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Canopy area vs knife edge coupling - 1/24/2004 8:25:25 PM   
crossup-RCU


 

Posts: 64
Joined: 7/22/2003
From: annapolis, MD, USA
Status: offline 		Chip, can you explain the relationship between lateral area placement and knife edge control coupling? I ask because I have a Texas Hurricane on which I replace the short fishbowl canopy supplied by Kangke which is either an old version or incorrect. Since the only photos of the real Hurricane I can find show a rather long canopy ala Extra 330L, I used one from a GP Giles that by chance is almost exactly the correct size and shape to match those photos. In reviews, I see the claim for practically no pitch coupling and NO mention of roll coupling. With my long canopy Hurricane I get pitch coupling and roll coupling. My first attempts at knife edge were done at moderate speed(thinking the plane was so light it didnt need to scream along). I was horrified to finds roll coupling requiring 50% aileron correction and pitch coupling so bad that I needed nearly full up elevator(low rates) to hold a line. With a good running start and full throttle (OS 160 , 18x6/10 Top Flite Pro she can just barely claw her way out of a hover) I am now using just 8% up elevator(Futaba 8u) and no aileron. But I have to think Ive caused it to need mixing because of the canopy. FWIW, my CG is about as far back as possible, not pitch unstable but pretty much hands off inverted and local experts commented they could tell she was pretty tail heavy watching it fly. Since I like the way she flys with aft CG I am inclined to leave it that way unless that is involved in my coupling.
Crappy webcam pic shows trial fit of the new canopy.. old one just covered the cockpit "hole" and was literally fishbowl shaped

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RE: Canopy area vs knife edge coupling - 1/24/2004 8:31:52 PM   
crossup-RCU


 

Posts: 64
Joined: 7/22/2003
From: annapolis, MD, USA
Status: offline 		another view

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(in reply to crossup-RCU)
       Post #: 2

RE: Canopy area vs knife edge coupling - 1/26/2004 4:51:27 AM   
Jleyland



Posts: 360
Joined: 11/22/2002
From: Doylestown, PA, USA
Status: offline 		Hey Crossup,
Read this attachement thats in GRH's (George Hicks') post. It's fascinating stuff. The article is excellent, and debunks a few popular theories. I'm not sure if this is exactly what you are after but judging by your post I think you will enjoy it. (the attachement is in post #11)

http://www.rcuniverse.com/forum/Low%2Dwing_aircraft_and_knife%2Dedge/m_120014/tm.htm

take care,
Jon Leyland

< Message edited by Jleyland -- 1/26/2004 4:50:13 AM >

(in reply to crossup-RCU)
       Post #: 3

RE: Canopy area vs knife edge coupling - 1/26/2004 12:48:58 PM   
Passport1


 

Posts: 719
Joined: 12/7/2002
From: Sierra Vista, AZ, USA
Status: offline 		The canopy has nothing to do with your coupling, it would have done it no matter what canopy was glued on. The roll coupling is caused by having the incorrect amount of dihedral, and the pitch coupling is due to your CG. Tailheavy planes pitch to the belly while noseheavy planes go to the canopy.

Chip
quote:

ORIGINAL: crossup-RCU

Chip, can you explain the relationship between lateral area placement and knife edge control coupling? I ask because I have a Texas Hurricane on which I replace the short fishbowl canopy supplied by Kangke which is either an old version or incorrect. Since the only photos of the real Hurricane I can find show a rather long canopy ala Extra 330L, I used one from a GP Giles that by chance is almost exactly the correct size and shape to match those photos. In reviews, I see the claim for practically no pitch coupling and NO mention of roll coupling. With my long canopy Hurricane I get pitch coupling and roll coupling. My first attempts at knife edge were done at moderate speed(thinking the plane was so light it didnt need to scream along). I was horrified to finds roll coupling requiring 50% aileron correction and pitch coupling so bad that I needed nearly full up elevator(low rates) to hold a line. With a good running start and full throttle (OS 160 , 18x6/10 Top Flite Pro she can just barely claw her way out of a hover) I am now using just 8% up elevator(Futaba 8u) and no aileron. But I have to think Ive caused it to need mixing because of the canopy. FWIW, my CG is about as far back as possible, not pitch unstable but pretty much hands off inverted and local experts commented they could tell she was pretty tail heavy watching it fly. Since I like the way she flys with aft CG I am inclined to leave it that way unless that is involved in my coupling.
Crappy webcam pic shows trial fit of the new canopy.. old one just covered the cockpit "hole" and was literally fishbowl shaped

It

(in reply to crossup-RCU)
       Post #: 4

RE: Canopy area vs knife edge coupling - 1/26/2004 3:07:19 PM   
aerobaticsdude


 

Posts: 69
Joined: 3/3/2003
From: USA,
Status: offline 		What about a plane that has opposite pitch coupling, pitches to the canopy one direction and to the gear the opposite?

