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Old Nov 02, 2011, 01:09 AM
G_T
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Freak-Horizontal V2 hinged stabilizer airfoil

Hello everyone,

Here is an update for the Freak-Horizontal airfoil, the V2. Generally this is an improvement but not in all ways for all planes.

Drag is reduced perhaps about 1%, hingeline is moved back to 50% which may make construction and mounting easier even though the new version is thinner, and drag is reduced notably at launch speeds in cases where the horizontal is not large and the CG is forward. The reduced stabilizer volume should allow a little lighter horizontal stabilizer to be made, reducing mass in the critical tail area.

As usual, free to use, commercial or otherwise.

The goals were to reduce drag for small negative lift coefficients at launch speed, and in general to allow a little smaller horizontal stabilizer and/or a little more forward CG if desired. As one reduces the size of the horizontal stabilizer, the horizontal ends up running at a more negative lift coefficient.

Smaller horizontal stabilizers obviously would have a little less drag due to the reduced area, but what is less obvious is the overall plane performance is boosted beyond just that small loss of drag. However, if one loses handling characteristics then the tradeoff is likely not worth it. This foil is in part to allow people to make the experiment with a more optimal foil that is hinged. HT-22 is not suitable and neither is Freak-Horizontal. Symmertic foils just do not behave as well in turns. I did not know of a suitable foil so I designed one.

Note the full flying Edge-Horizontal foil should not have issues with being sized a little bit small. It was already biased slightly in that direction. But there was nothing (that I know of at least) that was suitable that had a hinge.

One can tell from the shape that I played some games to get the desired performance. The main game I played this time was to establish the default position for the elevator portion of the airfoil to be slightly down. It is not down enough to cause flow separation in normal level flight. But, the biasing of the elevator position improves performance for up elevator settings.

Neither the Freak-Horizontal V2 nor the Edge-Horizontal is designed to support seriously shrunk stabs. Without doing the full analysis I'd suggest staying in the 170 - 200 cm^2 range for the Freak-Horizontal V2 with a stabilizer aspect ratio between 5.5 and 6, to work with a wing with an aspect ratio somewhere around 10.5 with normal tailboom lengths.

Gerald Taylor

PS - See post #19 for a version of the foil with only 151 points, for use with programs that can't handle the full set.
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Last edited by G_T; Nov 07, 2011 at 10:05 PM.
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Old Nov 02, 2011, 06:36 AM
Father of Fr3aK, DLG Pilot
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Good stuff Gerald. Thanks again for sharing. I'll get some significant flight reports back after the weekend. Initial flights were all positive. Would you like for me to share the shape and sizing of my first test part?
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Old Nov 02, 2011, 10:17 AM
G_T
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Sure, go ahead!

Gerald
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Old Nov 02, 2011, 10:33 AM
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Tom,

How difficult was it to hand sand this foil?

James
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Old Nov 02, 2011, 10:42 AM
G_T
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If one is looking to purely hand sand it, as one might do with a balsa core, then yes it certainly can be done. One should work with templates and try to be accurate. But deflect the elevator about -1 degree first. Then the shape is relatively conventional - otherwise one won't be able to shape it on the bottom side. Once shaped, put the elevator back down that degree.

This design should be hand sandable, unlike the original Freak-Horizontal, which was kinked on both sides no matter what one did for elevator deflection.

I'll let Tom answer on what he did which is a bit different.

Gerald
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Old Nov 02, 2011, 10:44 AM
Father of Fr3aK, DLG Pilot
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Um, that's a tough one. I hand-sand the blank and then press it in a mold, so my hand sanding is to a slightly oversized approximation.

As far as the shaping of the approximation, it was pretty easy. Roughly 15 minutes from blank sheet to ready to mold.

I'll post some screenshots of mine later today when I have the CAD files to reference. IIRC mine was a hair over 200 cm^2 and early flight testing shows it doesn't need to be that big.
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Old Nov 02, 2011, 10:54 AM
G_T
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Something I should mention here - one wants to get the mounting angle accurate, otherwise performance can be lost. To do this, just assemble, fly, and get the CG and trim speed set (cruise is good). Then check the elevator angle. If it isn't right around 1 degree down, then shim the tail until the plane trims out with that native angle.

Unlike conventional wisdom that exact trim angle may not matter for the elevator, in this case it does matter some. That's the cost of my pushing the performance limits a bit hard.

If the horizontal stab is appropriately sized and shaped, and the CG has it loaded within its performance band, then the DLG should be quite responsive to elevator input over the full speed range. If one is used to a conventional horizontal tail, particularly one with the hingeline farther back, don't be shocked about wanting a little less travel than normal. To do that, I recommend just drilling a new hole a little bit closer to center in the elevator servo horn.

Gerald
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Old Nov 02, 2011, 01:18 PM
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Thanks Guys
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Old Nov 02, 2011, 01:46 PM
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Thanks a lot, Gerald. This foil will definitely go on my new plane!
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Old Nov 02, 2011, 01:48 PM
Father of Fr3aK, DLG Pilot
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Here's a shot of Rhino when I started crafting the mold. Area was 218.5 cm^2, or about 33.8 in^2. I think this is true to what I built though I did round the tip just a pinch more before cutting(this was v7... cut was v12.)

I will be recutting another at about 190 cm^2.
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Old Nov 02, 2011, 03:24 PM
G_T
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I'd likely recommend a starting alpha of about -0.7 degrees relative to the wing's downwash and therefore a target lift coefficient for the horizontal tail in cruise of essentially 0. One would have to compute out the required optimal size/shape of the horizontal so it would trim out that way with the desired static margin for any particular plane design.

Then, during a turn, the alpha seen is greater due to the curved airflow...

This setup puts the airfoil pretty much in the middle of its sweet spot range, with some margin for CG adjustment either way. One can see this by looking at the appropriate graphs I posted earlier.

As one goes smaller than this, the mounting angle should be reduced a little bit more and one gets into expecting steady-state negative lift coefficients. One can do that with this foil, but don't go very far. It is better than the other published non-flying options but not enough so that one can go crazy! If the stab looks rediculously small, it probably is too small. If it looks a little smaller than average, then it is probably about right.

If we assume a design that has the tailboom relatively aligned with the downwash off the wing, then we should see the mounting angle of the horizontal stabilizers based on this airfoil to be slightly nose down compared to the tailboom centerline.

I did not design the foil for seriously negative lift coefficients in steady-state. One gives up a lot of performance for the tail airfoil if one does, and one also would lose pitch damping. I expect that pilots mostly wouldn't like such planes. I'm thinking this Freak-Horizontal V2 foil could allow stab designs that are still in the damping range that would be liked by most pilots, while allowing a little lower drag overall. Time will tell.

Gerald
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Old Nov 02, 2011, 04:46 PM
Aurora Builder
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Thanks Gerald for the foil and all the hot design tips! Thanks Tom for sharing your work! Will throw together a design over the next week or two and set it up to mount on my Akcent-2 along with my new wing design.
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Old Nov 02, 2011, 10:09 PM
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Gerald,

Thanks for publishing this. Every little bit helps

Can you do a ball park of stab size to wing area for trying this? I'd like to try it on a Blaster 3 and a Zone V2 3 panel, and the areas are quite a bit different, as are the tail moments.

Or I can just eyball Tom's plane this weekend and apply kentucky windage.

Gary
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Old Nov 02, 2011, 10:13 PM
Father of Fr3aK, DLG Pilot
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My stab is currently almost exactly 10% of the wing area. I'll be sizing it down a bit though.

I think Gerald was thinking a bit smaller.
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