Saturday, July 11, 2009

KR2S Flaps 01

Well this is a little unnerving, modifying work that had already been completed.
The previous builder decided to build the stub wings without flaps (KISS), but by all accounts the KR2 is a slippery little rocket and can benefit from additional drag in the landing profile to shorten up the landing roll and to help with visibility over the nose. I have decided the simplest and probably the most drag producing solution is a split flap arrangement. Hinged off the aft spar there is sufficient room for 2 * 600mm by 285mm (23" * 11") flaps giving a total of 3.5 sq feet of flap. Hopefully if I dangle them enough this should be enough. I've seen a report that split flaps are affective up to 60 to 70 degrees down, I'm going to go for 50 to 60. They will be actuated via a torque tube acting on the inboard end much like another builder here in Australia Darren Crompton. I'm thinking also of an electric actuator to simplify mechanical linkages.
This first photo has me cutting out the existing lower skin. As you can see I have left the hardened trailing edge in place to support the upper surface.

Here it is with the lid off. To get my flap as big as possible I will extend it all the way from the wing joint to the fuselage skin. You can see the original builder has put a PVC guide for the aileron cable near the root but if you look closely it is actually in contact with the cable. As I will need a space between this and the new flap skin I will have to displace this guide, fixing this small problem along the way.

Another shot of that guide, note the lightning hole that the cable was meant to go through, I will make a little filler to allow me to mount the guide lower.

Here I have hardened the inner skin just to protect it a little and to firm up the upper surface.

This is how I sandwiched a 10mm sheet of foam on both sides with a single layer of BID. I will use this in many places but for the flaps I will be using it for ribs and the centre spar.
Two spars cut from the foam sandwich. For now they are the full height of the aft wing spar but later I will trim them down to the height of the hinge. I have done it this way so I can get an exact profile of the original ribs before cutting them down a little.

This is a picture to remind me of where I have positioned the flap spar 5" out from the hinge gives me an almost 50/50 split fore and aft of where I will be applying the force to actuate the flap. My calculations show that I should be able to get almost 60 deg. drop by moving this point down 5" to 6". I should be able to hide the torque arm inside the wing fillet @ 6" travel.

Trial fit of the flap spar.

Spar and ribs trial fitted into place. Scrap foam has been set up to support a 10mm sheet of foam for the flap surface. (I later changed this to 27mm, the same as the hinge and the spar to simplify glassing it later.)

Hinge in place. You can see here I have dropped that PVC cable guide.

Here is the spar and the ribs bonded together. To strengthen all the edges and joints I removed about 5mm of foam and filled it with flox. Very strong and only 155 grams at this stage.

Here the centre section has been filled with foam. You may see the subtle curve which is the same as the original profile of the wing in that area.

Dry fit the hinge spar in place. It is made of 6mm (1/4") Hoop Pine marine 5 ply. I'll use the same materials on the Ailerons when I cut them out.

Cutting back the wing ribs so I can run the flap across the top of them. Amazingly there is still plenty of support for the upper skin and trailing edge.

A shot of the outer end cut down also. I will have to thin the flap around the aileron bell-crank mount but it should fit OK.

Here I'm starting to fill in the outer portions of the flap.
That is it for now, don't forget to send any comments you nay have. I always appreciate feedback.

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