(in reply to Passport1)
       Post #: 5

RE: Canopy area vs knife edge coupling - 1/26/2004 4:34:17 PM   
crossup-RCU


 

Posts: 64
Joined: 7/22/2003
From: annapolis, MD, USA
Status: offline 		Thanks Chip, I guess Im stuck with the roll coupling which is pretty minimal as there is no way that I know of to adjust dihedral with a wing tube involved. Would you suggest staying with the aft CG as a comprimise for 3D work and live with the mixing?.. or try to get the CG to where the plane need no mixing(Im mixing on a 5 point curve).. that I presume would make KE control less speed dependant , I would like to be able to do high alpha KE... which means slow I presume Thanks very much.

Jon: thanks- rather more detailed than I need but great info...

< Message edited by crossup-RCU -- 1/26/2004 11:51:36 AM >

(in reply to aerobaticsdude)
       Post #: 6

RE: Canopy area vs knife edge coupling - 1/26/2004 8:57:07 PM   
OUTCAST


 

Posts: 880
Joined: 1/5/2002
From: ., NJ, USA
Status: offline 		Crossup, MANY years ago, before computer radios we used some tricks to help with coupling. For pitch coupling the only cure I have found that works is changing the C.G. as stated by double-vision. For roll coupling however I have found that some correction can be made by changing the location of the radio gear. If you have a mild roll coupling to level raise the batteries and receiver to the top of the fuselage, and use light weight wheels, it doesn't sound like it will do much but you may be surprised..
I have a hurricane and it has almost no pitch coupling, and no roll coupling at all. My C.G. is in the forward 1/4 of the wing tube, my receiver battery is in the turtle deck behind the canopy and my ign. battery is at the top of the firewall.

Aerobaticsdude,What about a plane that has opposite pitch coupling, pitches to the canopy one direction and to the gear the opposite?
I have never seen this in normal knife edge, I have seen it only once in very high alpha knife and I believe it is due to the "P" factor as the plane always went left as viewed from the rear, just like it did on its take off roll if no right rudder was used.

(in reply to crossup-RCU)
       Post #: 7

RE: Canopy area vs knife edge coupling - 1/26/2004 11:19:06 PM   
aerobaticsdude


 

Posts: 69
Joined: 3/3/2003
From: USA,
Status: offline 		Outcast,
I have 2 planes that do it. Ones a CA Models Eclipse and the others a 42% Troy Built Extra. This is straight and level knife edge, no high alpha.

(in reply to OUTCAST)
       Post #: 8

RE: Canopy area vs knife edge coupling - 1/27/2004 12:07:11 PM   
Passport1


 

Posts: 719
Joined: 12/7/2002
From: Sierra Vista, AZ, USA
Status: offline 		Ok guys hears the trick to fix the belly pitching one way and canopy pitching the other. Fortunately I had a conversation last summer with long time pattern flyer Dave lockhart and this very subject came up. He claims that you can fix this by adjusting the stab and I believe him. I do not want to put out bad info., so I am going to contact him today to make sure I tell you the proper amounts and movements that it will take.

My planes have never needed that type of elev. mix, because its a given they are going to require up elev on knife edge. This is due to the location I put all of my stabs in, which is higher then most. To me when I fly a plane with 0 mix, there are certain attitudes and angles the planes feel very unstable in. I also do not like the way those planes enter and exit rolling manuevers because I never know what to expect from them due to the conditions upstairs. It doesn't mean they fly any worse, having 0 mix, but when you can't be inside it, the only thing you really have to go on is past experience and I find that its better when its always the same. Just the way I feel, please don't read anymore into it then that.

I'll get back to this ASAP

Chip
quote:

ORIGINAL: aerobaticsdude

Outcast,
I have 2 planes that do it. Ones a CA Models Eclipse and the others a 42% Troy Built Extra. This is straight and level knife edge, no high alpha.

