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Old Nov 06, 2013, 01:56 PM
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Knoll53's Avatar
United States, CA, Marina
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Originally Posted by andrecillo76 View Post
I found this video in my collection:
Andrés
I love it when the tell tale points forward ! Great video.

The cabin appears to be a very smooth streamlined shape but it is clear that has lots of separation during "maneuvers". I'm surprised. If you could just convince the pilot to lay down prone, you could put him IN the wing and not have this issue.

I've never put tell tales on one of my wings. Probably would learn, or at least see, a lot. You are getting a lot of mileage out of that key chain camera.
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Old Nov 06, 2013, 08:01 PM
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What exactly are you guys doing for yaw control, clam shell type, drag rudders?
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Old Nov 07, 2013, 01:39 AM
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Kent, "milage" is the right word. My kids use my keycams for filming onboard of their rc-cars. I have now tons of "miles" of video from the kids . I find this little cams neat. They are small enough to attach them outside. The shape of the cabin is indeed very streamlined, but as you mentioned, it suffers from separation at higher yaw angles. I knew this when I designed the cabin, but I did not expect that high yaw angles and the shape was chosen for a much higher Reynolds number. The actual problem is a too high dihedral inducing the dutch-roll. I learned a lot since this design... By the way it's a side-by-side cabin for two.

Iron Eagle, I use elevons for yaw control . Trying to control the dutch-roll could even end up in promting it. I've used asymmetric deployed spoilers, like the ones shown in the videos. They are efficient in affecting the yaw angle (onboard video of the Snow White at 6:50), but I do not need them really for flying .

Cheers,
Andrés
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Old Nov 07, 2013, 05:16 AM
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ohmite's Avatar
Eden NY
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Quote:
Originally Posted by andrecillo76 View Post
I found this video in my collection:

https://vimeo.com/78742987
It shows the flow separating off the cabin during the dutch-roll. By the way, our models tend rather to dutch-roll than big planes, because the inertia is much slower (frquency shifts to much lower values in big planes).

Andrés
Andres,
That video is great. I love the flow reversal and separation shown by the tufts . Very effective use of them and your small camera.
Regards,
-Eric
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Old Nov 07, 2013, 10:40 AM
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Quote:
Originally Posted by andrecillo76 View Post

Iron Eagle, I use elevons for yaw control . Trying to control the dutch-roll could even end up in promting it. I've used asymmetric deployed spoilers, like the ones shown in the videos. They are efficient in affecting the yaw angle (onboard video of the Snow White at 6:50), but I do not need them really for flying .

Cheers,
Andrés
Jack Northrup used drag rudders on his aircraft for yaw control I was just curious if you did something similar. That system couple with a gyro is what ended up being the standard method for yaw control and damping and I was wondering if you could do something similar.
Funny how it was mentioned about having the pilot lay prone in the wing that was done on several early wings.
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Old Nov 07, 2013, 11:15 AM
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On the Vampir - nothing Pretty stable in yaw and never really needed rudders at all on her, so I muddle through as-is. I've been toying with making a twice-size version over the winter - that would definitely include some drag rudders as there would be a lot of space in the outer panels then for some creativity.
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Old Nov 07, 2013, 11:31 AM
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Anhedral ???

Quote:
Originally Posted by andrecillo76 View Post
The actual problem is a too high dihedral inducing the dutch-roll.
The HXc also has dihedral. 1 1/2 degrees each side (last minute revision from the original scale version of 6 degrees ) I wish that I had made it dead flat.

Considering the bad yaw behavior induced by dihedral, in a variety of flight conditions, I'm wondering if we wouldn't be better off, in Horten style planes, with anhedral. The swept wing provides some roll stability, which if balanced by anhedral could yield neutral roll stability.....which is fine by me. Yaw instability, on the other hand, during landing, not so fine. Could anhedral reduce yaw instability at high angles of attack? Should I build a test plane with variable dihedral/anhedral?

Considering the fact that Horten style ships have no vertical fins/winglets to combat yaw instability, having a plane that has no bad yawing habits (especially when landing) would seem like a high priority. Of course, Design is a "study in compromise" and I'm no doubt missing something.




Also, I have often wondered if tip air brakes, such as the ones used on the H IV (the foot operated rudders), wouldn't be the perfect main spoiler for Hortens, inasmuch as they provide some drag for glide path control, but more importantly, they provide symmetrical drag at the wingtips behind the CG, thus providing lots of yaw stability. I've never tried them, but they appear to be be very effective at yaw stability. Blanketing the elevons with "spoiled" air is a problem for control, so multiple control surfaces would be a good starting point.

Kent
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Old Nov 07, 2013, 11:55 AM
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On the Vampir - nothing Pretty stable in yaw and never really needed rudders at all on her, so I muddle through as-is.
What sort of flying do you do with the Vampir?

Some times I just float around in nearly calm air, in which case, little yaw stability is needed. Other times, at the slopes, I'll get all crossed up in a hammerhead maneuver in gusty conditions, in which case, lots of yaw stability is good.

