Thorp Air Command - T18.net

Hi all
I have draw a flat layout of the aft tunnel based on J Thorps original drawing number 505.
My drawing is per plans except that I have attempted to modify the layout to incorporate a 1/2" radius on the top corners rather than the .09"R called for in the plans. I note in past newsletters that a few builders have attempted to do something similar with options along the lines of removing, reducing or rounding off the tunnel to get more space between the seats.

I hoped that by using a 0.5" radius I could without being too radical partly improve the comfort factor while still allowing for the standard flap handle to be fitted if needed, although personally I am inclined towards installing electric flaps.

Does anyone see any issues with this approach?
I am reasonably confident that there would be no interference issues and minimal change to structural strength, but I did have to modify the flanges slightly to accommodate the curves where the tunnel attaches to the aft cockpit frame.
Does anyone see any structural issues to be concerned about?
Should I try and increase the radius further and just go with electric flaps?

Any other comments or suggestions, I'd also be keen to know if I should be thinking of adding any access/ inspection holes .

Thanks

_________________
Stug

Stuart,

I built my 1984 T-18 with a semi-rounded tunnel as you suggest. I am extremely happy with the results. The tabs you mentioned were shortened accordingly. In addition, however, I reinforced the top and sides of the tunnel with a doubler that triangulates the corners, thus beefing up the tunnel to serve as a step. No stepping on the seats upon entry or exit!

I wish that I had rounded the forward tunnel, as well. While on tunnels, I would recommend that no wiring be routed through them as I had a near disaster with a wire coming in contact with a rudder cable. The cable nearly burned in two!

_________________
Bob Highley
N711SH
SN 835
KLAL

thanks Bob
It shouldn't be too hard to incorporate a doubler or some extra stiffeners to beef up the structure, so that sounds like a smart idea. How much clearance did you leave to the between the doubler and the elevator tube?
Would another option be instead of adding doublers to use thicker 0.032 in place of the 0.025 called for in the plans? But I'm guessing the doubler would be lighter, and even 032 may still need some reinforcing to cope with the weight.

It's been a while, but I believe I used .032. whith the doubler being .032 as well. I will try to get a picture for you. I am in the process of building the tunnel on another project I'm working on.

The doubler is mearly added to the inside of the tunnel with the exception of going at 45 degree angles when going round the bends. Tube clearance remains virtually the same. Picture to follow.

_________________
Bob Highley
N711SH
SN 835
KLAL

Updated drawing attached.
I have made a few subtle revisions and have also included one possibility for bracing. It isn't exactly what was suggested but it may be adequate reinforcing to allow for standing on. This approach was a relatively easy option for me to draw more than anything else.
Any pictures would be great.

Forum post for fuselage/empennage.

I took a couple of pictures of my tunnel out at the hangar today. Both the tunnel and stiffener are .025. Wouldn

Thanks Bob... as I've said before, the photographs on this forum are GOLD to those of us who didn't build the airplane we own.

Man, I wish I had seen how you did your seats before I did mine. Great idea! (Maybe it was in a newsletter and I missed it?)

D

After reading the comments about rounding the forward tunnel, I have just had a go at drawing the flat layout of the forward tunnel using the 1/2" radius on the top corners of the tunnel following the same approach as on my earlier drawing of the aft tunnel.
I have a bit of a problem working out what to do with the two upper forward tabs that attach the forward tunnel to the firewall. With a .5 radius the tabs end up on the curved section so I might need to modify them somehow but I am not sure what would work best.
Any suggestions?
Stuart

Stug,
The forward tunnel is not structural. Even with the rounded corners you should be able to get the same attach pattern called out in the drawings. One on each of the upper tabs and two on each side tab (6 total).

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

Lee,
I can can get the hole pattern to match but doing it the way I am, based on my limited sheet metal experience, leaves the tab with only about 1/4" of material joining it to the tunnel. The updated drawing attached shows what I mean. There might be another way of setting out the tab but I am not sure what it would be, the only other option I could think of would be to move the tab inward about 1/2" and attach them through the 2 angles that run up the firewall.

Stug,
I would not be afraid to go through the angles but a better solution may be to continue the corner reinforcements shown in Bobs pic an just forget the upper tabs. It's not a structural piece, all you need be concerned with is that its rigid enough to lean your leg against. My current project uses an angular tunnel, I only have 2 an-3s on each side, I do not have the upper attach at all.

_________________
Lee Walton
Houston, TX
N51863,N118LW
KEFD

Replace the two tabs with stainless steel strips cut about 1/2" wide. Bend them to the proper angle and rivet them to the top of the tunnel. Run them aft on the tunnel far enough to place two 1/8" rivets. The part next to the firewall should have nutplates for ease of installation. As Lee said, the tunnel is not structural -- just keeps your feet out of the rudder cables and, in my case, the fuel lines.

_________________
Bob Highley
N711SH
SN 835
KLAL

I can manage the no tab idea or sort out some separate brackets.
Bob with the nut plate suggestion I assume you mean to substitute nut plates for the Tinnerman nuts on the forward flanges of the 504 tunnel. I would have thought that the Tinnermans would already have been pretty easy to install am I missing something, is there more to the story?

Stuart

Tinnerman nuts seem to always get missaligned at the worst time. I have systematically removing Tinnermans since I first built my plane. Put in the proper hardware in the first place.

_________________
Bob Highley
N711SH
SN 835
KLAL