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Marty_d

701 elevator / flaperon control system

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Hi all,

 

I need some advice from those who've actually flown the CH-701.

 

As you know the plans show the main flaperon torque tube with brackets welded to it which hold the elevator bellcrank.  This means when you move the stick sideways for bank, the elevator bellcrank rotates with it.

 

In the zenith.aero site there's lots of discussion about "divorcing" the elevator control from the flaperon torque tube, so it doesn't rotate.  Several people have done this by various methods.

 

My question is... is it necessary?  When you're flying the 701, do you notice any binding or control conflict when you use flaperons and elevators together?

 

I'm at the stage of getting the various controls ready for welding, and I'd rather not get the brackets welded to the torque tube if I'm going to do an alternative option.  On the other hand, I don't really want to second guess the designer if it's not necessary.

 

All input gratefully accepted.   Thanks!

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Sometimes, although mostly not, it is a good thing to second guess the designer. I read what I could and spoke to whom I could while building. I found Hans who flies at Kilcoy incredibly helpful. Net result was I changed a few things whilst building. I have the divorced brackets for the elevator. I also used streamlined tube for the struts, a Savannah style tailplane/elevator instead of the inverted aero foil one and I have bolted rather than riveted the bracket for the cables on the rudder and the bottom plate to fuselage on the nose leg. Hans told me, from experience, that those rivets will work loose. It is a known problem with the standard 701 tail that the nose can drop in the flare. Everyone, including Zenith recommend fitting vortex generators on the elevator to solve this. With the Savannah style tail this is not a problem. Again, Hans has this style of tail. I really don’t know what I would have thought of the aeroplane built 100% by the book as it were, but I was interested in building and flying, not stuffing around changing things afterwards. I am very happy with the aeroplane as it now is. I think listening to people who have built and flown for a lot of hours ( not me, I only have about 100hrs on mine!) is well worth while and certainly worked for me. The 701 was the very first all metal two seat ultralight design and was originally powered with two cylinder Rotax two stroke and hence was designed as light as physically possible. Things have evolved a lot since then and some things are worth changing. I also have additional L angle diagonals on the fuselage sides to cut down oil-canning (also a known problem) and wish I had put them on the top and bottom also since it still rattles like an old tin can when I get down to 50kts on approach. Sorry for the verbosity but hope this is helpful!

  • Informative 1

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Marty I don't have the flying experience you asked for, but I got the undivorced hardware with the divorced instructions, so I got to think about this while I sourced some undivorced instructions.

 

My thought was that, given the length of the cable runs, the effect of aileron action on elevator would be minute.

However, in the undivorced version, the tension of the elevator cables is exerting a strong and constant pull on the torque tube, and in the Sav you can see where the torque tube mounts have been beefed up to counter this. And I was still surprised at the very thin collar at the front of the tube that is the thrust bearing in all this.

 

It seems to me that divorcing these controls will, if anything, result in slightly more controls interaction. But I still think it would be negligible. And I do like the idea of the cable tension being maintained by something other than the torque tube. Though to do that, the frames and floor forward of the belly hatch need considerable beefing up.

 

DSCF1978.JPG

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...and thanks...I found my missing podger........)

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The other thing that I changed, again on advice from others, was the rubber blocks on the u/c spring which will extrude out. I was advised to use ‘virgin teflon’ which thus far has worked well.

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Incidentally, if you go the ‘divorced’ route you will need rod end ball joints on the ends of the elevator push/pull tube. Question, in the photo, what aeroplane is that? I ask because the torque tube is different to my CH701.

Edited by derekliston
Additional text

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The standard setup has to be rigged with enough slack in the elevator cables to allow full right aileron deflection and then the left deflection makes the cables loose.

I think the rigging instructions are deliberately vague because the simple to construct design has these issues. Another not obvious rigging situation is the rudder tensions, which need setting with the nose unloaded because the cables go slack with nose strut compression.

