A Modestly High Aspect Ratio Plank - RC Groups
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Aug 01, 2017, 11:01 PM
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EdSoars's Avatar
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A Modestly High Aspect Ratio Plank


I can't get Carolyn's Calliope out of my mind. So after sorting through the remainders of some past sailplanes, I found a very nice fuselage from a TopModel Gin. The original was a 2.3 meter conventional tail, and flew very, very well, until it met the Rocky Mountains on a face to face basis. The wing was trashed, but the fuselage was in pretty good shape. I wanted to fly the same slope-thermal conditions the Gin was good at, but with a bit more speed and maneuverability.

The power system is an Arthobby 2400: 650 KV, on a 3200 mah 3S battery, with a 16x8 prop. It climbed very well indeed, always nice in downdrafts over rocks or forests. It has over 80 oz of static thrust, and I expect the weight to be 52-55 oz.

The tail seemed superfluous, especially after following Carolyn's reporting on the lovely Calliope. So I sketched out a few planforms, found a set of 48" foam blanks already cut to a 9" to 6" taper, and plotted out some templates. I already had PW-51 templates, but wanted to try Calliope's airfoil: the MH-22, which is more oriented toward thermal flying. The span is 96" or 2.4 m. The aspect ratio is moderate at 13:1.

It should work out at 11 to 12 oz/sq ft. Sorry, you metric folks. I'm stuck in neolithic units. The wing structure will be dead simple: 1/16" balsa over blue foam with FG reinforcements here and there, and a balsa sub spar at the control surface leading edges.

I chose outboard elevons and a central flap (not elevator) for crow, and for high-speed down-trim. This will keep the tips in a washed-out condition at all speeds except cruise, which will have all surfaces centered.

Since I bobbed the Gin's tail at the very beginning of this build, I'm referring to it as the Bobtail Cat, a local feline known to be very fast on its feet.

Here's a sketch.
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Aug 02, 2017, 04:26 AM
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Carolynne's Avatar
Dear Ed,

Come on in! The water's warm!

Just FYI, the Calliope is still doing its thing. The CG has slowly been moving rearwards and it now flies happily with the elevator faired with the aerofoil in a cruisy sort of glide ( not super floaty and not super fast, somewhere around good L/D I guess). About the only issue I've had is the carbon fuselage blanking the antenna when it was routed through it ( the antenna is now taped to the bottom).

Carolyn
Aug 02, 2017, 09:19 AM
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EdSoars's Avatar
Carolyn, thanks for Calliope's update. Good to know the MH22 will trim out with positive pitch at zero deflection. Our air here at 2000+m between 4000 m peaks is usually bumpy and breezy, so a cruisy glide is what we normally fly, rarely having any need for minimum sink. Perfect! (See? I can do metric!)

A few burning questions related to the higher-than-I've-tried-before aspect ratio:

Did Calliope finish up at the projected 2 kg? That would give her a fairly low loading of 10 oz/ft2 or 30 gm/dm2, which means the MH22 is, as expected, a fairly slippery airfoil. Good news! I'd be happy at anything from 10 to 12 oz/ft2.

What static margin did she trim out at? I've done my balance calcs at 3%.

Did you use any dihedral? Your photos look like nearly zero. I'm building the Bobtail wing in one piece, so would like to get by with whatever dihedral I can cut into the foam blanks. That's only about 1/3 degree: next to nothing, but I've found only 1 degree provides more stability on a plank than on a conventional planform.

Last question: where are the videos?

Ed
Aug 02, 2017, 10:33 PM
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Carolynne's Avatar
Hi Ed,

Well done on going metric! That didn't hurt at all did it?

The Calliope:

Weighs just under 2kgs
I cant remember what the static margin is, but it currently balances at 44mm aft of the LE. Please bear in mind that the Calliope is effectively swept forward. If you are going to do funky calculations based on the original drawing I posted, bear also in mind that the outer panels were stretched by about 100mm each at the tips in the final aircraft.

Dihedral was set by eye to what looked nice and not too droopy. I'm not in favour of huge amounts of dihedral as I find that it complicates directional control a bit. I would guess that it is about 2degrees?

Also, FYI, because it made jigging the wing easier, there is 3mm of washout across the outer panels.

