

Alp,

Yes that is the idea. This "fix" should only be used when the model pitches with top rudder in one direction and does not in the other. The adjustment must be made in very small increment like half turn at a time on the adjuster. Small adjustments such as this to target a specific flaw, need to be made with your eyes wide open such that other flight regimes are not compromised.

Some models, perhaps most, can't be trimmed to fly perfectly at all flight regimes and will probably require some small amount of electronic mix. What you are after is the minimum amount of electronic mix possible for that particular model.

For what it's worth, the spiralling airstream produced by the propeller causes an unbalanced effect that is very difficult to eliminate all effects from. If you don't believe the effect of the spiralling airstream, try to hazard a guess of the amount of air tonnage (yes, tonnage) moved in a typical pattern flight. Air moved becomes huge (hundreds of tons) over the span of a 10 minute flight. The reactive forces from that much motion are equally large; it's quite a feat to get our models as close as we do

MattK

RE: Knife edge trimming question
 

MattK,
What you suggest is interesting and worth trying. I will definitely try that and see what happens. I will write the result here.

Thanks
Alp

RE: Knife edge trimming question
 

Alp,
Read Bryan's article again, he provides a very good explanation as to why you don't want to trim differential into your stab.

RE: Knife edge trimming question
 

I was at the flying field yesterday for some practicing and some more trimming.

After some more trimming, the plane is now perfect. I was able to take out the rudder to elevator mix. At the moment I have a plane which do not need any electronic mix and I'm very happy with that.

I would like to thank Bryan for that article which helped me a lot. I printed it and it is in my flight box all the time from now on.

Alp

RE: Knife edge trimming question
 

It is good to know that worked for you. What are the final values of CG position, incidence, thrust line, right thrust and stab? Also we would like to know the initial values before you started the triangulation process.

Thanks,

Vicente "Vince" Bortone

RE: Knife edge trimming question
 

Here are the before and after values.

Wing incidence:
Before: +0.5
After: +0.98
I increased the incidence in very small increments (quarter turns at a time)

Stabilizer:
0 degrees (no change)

Engine:
2 degrees of right thrust
1 degrees of down thrust.
(No change. Values are as suggested by the manufacturer/BN Models)

CG:
Before:Back of the wing tube.
After: Close to front of the wing tube
(I can't give a specific value for the CG as I have not measured it)
At first, I installed the wheel pants which weights 60 grams (2.1 oz), this moved the CG a little forward. Second, I replaced the 800mah lipo battery with a 2200 mah lipo battery(73 grams/2.6 oz extra weight). Finally I moved the receiver and the BEC circuit a little forward. If I needed to move the CG more forward I was planning to replace the carbon fiber spinner with an aluminum one, but it was not needed.

Weight:
Before: 4505 grams
After : 4638 grams

As you can see I didn't have to modify the engine thrust line and the stab. I was able to solve the problem only by changing the wing inc. and CG.

Alp

RE: Knife edge trimming question
 

In addition to my previous post, to my surprise, these changes affected the left rudder knife edge problem. Straight flight is still straight, inverted flight still needs a little down elevator, uplines are straight and vertical. Only other change happened in the downlines. The plane used to pull to canopy after going straight down for some time but after the changes it does not pull to canopy as it used to. Now it needs to go down some more to pull to canopy. Usually during the P09 flight I don't have enough space for it to pull to canopy. So basically it does not pull to canopy at all.

Alp

RE: Knife edge trimming question
 

HI Alp
I`m very glad you chose to use My trim article" Triangulation Trimming " and stuck with it to the end
most guys give up and punt :)
this article comes from Years of practicle knowledge, 15 or so designs and many many hours of tweaking on hot summer days.
I will be doing a video later with more in depth methods
thanks for sharing your findings with everyone it will probably inspire many more guys to use these trim setups.
now my only problem is,,, my ace has been played <G>
Bryan

RE: Knife edge trimming question
 

Degrees
One degree of incidence?
one degree in one foot is almost 1/4"
do you guys actually do this?
I must be daft as I find that typically anthing over 1/16 "incidence change is a LOT
By 1/16" I mean the existing centerline of the wing (the big one -up front) if shimmed up or down at one end of root airfoil. Most trimming -I always found was to correct a botch up in building or balancing.
I still get guys wanting to take their 3D thingies and get a solid predictible performance -with same CG.
after flying stuf upto inc 42% I found it simply is a waste of time
you have to decide which you prefer then rebalance to get best characterisic of THAT particular CG
when it's all said n dun I still go back to an old friend's advice -(he flew and was proficient in full scall helicopter /multi and single )
I asked him how do you trim it?
he said "well I really don't I just fly it and compensate as needed ."

Iwas doubious-at best then foundthat depending on weather conditions - the required inputs varied - so MY best setup always was one which was easiest to correct and was on the 'stable " side .
in a down line a litle down pressure - etc..
With my DX7 this requires LOTS of expo to prevent overcontrolling - with my X9303 - the "setup" in the TX is smoother . All tx are NOT the same -and it ain't slop in the system .
rollers on th DX7 get wavy -till you get used to response (fast as hell)

so "trimming" has a lot to do with the radio it's self unles you believe that thru some devine intervention - the proper trim will allw th model to fly it's self. (correct typing as req'd)

RE: Knife edge trimming question
 

Hi Dick,
it seems we have had this aurgument together about 10 years ago
Nat told Me the same thing about 20 years ago.
I think you need to wait for the video:)
yeah!,you can make it fly with a 1/4 deg of inc.<in the big wing up front> as you put it , But ,OH! No!
I can`t do a knife edge loop without tons of mixing or an upline snap and stay on line,
Oh Yeah and my left rudder knife edge wanders all over the place
come to think of it so does my upline and the down lines
in the world of FAI these things have to be perfect or ,you give up points
only a few guys in the US can make it look" Good" with that set up ,Brett ,Jason, Chip
don`t forget ,,I owned one of your TIPO`S [&o]
it`s where I started this Effort 20years ago
I tell you what ,,,go try my method , give it an Honest effort and, report back to the list, then poo! poo! it if you can,
everybody I have helped has had solid results. with a very skepticle reaction at first, you might even be able to come off of your expo a little.
I already used your method ,,,it don`t work, so I developed mine :D
Sorry Dick, it`s a different Era !
may-be you would be more help teaching me how to spell[8D] ,,no that s a loosing battle LOL!
Bryan

RE: Knife edge trimming question
 

{Quote}
*******"it`s a different Era ! "

{Quote}

True.
But our thumbs were faster.:eek::D[:o]

RE: Knife edge trimming question
 

LOL!
yeah Mine too,,but not fast enough
all the more reason to use my trim method <G>
I blame it all On Dean P. and Don L.
they really lead me in the right direction and I could not stop tinkering
Bryan