Incidentally my initial CAA inspector was puzzled with the tightness of the cables and thought it might be due to the plane being rigged in the Czech  winter and now its summer in NZ and aluminium fuse has expanded....I still laugh about this.

Rather than second - guess the design I just call it a backup feature that holds the rudder firmly on - anyone familiar with 701 Hstab and rudder attachments will know I'm serious about this.

If you are at the fabrication stage I'd do the mod to not have the elevator yoke rotating on an offset to the aileron tube.

I've been flying mine since 2004 and the remedy to reduce the aileron bias (to the left) was to alter the position and tension of the bungee inside rear fuselage so it pulls the upper cable to the right. this by no means cures the problem just makes flying less tiresome - it holds a bit of trim tension the way that centralises the stick.

 

Ralph

701 cable bias.JPG

701 fuel c.jpg

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12 hours ago, IBob said:

Marty I don't have the flying experience you asked for, but I got the undivorced hardware with the divorced instructions, so I got to think about this while I sourced some undivorced instructions.

 

My thought was that, given the length of the cable runs, the effect of aileron action on elevator would be minute.

However, in the undivorced version, the tension of the elevator cables is exerting a strong and constant pull on the torque tube, and in the Sav you can see where the torque tube mounts have been beefed up to counter this. And I was still surprised at the very thin collar at the front of the tube that is the thrust bearing in all this.

 

It seems to me that divorcing these controls will, if anything, result in slightly more controls interaction. But I still think it would be negligible. And I do like the idea of the cable tension being maintained by something other than the torque tube. Though to do that, the frames and floor forward of the belly hatch need considerable beefing up.

 

DSCF1978.JPG

Hi Marty regarding the fuel collector tank. After reading some comments on the Savannah owners facebook site I have just finished modifying my fuel system to have the feed lines from the two wing tanks feeding into a 3/8" 'T' and into one of the collector tank connections in the top. The other collector tank connection nipple is reduced to 1/4" and that hose is connected to a 'T' in the top of the fuel tank contents sight tube of the Stb wing tank.  This allows the air to vent and the fuel to run to replenish the collector tank. At low fuel I find the replenishing slows and I believe this is due to the air in the collector needing to rise up to the tank which slows the fuel refilling rate. I can see the fuel rate through the filters and the collector tank at alltimes during the flight.  A couple of sav owners have done this set up including Mark Kyle.  The idea of returning to the top of the sight tube was his. Just passing on in case its of value to you. Cheers.

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You can see my fuel manifold and the single pipe down to the collector and the other pipe is the breather coming up going to the fitting at the top of my sight gauge. This has been like this and is the existing gear I made for the last 7 years and no issues ever except when the tank filler cap breather tube got blocked by a insect...then I changed them so no drama anymore

 

 

IMG_6668.jpg

IMG_6667.jpg

IMG_6669.jpg

Edited by Kyle Communications

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13 hours ago, derekliston said:

Incidentally, if you go the ‘divorced’ route you will need rod end ball joints on the ends of the elevator push/pull tube. Question, in the photo, what aeroplane is that? I ask because the torque tube is different to my CH701.

Yes I've seen that others who've "divorced" them have used ball joints to allow for the new twist in the elevator tube that wouldn't be there if it were built the traditional way.

 

In answer to your question about what plane that is with the dog-leg torque tube, Bob's building a Sav.

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14 hours ago, IBob said:

And I do like the idea of the cable tension being maintained by something other than the torque tube. Though to do that, the frames and floor forward of the belly hatch need considerable beefing up.

One method I've seen of doing this is to sleeve the bracket tube over a reduced-diameter pipe section bolted to the torque tube.  The elevator brackets are held vertical by another bracket at the top.  (Photos below sourced from zenith.aero).

This would maintain the cable tension on the torque tube as normal.

 

modified elevator bracket 1.jpg

modified elevator bracket 2.jpg

modified elevator bracket 3.jpg

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