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Elevator, aft center section, and fuselage bits
The joiner tube guide sleeve was aligned to the fuselage and glued in using Loctite EA9462 epoxy adhesive.
The aft center section and elevator was made with a blue foam core sheeted with obechi. An embedded strip of kevlar formed the skin hinge. The sides were faced with birch ply. A pair of plywood gluing tabs were added because it would be a real bummer if the back end of this comes loose. A nice long G10 control horn was glued in through the bottom skin and extending to the top skin. The elevator center section drives the wing segment with a tab of G10. The elevator servo mount is a plywood plate glued to the fuselage and wing joiner guide sleeve. The center section was glued to the fuselage and then glassed inside and outside. The turtle deck was glued and glassed to the fuselage since there was no need to make it removable anymore. Polyfiber SuperFil epoxy filler was used to reform the wing fillet and fill in some low areas. ImagesView all Images in thread
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Balancing and flight report
Curtis Suter's Flying Wing Calc for Planks spreadsheet was used to find static margins of 5 and 10%. I shot for a 6% static margin for the maiden flight. This turned out to be rather nose heavy. I'm not sure where the problem is, but it was good enough ,and I progressively removed all of the 4.5 oz of free nose weight. By the last flight I had 2.5 oz taped onto the rear of the fuselage and it still had considerable up trim in the elevator and a positive dive test. It also only changed the up trim by 3-4 clicks which indicates there is quite a bit more that can come out of the nose.
Towing was uneventful. One of the tows went fairly high and at release I lost orientation. I'm not sure what it did but one of the other guys described it as a bug splat release. A white plane in a slightly hazy sky is a bad combination. I caught up to it after a few seconds and got things straightened out in short order. There were a couple of extended thermal flights. It is needing a lot of up elevator in thermal turns which is consistent with being nose heavy. Adverse yaw was present on all three of my aileron differential settings. I think the highest is about 60%. Starting the turn with rudder takes care of the adverse yaw which is the same as all of my other scale planes. The split flaps worked great on the first flight, I think because I came in hot and actually had some down elevator. On subsequent flights with a lot of up elevator subtrim, I lost virtually all of the down elevator travel and there wasn't enough down pitch control to overcome the nose up pitching with full split flap deployment. I know Kent had a strong up pitch on his plank. Something to work out once the CG is sorted. I'm planning on borrowing the paint scheme from a British AV-36 which has a red, white and blue sunburst pattern. It will be awhile as I'm in the middle of another project at the moment. |
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6% static margin is certainly a good starting point for the first flight test. I am baffled as to why Suter's calc. did not yield a nice handling plane on the first flight. From the sounds of your flight report it appears to be over 10% static margin. One thing for sure, once you start pushing aft of 4% static margin, things start getting wild quick. You will know when your find that spot. Planks are so predictable. Every plank I ever built ended up at 4% static margin.
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BTW, that WAS a hot landing! |
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A little history
Look what I found..............
Its a landing video of Ken's original AV36. I think that there may be a more exciting landing video buried somewhere. I will always remember Los Banos as the place scale ships go to die. I don't know why, but there were a lot landing crashes. so much so, that some scale pilots swore off ever flying there again.
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My guess is that this NACA 23112-75 version will turn out to be a super stable ship, which is a little surprising inasmuch as it...........well........... has no tail. Last time I was at the Visalia aero tow, one guy with a freshly built vintage plane had nothing but problems...........even with a tail.
I did always have some pitch instability with the Plank 368 when the bottom flaps were fully deployed. Never got that bit sorted out, but otherwise it was docile like a trainer. |
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Any comments on my theory? |
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NACA-TM-1033 describes the work they did on dive recovery flaps in WWII for use on fighters that were losing elevator authority in high speed dives. Basically it describes moving the hinge of a split flap forward while keeping the chord constant. With the hinge at 80%c lift and pitching moment are comparable to a plain flap but with more drag. As you move the hinge forward both lift and Cm decrease until somewhere between 60% and 40% of the chord Cm goes to zero. There is still some lift from the flap and quite a bit more drag than a plain flap. I can't give a specific location for the flap hinge for zero Cm because airfoil and planform may have a affect on exactly where it will be. Anyway NACA (and other researchers since then) found that moving the hinge farther forward produced a positive moment, which is what they were looking for to recover from the those high speed dives. Also if you move the hinge all the way to the leading edge you have to call it a Kruger flap.
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Very interesting discussion, thank you. I recently came across this photo of a full size AV-36 and discovered that there are fenestrations along the hinge line. I'm wondering what effect that has.
And another thing. The vertical fins/rudders have an asymmetric airfoil lifting toward the center of the plane. What advantage does that have over a symmetrical airfoil? |
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A perforated bottom flap would create more drag................me thinks.
I have a theory re the "lifting" fins. It is to compensate for the disturbed airflow caused by the pod. No, no, no........scratch that. It is to increase the air flow over the elevator to make it more effective. No, no, no........scratch that. it is to reduce the air pressure at the pod, thus improving the mood of the pilot. No, no, no........ scratch that. it is because symmetrical airfoils are racist. |
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Spoilers on full size sailplanes are perforated to reduce the lag between deployment and the drag buildup. Yep, solid spoilers don't produce their full drag effect for a few seconds because of a starting vortex that makes it look to the free stream are as if the airfoil is getting thicker. The air that blows through the vent holes breaks up the lag vortex so the spoilers start doing what they're supposed to faster. Most of the pictures of full size AV-36s with the flaps down that I've seen are not ventilated which makes sense if you want it to function as a flap (leaky flap hinges increase drag but reduce lift). The tech report I mentioned in post #26 mentions that the dive brakes caused some vibration of trailing edge control surfaces and that a hinge line gap reduced this.
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Last edited by Knoll53; May 01, 2021 at 07:28 AM.
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