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Feb 08, 2017, 10:02 PM
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EdSoars's Avatar
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Tiger Moth revisited


After a year of messing around with swept wings only, it’s time to try a plank again, in the eternal quest for speed, durability, ease of construction, durability, great handling, and yes, durability.

Here is the latest distillation:

Fuselage: EPE, expanded polyethylene. As tough as EPP, but very fine-grained, easy to sand smooth, needs no finish. Plus I have some on the shelf. Only the battery and pin switch will stay in the fuselage, with 4 AA cells in-line. It was already made up and on the shelf, next to the EPE. A 1/4 ” carbon tube sits in a slot along the bottom surface, running from 4 inches back from the nose, to the tail. A hardwood block was fitted to the nose to protect the foam from rock cuts.

The radio system: 4-channel receiver, battery and three Dymond servos: a D60 for the elevator because of its high accuracy, return to zero, and zero slack in the gear train. The aileron servos are D150 MGBB. They are also good quality and good value, but have too much play in the gears for plank elevator function.

The wing: 1.9 lb EPE from leading edge back to the thickest point. The spar is made up of a 1/8” light-ply web with carbon tow caps. This turned out to be more flexible than desired; the next one will be much stiffer. From the spar to the subspar is 2 lb EPS. The sub-spar is 1/4 x 3/8 inch balsa. The control surfaces are 1/4 x 1 ½ inch balsa trailing edge stock.

The fin is 3/16 inch balsa sheet.

Plans and more photos to follow.
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Feb 09, 2017, 02:01 PM
You know nothing....
Stuart A's Avatar
Nice looking wing!Can I infer that you have built other versions of this?
Feb 09, 2017, 07:48 PM
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EdSoars's Avatar
Stuart: On the Slope forum, there are three threads: Tiger Moth (Dec, 2007), and Tiger Moth Revisited, (May 2013) . I'd forgotten about the "revisited" thread. Bad choice of names for this one!

And I just found "Tiger Moth Again" (Jan, 2014)! That one is still on the build shelf. Don't ask me why, I don't know.

I'm not going to sleep until this one flies.

Ed
Feb 09, 2017, 09:16 PM
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Well, I got as far as a bench, or garage floor, maiden before bedtime. I have to say the Tiger performs very well... on the floor!

Looks like the weight will be 20-21 oz, for a wing loading of a little over 7 oz/sq ft. Quite low, so maybe self destruction can be held off for a while.

I'm guessing the maiden flight should have a CG at 3% static margin. Any suggestions?
Last edited by EdSoars; Feb 09, 2017 at 09:30 PM.
Feb 10, 2017, 03:46 PM
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Knoll53's Avatar
Quote:
Originally Posted by EdSoars
Any suggestions?
Why yes......I do have a suggestion. Your control surfaces look smallish, what 15% - 20% of the local chord? For the first flight, just to be safe, program all 4 surfaces to act as elevators. I'm sure that flight testing will prove that only the root based elevator will be enough even with minor deflections, but better safe than sorry for flight #1.

Any chance of a video of test flights? The other day I strapped the Sony Action cam to my head with a #64 rubber band. Worked great.
Feb 10, 2017, 08:47 PM
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I'll try my helmet mounted camera for a maiden video in spite of the bad juju that is bound to invoke.

Thanks for the advice. I'm thinking I'll also have a mix version that is pure elevon, leaving the central elevator fixed.
Feb 12, 2017, 09:05 AM
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Kent, the elevator surface doesn't look all that small compared to some others, e.g. the Mandarin on p.3 of the High Aspect Ratio Plank thread. I'm trying to get away from tiny elevator movements and those critical control linkages.

For the early flights I could add a tab to the elevator to increase its chord, and then remove it if there are no issues with elevator authority.

The center elevator adds enough complexity to the build that it will have to be a big improvement to be justified.

I want to try the straight trailing edge with outboard elevons concept, like Bruce's Manta on p.4 of the High Aspect Plank thread. That layout should add washout with up elevator, a good thing, but not reduce the camber in the center section. This should reduce the stall speed, also a good thing. I will also stretch the span to 72" to increase the aspect ratio to 10.