Videos? what Videos?

Carolyn
Aug 02, 2017, 10:57 PM
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EdSoars's Avatar
Okay, so videos are digital or analog magnetic recordings of events involving movement of objects of interest to widespread audiences. Nothing could be more interesting than the flight of aircraft most resembling birds. There.

Those 100mm at the tips are the source of discrepancies in some calculations. Thanks!

Another question: In your Report #41 in the Calliope thread, you say that your friend Klaus's wing had good elevator response from a swept forward (straight LE) planform, with outboard elevons. That would make me lean toward less swept-aft planform on the Bobtail, i.e. straight LE or straight 25% chord line.

I do like an axial roll as far as handling goes, although it isn't the best for thermal turns. Any thoughts? Plank controls need more attention on this forum than they get, outside of our German contributors.
Aug 03, 2017, 12:37 AM
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Carolynne's Avatar
Dear Ed,

From memory, Klaus' wing has a constant chord centre section and tips with a swept leading edge. the entire trailing edge is straight. ( Klaus, if you're happening to be reading this and I've inadvertently gotten this wrong, please feel free to laugh at me!). Controls are outboard elevons and rudder. As previously mentioned, Klaus finds the elevator response very 'snappy' which is why the Calliope has the extended centre section and appears to have a better damped elevator response.

As mentioned in the Calliope thread, I found with my Little plank needed a small amount of negative differential for a more or less axial roll. The Calliope has no differential, but an amount of pro-roll rudder mixed in. The Calliope rolls into and out of turns very axially. In fact, this was something that Tim commented on when he saw it fly. Tim is very keen on 'clean controls' and likes to set his aircraft up so that controls don't interfere with each other. Come to think of it, I have the same preference.......

What does mean for your aeroplane? Let me say that I like the shape of your bob tailed beastie! Beyond that, I can only refer you to my comments regarding the reasons for the control set-up of the Calliope in that thread. The Calliope was designed to do certain things and very much of the shape and layout were controlled by those requirements. In fact, very little of the Calliope was dictated by aesthetics alone ( The pointy nose was aesthetics.......). Of course , what I like may not be what you like.......or want...... or need.


Carolyn
Aug 03, 2017, 08:35 AM
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EdSoars's Avatar
Thanks very much Carolyn.

On a high-aspect-ratio wing, if the sweep is kept to the limits of a straight LE or TE, it shouldn't change the elevator response much. I will probably just keep the 25% chord line straight.

As to roll and aileron differential, how much is needed depends on the airfoil and aspect ratio, I think. It's surprising that Little Plank needed negative differential. Low aspect ratios don't usually need any differential, and then usually positive. I'll just have to experiment. At present, no rudder function is in the plans, but that could be changed easily. Combined rudder and split rudder might take some thinking, so I may get back to you in future if axial rolls get hard to manage with differential, and the wing needs more drag for landing.

Currently gluing up skins and cutting hotwire templates.
Aug 03, 2017, 04:30 PM
Herk
HerkS's Avatar
On dihedral I'm with Caroline - Zero or negative just doesn't look good to me. It always makes the plane seem to appear droopy somehow.

Differential in the aileron control has had an undesirable elevator effect the times I have tried to use it with elevons. I think a bit of rudder mixed in would be my choice.
Aug 04, 2017, 12:32 AM
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EdSoars's Avatar
I can see where the up-elevator would come from with differential aileron. So maybe use the center surface for elevator, but use the ailerons for up-trim to keep from reducing center camber and producing wash-in during slow flight. Down-trim and crow would be on the center surface, but the elevator function would have to be switched to the ailerons in crow mode. That seems less than optimum.

Well, there's always the rudder.

I cut the cores allowing for 1/16" skins, which cut a wedge out of the trailing edge in the beds. So I'll cut separate beds for construction, which will let me cut in over one degree of dihedral in the 2" thick foam blanks. The appearance of droop is partly due to leading edge sweep. Straight leading edges often look to me like dihedral, straight trailing edges like droop. It's probably best to have an appearance in flight that best indicates the flight attitude. Like "clean" controls, careful design is needed sometimes to produce simplicity in the air... especially at long distances and altitudes!

