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Old Mar 11, 2013, 01:49 PM
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Horten "Sickle"

Hi,

I've been dreaming of a high aspect ratio "utility" Horten and came up to following preliminary data:
  • Span: 2.3 m
  • Area: 0.36 m²
  • AR: 15
  • Sweep back: 30°

The aim is to obtain a versatile wing, which has a high diving speed, but reaches still an excellent glide ratio of over 20. The airfoil is still quiet experimental and is the one I came up in the T6 prototype. It will be thin, between 10% at the tips and 14% at the center (to house the batt and motor), and has a low minimal drag around a CA at about 0,05 to 0,1. I'm still working on the design, but it should be more or less as in the attached picture below. As always, I'll use an electric motor for climbing (no slope around here ). I chose already a motor setup for it: Kontronik Kira 400-39 with a 5,2:1 gear and a 13x9 propeller on a 3S Lipo. That should bring more than 400 W of power (output not input ).

I'm checking the design against the small Schapel, which has a quiet small drag at high speeds (originally designed for speed flights). We've measured speeds of about 180 km/h (110 MPH) during diving without motor in the version with a cabin. The motorized cabin-less version has been measured at about 240 km/h with "only" 750 W of power (input ).

Though the minimal drag is slightly higher, I expect the Sickle to reach similar speeds during diving (has no cabin). To give you an idea, here's a video of the small Schapel during some acrobatic diving manouvers (reaching the mentioned speed):

Flying wing Schapel SA-882: Dynamic flight (6 min 36 sec)



I have not decided on the type of construction, but I love wood . I'll try to use as much as possible of it. Because the wing is thin, I'll probably will have to use some sort of carbon spar caps, though . The structure will be more or less similar to the above Schapel. The main difference is that I will make a three pieces wing.

Cheers
Andrés
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Old Mar 11, 2013, 03:05 PM
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Australia, QLD, Maryborough
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Lookin' good Andres.



Greg.
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Old Mar 11, 2013, 03:15 PM
I don't like your altitude
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I await the sound of axe striking wood.
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Old Mar 11, 2013, 09:05 PM
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Perfecto indeed.

That Schapel DOES move out nicely when gliding. If you're going to fly that fast, we'll have to all chip in and buy you some wipers for those elevons. So the Schapel has a 14% thick wing? I wouldn't think that such a thick section would allow such speed....nice! I'm guessing at least 50 gm/dm wing loading.

The new Sickle is a sleek looking ship.. Simple and clean....Designed for Speed?...Count me in!... Maybe 15 AR is the perfect sport shape. Sexy looking but not that hard to build or too heavy to pack to the top of the mountain.

The foam wing construction is going very well with the Plank 2.5 and foam would be my preferred method of construction, so only a small wood Center Cabin. Any spar strength can be built up with CF tow and balsa shear. Really quite quick. As an experiment, birch plywood could be vacuum bagged on to the foam if the wing twist wasn't too wild. That could save some time and remove the cost of buying and making mylars. The money I saved in laser cutting I could put towards a teak LE.

Of course the foam version would lack the quality lift distribution of the all wood version, but maybe it wouldn't be too far "off the mark."

Cool design Andres.

Kent
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Old Mar 12, 2013, 03:06 AM
I don't like your altitude
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How many panels would you use for the outer wing?Would 3 be enough to make a decent likeness of the shape?
I must admit I could get hooked on foam wings.I keep looking at Verns Colibri builds,one of these with a built up centre would be cool.
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Old Mar 12, 2013, 10:29 AM
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Thanks guys!

There is still some tuning to do. I have to choose the thickness keeping the wing as slim as possible but having space for the battery, motor, and for the elevon servo, which is tricky. The wing is only about 12mm thick there .
I also have to decide on the sizes of the elevons and flaps.

@Kent: The Schapel has a thickness of about 13% in the center. I also invested quiet a lot of time to achieve low trimming drag in that design. That has as a consequence that the Schapel performs less good at high lift conditions. Thermalling is imposible with it, as it just flies through the thermal without showing a sign . The Sickle will hopefully be a compromise without increasing too much the drag in the low lift condition. The wing loading of the Schapel is something between 50 and 60 g/dm², which is higher than I expect for the Sickle (hopefully 40 to 50 g/dm² in a motorized version, glider should be 30 to 40 g/dm²).

@Stuart: I'll check the amount of pieces needed. But I think three are a minimum to keep the shape more or less as desired. The center section would be a separate piece anyway, as the Horten tail makes it hard to cut foam without needing too many pieces.

Cheers
Andrés
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Old Mar 12, 2013, 11:35 AM
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Quote:
Originally Posted by andrecillo76 View Post
Thermalling is imposible with it, as it just flies through the thermal without showing a sign .
I could thermal it !