(in reply to aerobaticsdude)
       Post #: 9

RE: Canopy area vs knife edge coupling - 1/28/2004 1:55:30 AM   
crossup-RCU


 

Posts: 64
Joined: 7/22/2003
From: annapolis, MD, USA
Status: offline 		You may have put your finger in the real problem.. since Chip has eliminated the canopy as a culprit... I have two thing done very differently than you: my OS engine very is light..and of course that means less weight up high(and no ign. battery weight) and I have a 24 oz. tank UNDER the wing tube. So I guess my upcoming switch to my ZDZ 40 may help. And I might be able to redo my tank mounts and get it higher, in front of the wing tube. If memory serves me my CG is at the back edge of the wing tube... what can I say? all my planes are setup with maximun rear CG....so far its been clearly more to my liking that way...then again maybe that helped me lose my Giles which snap rolled on a touch and go..

quote:

ORIGINAL: OUTCAST

Crossup, MANY years ago, before computer radios we used some tricks to help with coupling. For pitch coupling the only cure I have found that works is changing the C.G. as stated by double-vision. For roll coupling however I have found that some correction can be made by changing the location of the radio gear. If you have a mild roll coupling to level raise the batteries and receiver to the top of the fuselage, and use light weight wheels, it doesn't sound like it will do much but you may be surprised..
I have a hurricane and it has almost no pitch coupling, and no roll coupling at all. My C.G. is in the forward 1/4 of the wing tube, my receiver battery is in the turtle deck behind the canopy and my ign. battery is at the top of the firewall.

Aerobaticsdude,What about a plane that has opposite pitch coupling, pitches to the canopy one direction and to the gear the opposite?
I have never seen this in normal knife edge, I have seen it only once in very high alpha knife and I believe it is due to the "P" factor as the plane always went left as viewed from the rear, just like it did on its take off roll if no right rudder was used.

(in reply to OUTCAST)
       Post #: 10

RE: Canopy area vs knife edge coupling - 2/19/2004 2:01:39 AM   
mithrandir



Posts: 683
Joined: 7/16/2003
From: adelanto, CA, USA
Status: offline 		Somewhere in here, in another thread, somebody took his CAP. which yaw coupled into pitch terribly, lowered the stab on the fuse, and it got better. I have flown 3 different versions of the Sukhoi 31. 2 of the 3 had the stab in the "Near Scale" location, and pitch coupled. The new CERMARK SUKHOI 31 has the stab lowered on the fuse, and half power 45 degree "Knife Edge Harriers" with NO COUPLING! They also made the top of the wing flat because the scale "ZERO" dihedral configuration tends to suffer adverse roll coupling with rudder. This CERMARK SUKHOI would not necessarily qualify as a 3D plane, but it LOMS for days... keep your foot in it and it will lom to the ground!!! Decent harriers with the CG aft. I guess I am trying to say that I have observed a corrolation between pitch coupling and stab location on the fuse. I had assumed that is why Quique put the stab so low on the TOC Extra....

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(in reply to crossup-RCU)
       Post #: 11

RE: Canopy area vs knife edge coupling - 2/19/2004 10:24:20 AM   
Passport1


 

Posts: 719
Joined: 12/7/2002
From: Sierra Vista, AZ, USA
Status: offline 		YOu are abnsolutlely correct about the stabs effect on knife edge coupling, and the other factor is CG.
quote:

ORIGINAL: mithrandir

Somewhere in here, in another thread, somebody took his CAP. which yaw coupled into pitch terribly, lowered the stab on the fuse, and it got better. I have flown 3 different versions of the Sukhoi 31. 2 of the 3 had the stab in the "Near Scale" location, and pitch coupled. The new CERMARK SUKHOI 31 has the stab lowered on the fuse, and half power 45 degree "Knife Edge Harriers" with NO COUPLING! They also made the top of the wing flat because the scale "ZERO" dihedral configuration tends to suffer adverse roll coupling with rudder. This CERMARK SUKHOI would not necessarily qualify as a 3D plane, but it LOMS for days... keep your foot in it and it will lom to the ground!!! Decent harriers with the CG aft. I guess I am trying to say that I have observed a corrolation between pitch coupling and stab location on the fuse. I had assumed that is why Quique put the stab so low on the TOC Extra....

(in reply to mithrandir)
       Post #: 12

RE: Canopy area vs knife edge coupling - 2/22/2004 10:41:11 PM   
crossup-RCU


 

Posts: 64
Joined: 7/22/2003
From: annapolis, MD, USA
Status: offline 		Chip, have you been able to contact Dave Lockart ?

" ...Dave lockhart and this very subject came up. He claims that you can fix this by adjusting the stab and I believe him. I do not want to put out bad info., so I am going to contact him today to make sure I tell you the proper amounts and movements that it will take."

quote:

ORIGINAL: Double-vision

YOu are abnsolutlely correct about the stabs effect on knife edge coupling, and the other factor is CG.
quote:

ORIGINAL: mithrandir

(in reply to Passport1)
       Post #: 13

RE: Canopy area vs knife edge coupling - 3/14/2004 2:04:33 PM