And I like the idea of having a rudder to do true hammerhead stall turns while flying halfpipes! May have to lengthen the rudder moment arm.
Aug 05, 2017, 11:05 AM
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EdSoars's Avatar
A "pre-bench-maiden" photo.

As time permits, the next step will be to reinforce the skins with CF tow and 2 oz FG. There will be no spar.

ed
Aug 05, 2017, 11:41 AM
B for Bruce
BMatthews's Avatar
A couple or three degrees of dihedral also aids with keeping the wing tips a touch higher and less snag prone on landings.

Ed, the plank is looking rather sporty. It'll be interesting to read about how well this broken fuselage inspiration works out. At the wing loading you're expecting I think it won't be a floater. But if you thermal soar in windy conditions it should be a real winner.
Aug 05, 2017, 06:37 PM
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EdSoars's Avatar
Brian: Right about tip snags. There is 1 degree cut into each core bed to make it easy to build as one-piece, but I can always prop up the tips.

What "broken" fuselage? It was in one piece after its final landing, so I was forced to cut off the end of the tail boom. It's rare for me to be able to do that!

Yep, no loafing around our skies! If you don't like the lift in one spot, scamper over to another part of the sky. It's all about good L/D at 30-40 mph. And at 7000 -10,000 feet (okay, 2000-3000 meters!), the speed needs to stay up or stalls happen fast.

Photo: I mark out the CF tow reinforcement locations, then tack them in place with CA glue. I was planning to use Polyurethane glue to adhere the skins, but I'm not sure about how well it will work with CF and fiberglass.
Aug 06, 2017, 11:29 AM
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Knoll53's Avatar
You mentioned that you have no spar, but you do. It is a built up spar with cf tow spar caps attached to a foam shear web with PU glue. It will be quite rigid in bending, but rigidity and strength are two different things. The cf tow is very rigid but the foam is not. This creates a stress concentration between the two dis-similar materials. One approach is to engage as much of the foam (the weaker material) as possible. That is, use a thin spread tow cap that has a lot of surface area in order to maximize surface area for the PU glue joint.

To maximize strength, a full depth glass shear web would probably double the bending strength. kevlar is even better. I used a kevlar shear web on the SW11 and transitioned to glass at the wing tips and you could see it bend at the transition when loaded.

A 100" wing span is pretty big. Especially when flying 80mph. Whatever you do, don't let me fly it. I love trying to break spars in the air!

Kent ....(the King of video)

BTW, if you are rolling in dough ($289), a great option for shooting video is at Costco in the form of a Gopro Hero 5 session bundle that includes a brilliant head band set up. Comfortable and it works great right out of the box. I shoot 2.7K video with a normal lens with that set up. If you can tolerate wide angle, then it is 4K.
Aug 06, 2017, 02:35 PM
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EdSoars's Avatar
Kent, you're on my wavelength: I'm spreading the tow thinly over a wide surface, tapering to the tips. Given the 7% thickness of the MH-22 airfoil, a non-compressing shear web is probably in the near future. I'm thinking vertical grain 1/4" balsa with 45 deg bias FG. that motor and battery is heavy, and a fast pullup from a dive is NOT going to be forgiving to the long span.

Got no budget right now for a GoPro, but will definitely use my old pocket camera on a hard hat for that video when the time comes.

Ed
the king of impact tests
Aug 06, 2017, 04:10 PM
less is more
Knoll53's Avatar
Costco would be delighted to let you use the gopro for 90 days in the hope that you would not return it...........for a full refund. It is try before you buy at Costco.

I am having good luck with my 50K spread tow spar cap on an open framework swept wing. vertical grain balsa shear web. There is certainly very little structure there. For right side up flying use double the cf tow on top, the compression side, for even stresses.

the main complaint about carbon fiber is it's poor ability to handle impact loads. As long as you don't put it on a 12V winch or use a giant ballast box, you should be in good shape. My little Plank 101 carries 16 oz. no problem. On your ship, 16 oz. would be barely noticeable..........as long as the wind is blowing.

Your plane should haul ass..............even without the motor.

I don't think that we have ever seen a landing at your legendary site. Landing this plane should prove interesting.


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