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Knife edge trimming question - 5/9/2008 9:29:02 AM   
alpbak


 

Posts: 41
Joined: 4/27/2006
From: istanbul, TURKEY
Status: offline 		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

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       Post #: 1

RE: Knife edge trimming question - 5/9/2008 10:56:29 AM   
Mike Pascale


 

Posts: 10
Joined: 12/14/2007
From: Lake Charles, LA, USA
Status: offline 		I have found this very helpful.very nice bird

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

(in reply to alpbak)
       Post #: 2

RE: Knife edge trimming question - 5/9/2008 12:56:26 PM   
alpbak


 

Posts: 41
Joined: 4/27/2006
From: istanbul, TURKEY
Status: offline
quote:

ORIGINAL: Mike Pascale

I have found this very helpful.very nice bird

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

That is a very helpful article. Thanks.

(in reply to Mike Pascale)
       Post #: 3

RE: Knife edge trimming question - 5/9/2008 3:21:50 PM   
BaldEagel



Posts: 4088
Joined: 10/12/2005
From: Wychling, Kent, UNITED KINGDOM
Status: offline 		Another usefull trimming chart, this one I can vouch for personnaly, well it worked for me: http://www.mini-iac.com/Portals/57ad7180-c5e7-49f5-b282-c6475cdb7ee7/PGoldsmith%20on%20trimming.pdf

Mike

_____________________________

No matter what anyone says 100% is the maximum you can get.

(in reply to alpbak)
       Post #: 4

RE: Knife edge trimming question - 5/9/2008 3:48:28 PM   
Dean Pappas


 

Posts: 163
Joined: 1/27/2004
From: South Plainfield, NJ, USA
Status: offline 		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

(in reply to alpbak)
       Post #: 5

RE: Knife edge trimming question - 5/9/2008 5:02:32 PM   
flyncajun


 

Posts: 117
Joined: 12/25/2006
From: DENHAM SPRINGS , LA, USA
Status: offline 		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

(in reply to Dean Pappas)
       Post #: 6

RE: Knife edge trimming question - 5/9/2008 8:21:15 PM   
Dean Pappas


 

Posts: 163
Joined: 1/27/2004
From: South Plainfield, NJ, USA
Status: offline 		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

(in reply to flyncajun)
       Post #: 7

RE: Knife edge trimming question - 5/10/2008 8:57:46 AM   
alpbak


 

Posts: 41
Joined: 4/27/2006
From: istanbul, TURKEY
Status: offline 		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

(in reply to Dean Pappas)
       Post #: 8

RE: Knife edge trimming question - 5/10/2008 12:40:17 PM   
vbortone


 

Posts: 192
Joined: 4/6/2004
From: Olathe, KS, USA
Status: offline 		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

(in reply to flyncajun)
       Post #: 9

RE: Knife edge trimming question - 5/15/2008 9:39:17 AM   
alpbak


 

Posts: 41
Joined: 4/27/2006
From: istanbul, TURKEY
Status: offline 		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

(in reply to vbortone)
       Post #: 10

RE: Knife edge trimming question - 5/17/2008 3:53:04 AM   
MTK


 

Posts: 460
Joined: 5/14/2004
From: Whippany, NJ, USA
Status: offline
quote:

ORIGINAL: Dean Pappas

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

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

(in reply to Dean Pappas)
       Post #: 11

RE: Knife edge trimming question - 5/19/2008 3:52:42 AM   
Rendegade



Posts: 839
Joined: 3/19/2003
From: Perth, AUSTRALIA
Status: offline 		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.

_____________________________

It's easy to fly, just throw yourself at the ground and miss!

(in reply to MTK)
       Post #: 12

RE: Knife edge trimming question - 5/19/2008 11:09:18 AM   
alpbak


 

Posts: 41
Joined: 4/27/2006
From: istanbul, TURKEY
Status: offline
quote:

ORIGINAL: MTK
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


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?

(in reply to MTK)
       Post #: 13

RE: Knife edge trimming question - 5/19/2008 3:14:29 PM   
MTK


 

Posts: 460
Joined: 5/14/2004
From: Whippany, NJ, USA
Status: offline
quote:

ORIGINAL: alpbak

quote:

ORIGINAL: MTK
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


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?

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

< Message edited by MTK -- 5/19/2008 3:25:27 PM >

(in reply to alpbak)
       Post #: 14

RE: Knife edge trimming question - 5/20/2008 12:00:02 PM   
alpbak


 

Posts: 41
Joined: 4/27/2006
From: istanbul, TURKEY
Status: offline
quote:

ORIGINAL: MTK
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


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

(in reply to MTK)