But I have to admit....I cheat....I fly in Big Sur from on top of a 600m mountain so any glider you toss off the mountain is right in the middle of a well developed thermal. It's great encouragement for the aspiring "thermalist'. I just look for a wing tip to be bumped up and then turn towards that wing tip to center the thermal. It's not much of a strategy, but it's all I've got.

The Sickle looks fast and, in my book, that's a good thing for thermalling. Every thermal has sink surrounding it and when in sink, I like to get out of there fast. Also, if the wind picks up when you are way down field, having those "legs" (speed = good L/D?) to get back is a big plus.

12mm thick is certainly do-able for an elevon servo. The Hitec 125MG would fit or the every popular 82MG would fit with a slight bump....that would be a faired bump. Even if the wing tip gets very thin, there is always the option of a bellcrank.

Are you planning on neutral flaps for the sole purpose of increasing lift while thermalling or for glide path control? I'm not familiar enough with the aerodynamic trade offs with a Horten style wing so I ask as a pure newbie.

If I get a shot at building this one, my build will probably be the 40 gm/dm version...Also, I may intentionally transpose the span and build a 3.2m version. Have fun tuning and tweaking your design......we'll be watching....

@Stuart: The Horten "Rump" is why I showed an all wood Center Cabin. It is easy to achieve a beautiful curved TE with wood construction. Also, the flare from a 10% thick wing airfoil to a 14% thick root airfoil ( 1.92" thick) at the cabin is dramatic and deserves the beautiful detailing that is only achieved with traditional plywood and rib construction.

Kent
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Old Mar 12, 2013, 03:31 PM
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Sickle vs. Manatee....for those who have been following along. The Sickle is really going to move...........

Suggested foam wing panel joints in green. Must not forget those straight hinge lines.

Kent
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Old Mar 13, 2013, 11:18 AM
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Looks already very nice in that picture! The joints of the foam pices will have to be selected depending on the change of airfoil geometry. I'll check this later, when the design is ready.

Andrés
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Old Mar 13, 2013, 11:21 AM
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The attached picture shows another comparison between the Sickel and the Manatee (centeral rib).

Andrés
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Old Mar 13, 2013, 11:47 AM
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Quote:
Originally Posted by andrecillo76 View Post
The joints of the foam pices will have to be selected depending on the change of airfoil geometry.
So many competing interests.....I'm happy to have you making these comparisons and trade offs.

Attached is the only panel break that I would introduce, that being the break between the curvy wood Center Cabin and the foam wing.

Kent
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Old Mar 14, 2013, 01:53 PM
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Since you are still designing.......

I have a design question (proposition) for you.

When designing flaps, you can position them so that they have a neutral affect on pitch OR you could move them outboard a bit which would require some up elevon to compensate for the nose down pitch created by the flaps, similar to crow.

This later condition would create additional drag at the wing tips and thus help reduce Dutch roll when flying very slowly (with flaps). Just writing this out I can see that this is a high price to pay for a little more stability.

BTW, I tried this attempt at crow on my Horten X-c with small flaps and, with my initial trim settings, it did not slow down much and actually seemed to glide better.....not great for landings.

Personally, I have no problem with fast landings.

Maybe a better question would be "What is the purpose of the flaps"?

Kent
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Old Mar 15, 2013, 01:33 AM
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Hi Kent,
thanks for your comments. I'll consider this when I lay out the flap position and size. The flaps are to increase the slow flight performance and extend the mininal speed. They could also be used as spoilers to create more drag by "over-deflection". But the main purpose is the slow flight performance for thermalling. The Dutch-roll can be fought against in othe ways, such as via dihedral.

Cheers,
Andrés
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Old Mar 15, 2013, 09:30 AM
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I'm beginning to see the value in pitch neutral flaps. They can be used at any time to modify lift and speed with minimal drag WITHOUT creating an unwanted pitch that must be balanced by elevon deflection, an thus increase drag. I've never had the pleasure of flying a thermal ship with such capabilities. This plane is already fast, so being able to slow it down and "float" will be nice. Dutch roll was noticeable with the HX-c (small amount of wing sweep), but not a problem.

Kent
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Old Mar 15, 2013, 02:36 PM
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Hi Kent,

I think I found a good compromise of elevon and flap size/position. The elevon changes only weakly the pitch attitude. The flap has a weak pitch up effect. Though the steady flight attitude does not vary much, a transient change of any control surface will produce short time reactions during equilibration, for example down or up, until the steady state attitude is assumed. I think additional spoilers will be a must to reduce the glide performace for landing.

An upscaling to 3.2m is uncritical. Smaller size is another story...

Cheers
Andrés
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