Kent
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Old Nov 07, 2013, 12:02 PM
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For sure - completely depends on what the airframe was designed for flying-wise. The Vampir isn't really a big-air machine, so the most she see's is the odd roll and maybe some half-hearted half-pipes. Looks cool enough just mooching on the ridge
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Old Nov 13, 2013, 01:33 AM
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Heidelberg
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I did some diving tests with the Snow White and shot an onboard video for your pleasure :

Diving the Witch (4 min 59 sec)


The logged speeds ranged something between 120 and 150 km/h (75-95 MPH):



Even more should be possible by diving deeper (it was only 30 to 40 degrees this time). Maybe next time

Cheers,
Andrés
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Old Nov 13, 2013, 11:12 AM
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Oh Boy ! Diving the Witch!!!......( I mean Miss White)

My favorite subject. So these are shallow dives with what appears to be similarly shallow pull out angles.. It DOES appear to get moving rather quickly. I'm surprised again at the apparent low drag of this thick wing. I probably reach similar speeds by putting my feather weight SW-2 into a vertical dive. By most any standard, the Witch is a heavy plane. So the load on the spar is significant during that pull out, unlike the SW-2 which simply does not weigh enough to create much bending moment. I shudder to think what speeds the Witch could attain if you put it into a vertical dive, which I hope you never do.

Do you have any sense of what the terminal velocity might be with this wing, say for example a 5 second vertical dive? 250 km/h?....350km/h? Drag increases exponentially so it would stop accelerating at some point.

Due to your very deep spar, your wing is rigid, except for maybe the thinner wing tips. If you were to load it to failure, I'd guess that it would deflect very little before it when "snap". Unlike this skinny long EB29 wing which was quite flexible, yet strong, which had a massive carbon spar..

Most structures will "crack, snap and pop" a bit before failure, thus giving some warning. Your sound quality was good in the video, since the camera was inside the cabin, so you can hear the wing structure talk to you.....which it didn't. Not a peep from your wing.

I've broken a few wings in the air and, not surprisingly, each time was when I added ballast and then "flew normally" which for me includes foolhardy vertical dives and such.

It's always exciting to fly a heavy plane......so much more to think about. Great video of the dive. Thanks for keeping up posted.

Kent
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Old Nov 13, 2013, 12:14 PM
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Kent,

the wing loading is more than for your SW-2, but is still with 45 g/dm² not very high for a motorized wing. Lothar's big XII has about 70-75 g/dm² . I will estimate the drag and will be able to give a figure for the "terminal" speed (more than 5 sec. diving should be needed). The wings of the Snow White are very stable. The bottleneck at the moment are the joiners: two pullwinded CFK tubes ø16 and 14 mm with 1mm thickness. Not much considering the size of the wing . I did not design her for flying fast and doing acrobatics. Anyway, I need a figure of the terminal speed for other reasons... The sounds heard in the onboard video, mostly in the last dives, come from the high pressure inside the cabin pushing the canopy up. The wing has an air inlet right at the tip, where a stagnation point builds up. When the wing flies fast, the stagnation pressure is very high having as a consequence that pressure builds up inside. The canopy is fixed by neodym magnets. When the pressure difference is high enough, the canopy opens sligthly and air escapes. The pressure in the cabin falls and the canopy closes. A new cycle starts... This sounds like plop, plop, plop in the video. You can see the camera going up and down with the canopy. If you look at the wing, it seems to be unaffected. For the other flights, I sealed the canopy with adhesive tape .

Cheers,
Andrés
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Old Nov 13, 2013, 01:49 PM
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Knoll53's Avatar
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Kent,

the wing loading is more than for your SW-2,
hahaha.......you can say that again! the SW-2, at it's heaviest is 20.31gr/dm2. A mere feather compared to yours. I have one plane, that with ballast, is about 70 gr/dm2. It is really fun to fly in good ridge lift.

Looking forward to your terminal velocity calc.

Kent
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Old Nov 15, 2013, 10:56 AM
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The wing has an air inlet right at the tip, where a stagnation point builds up. When the wing flies fast, the stagnation pressure is very high having as a consequence that pressure builds up inside.
I have to admit that there is a one percent chance that I am not quite following what you are doing here.

You have an "air inlet" on the wing in order to change the aerodynamic behavior of the wing at high speed ?....Really !

and a secondary ancillary effect of this air inlet is that it pressurizes the cabin causing the canopy to "chatter" ?...... Again, Really ! ..... So you could button up the canopy and install a "relief valve whistle" in the cabin and estimate your air speed by the tone of the whistle?

I would certainly be interested to learn more about this.

regards,
Kent
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Old Nov 17, 2013, 12:51 PM
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Originally Posted by Knoll53 View Post
You have an "air inlet" on the wing
Yes:



Quote:
in order to change the aerodynamic behavior of the wing at high speed ?
No, the effect at high speed is unwanted and the inlet increases drag. The true purpose is to cool down the battery, motor and ESC, a must for any motorized wing . The air should in principle flow out near the propeller, led through holes. By the way, the size of the inlet corresponds to 1/4 of the size of air inlets in real sailplanes, such as DG-1000, ASW-21, etc .


Quote:
and a secondary ancillary effect of this air inlet is that it pressurizes the cabin causing the canopy to "chatter" ?......
Yes, the path to the propeller is long and has a good amount of resistance producing an overpressure in the cabin. This overpressure is known from real sailplanes. See for example http://www.dg-flugzeugbau.de/index.p...dl-absaugung-e, where an inverted NACA inlet is proposed to produce a vacuum instead to stabilze the flow around the cabin.

Quote:
Again, Really ! ..... So you could button up the canopy and install a "relief valve whistle" in the cabin and estimate your air speed by the tone of the whistle?
Yes, nice idea . In principle the whistle would give a representative figure of the speed. Some of the stagnation pressure is lost to other paths, such as the one at the aft of the cabin near the propeller, though.

Cheers,
Andrés
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