Too many ideas, too little time.

ed
Feb 12, 2017, 09:18 AM
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Originally Posted by EdSoars
Kent, the elevator surface doesn't look all that small compared to some others, e.g. the Mandarin on p.3 of the High Aspect Ratio Plank thread. I'm trying to get away from tiny elevator movements and those critical control linkages.
ed
I think that you'll get your wish on that score. Such an approach worked out good on the Plank 368. It's central elevator is plenty authoritative.
Feb 13, 2017, 12:16 AM
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Kent, with the Plank 368 in final trim, what was the static margin? 5% seems pretty big for a plank, and I wondered if the central elevators alone were effective at that margin, or was that after you moved the CG back? What was the final SM?

I would guess your elevator function on the outer panels wasn't all that much effective, since with the forward sweep, they are very close to the pitch axis.

What do you think made the Plank 2.5 susceptible to tip stall? Do you have a thread for this ship?

I just cut the cores for the straight-trailing-edge, elevon only, version of the Tiger Moth, but could not resist stretching the span. It will be 2 meters, which moves the aspect ratio up to 11.7. This should be the departure from the Tiger Moth name. I'm thinking Hyper Moth: what the hey, it rhymes.

The 60" Tiger Moth should be flyable tomorrow. Then if the snow will abate, a maiden might just be in the cards this week. I do envy Kent's sandy test glide area.
Feb 13, 2017, 09:01 AM
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I use normally 4% static margin in the Ranis Flying programme for such a plank. That is normally very very close. What airfoil do you use?

yours Peter
Feb 13, 2017, 10:17 AM
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Knoll53's Avatar
Ed, the Plank 368 has not seen final trim yet inasmuch as there have been only a handful of flights so far. From what I've seen, 4% SM should be perfectly safe. It may be at 4% right now. The big central elevator was quite effective from day one. Flying Dennis has, I believe, smaller central elevators. Regardless of elevator sizing, I'd suggest that you use your best servo on the elevator, with stiff no-slop linkage.

On the Plank 368, the use of elevons for elevator function should be effective with the very minor amount of forward sweep. My "Nosey" project indeed had ineffective elevons due to it's significant forward sweep. Those elevons were indeed pitch neutral.

The Plank 2.5 thread has all of the flight testing and design drawings. You can draw your own conclusions. Normally I would blame operator error, but it happened so many times that I look to the build (which is a form of operator error). The wing does have some sweep, which may contribute to the tip stall or maybe it is the relatively high wing loading. Tip stalls would typically occur while floating around at minimum sink. I did have a high speed tip stall on bungee launch as well, but that could be a function of tow hook location. Otherwise it was a well built model, so why the fully vetted PW51 with no wing twist would tip stall is a mystery.

Bad juju aside, with a new prototype, if I didn't have sand for hand toss test flights, I'd probably just bungee launch it up 100' and use the altitude to get a handle on whatever challenges might exist. I find bungee launches to be quite predictable. Place the tow hook well forward of the cg, pull it back to 20lbs, get the wings level and let her rip. Of course just hand launching into great slope lift works as well.
Feb 13, 2017, 10:40 AM
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Originally Posted by EdSoars
I'm not going to sleep until this one flies.
Have fun with the test flying!
Feb 13, 2017, 08:33 PM
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Originally Posted by EdSoars
I do envy Kent's sandy test glide area.
This should make you absolutely GREEN.
Feb 14, 2017, 08:07 AM
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That Plank 2.5 crash was the reason I don't make the wing-fuselage joint bomb-proof. I make it STRONG in the vertical, pull-off-fuselage direction, but WEAK in the rotational , yaw direction. There isn't much force in that direction during flight, until you reach the edge of the air! My idea is to separate the inertia of the fuselage from the wing and so hope to reduce the damage to both.

Thanks! Great video and build log.
Feb 15, 2017, 09:54 PM
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Ready to Roll


Snow is on the ground or in the air, so I played with the trim... colors, not controls.

Final weight, balanced between 3-4% static margin for test tosses: 25 oz. =8.8 oz/sq ft. I just need to find that tall grass.. or maybe deep snow.

Photos hopefully show the control throws for the Dreadful Maiden. Do all these control travels look decent to you guys? This is all at high rates. Low rates are 70% for aileron, 60% for elevator.

The elevator throws are at least twice what would be used for full-span elevons at best CG point.

Thanks for the comments!

Ed


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