KH T-28 Flap Fix (not a full build thread!)

[b757captain]

Hiya gents,

 

I'm well underway on the build of my KH T-28C and though going well so far one thing about the kit has really bugged me. The flaps!! Kitty Hawk did a pretty good job overall on the rest of the kit but the flaps are just plain wrong!

 

What am I talking about? Well, Flaps 101 (basic lesson) there are 4 (generally accepted) types of flaps - plain, split, slotted and fowler. Plain are what you see on a P-51, split on a Spitfire or P-40, and fowler flaps on a P-38. The T-28 has slotted flaps which means the flap pivot point is somewhere close to the lower edge of the forward part of the flap body. As the flap is extended the flap body rotates around this point, the upper radius tracks along the upper wing flap fairing edge and the lower part rotates up into the flap cavity.

 

Here are pictures of the flap installed on the kit in the retracted position:

 

Lower view:

 

 

And upper view:

 

 

The flap doesn't fit well in the retracted position as it is too long for the cavity. I had to sand the outer edge down just to make it fit for the pics.

 

Most photos of parked T-28s seem to have the flaps extended so that's what I'm doing. Unfortunately this is what the flap looks like if built per KH:

 

Lower view:

 

 

And upper view:

 

 

This is what they should look like:

 

 

 

So what causes this, and how do yo fix it? Stay tuned.

 

Cheers,

 

Mark

 

 

[b757captain]

Kitty Hawk made several errors in the flap and aileron shape.

 

First problem: the lower leading edge of the flap comes to a sharp point:

 

 

Second problem: The inner edge of the aileron (and adjoining edge of the flap) are angled so as to be vertical to the ground in relation to the wing dihedral:

 

 

From an engineering standpoint, these angles are impossible - the aileron would bind with the flap when deflected upward.

 

Third problem: the flap hinge point are also angled relative the ground. Fourth, the wing cavity for the flaps has a really odd shape which will be difficult to fix. I missed getting photos here, but both of these problems are not an issue with the fix.

 

The Fix:

 

I wasn't trying for accuracy in the flap well, just a better representation of the proper flap position when down. So out with the sandpaper! I fixed both flaps in about 30 minutes.

 

First up was to sand the aileron edge so it was perpendicular to the wing chord:

 

Before sanding - line shows what to remove to square it up:

 

 

There was plenty of material here, I didn't need to add any plastic stock. Next sand the corresponding edge of the flap to match.

 

When the test fit looks good (with the flap in the retracted position) now it's time to round off the sharp edge of the flap:

 

Sorry for the blurry pic - I thought it came out better and now it's too late:

 

 

The entire leading edge should be rounded. Not too much, just enough to allow the leading edge to tuck up into the flap cavity. Again, there was plenty of material to take off. 

 

Next up is plenty of test fitting. I had to trim a small portion of the flap hinges on the wing. Keep rounding the leading edge of the flap until the upper camber of the flap rests against the underside of the upper wing skin. The plastic could be thinned here if you want but I didn't think it was needed. A very small portion of the inner forward edge of the flap needs to be trimmed to fit against the fuselage.

 

Here's a pick of one flap per the kit install and the other after the fix:

 

 

And closer pics - lower view (look close and you can see the sliver I had to trim against the fuselage):

 

 

And upper view:

 

 

And both flaps permanently installed:

 

 

To my eye this makes a huge difference in the overall look of the plane. I decided against going all out and detailing the flap well and I still need to clean up the hinges, but this won't bug me anymore and now I can sleep tonite!

 

Cheers,

 

Mark

Edited August 11, 2018 by b757captain

[Barry]

Nicely done Mark!

 

Barry

[Iain]

Great work/research Mark - I think my T-28 will be shouting 'build me' one I finish my current Spit project - so this is really useful!

 

Iain

[Hubert Boillot]

IIRC (and happy to get corrected there by those more knowledgeable  ), the T-28 flaps are always down on the ground as the they have built in them the first step to get on board. So absolutely needed fix !

 

Hubert

[b757captain]

On 8/11/2018 at 11:08 AM, Barry said:

Nicely done Mark!

 

Barry

 

On 8/12/2018 at 1:09 PM, Iain (32SIG) said:

Great work/research Mark - I think my T-28 will be shouting 'build me' one I finish my current Spit project - so this is really useful!

 

Iain

 

Thanks guys!

[b757captain]

On 8/12/2018 at 3:07 PM, Hubert Boillot said:

IIRC (and happy to get corrected there by those more knowledgeable  ), the T-28 flaps are always down on the ground as the they have built in them the first step to get on board. So absolutely needed fix !

 

Hubert

 

The last time I was up close to a T-28 was about a year ago and I forgot to look for the entry process. I don't remember seeing a step but it could be a recess with a hinged door in the flap. Makes sense since I've never seen a retractable step and the plane is tall and (relatively) huge up close.

[Guest Maxim]

Well that certainly is an improvement. Thanks for the tutorial on how to fix them.