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Old May 04, 2010, 12:03 PM
G_T
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a Freaking hinged stabilizer airfoil

Hello everyone,

I have received numerous requests for an airfoil for the horizontal stabilizer that uses a hinge instead of being full flying.

Now I like full flying stabs, but they require more maintenance. If there is any slop at all at the pivot pins then stability suffers greatly. For this reason I'm likely to move away from full flying even though it can be a little better aerodynamically.

I have a tentative new airfoil for hinged horizontal stabilizers, as a substitute to the venerable HT-22. The new foil drastically drops drag at large up elevator deflections which is the weak point of hinged systems. I'm in the tweaking stage trying to get another percent or so improvement, but it is very close. I should be done in a few days. Current version is 5% thick and somewhere around 1% camber. Hingeline is at 40% from the leading edge. All of that is like the HT-22.

The shape won't look like what you are used to seeing though. I've not seen another foil with this sort of shape.

More details when I'm done.

Gerald

Edit - Foil on second page of discussion. http://www.rcgroups.com/forums/showp...5&postcount=17
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Old May 04, 2010, 12:12 PM
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Cool. I was already waiting for that as well.
Looking forward to testbuild it with my friends...

Regards Klaus
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Old May 05, 2010, 06:35 AM
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Therwil, BL, Switzerland
Joined Nov 2006
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Hi Gerald,

That's interesting, I would have guessed you'd be the last one to consider switching to hinged elevators. But I can understand the practical reasons. DLG construction involves a lot of compromises, some aerodynamic ones but also a lot of practical, handling and mechanical ones.

For example, I recently build a SALpeter and I really like the simple mechanical design and agree with most of the compromises the designers made. I still think the flaperons should not stop that far from the tips, though.

Looking forward to your new development.

Jan
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Old May 05, 2010, 12:28 PM
G_T
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Joined Apr 2009
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On practice day at IHLGF both Phil and I found our Edges to be more difficult to read and to trim. We were thinking they needed a larger stabilizer. In the hotel room in the evening we were checking the planes out and found we had each developed slop in the mounting such that there was about a half mm free play at the trailing edge of the stab. We tightened that up and the trimming and air reading issues were gone. I also found the same issue on my backup plane.

I like full flying stabs, but it is looking like the mounting arrangement involves either maintenance or bushings/bearings/preload-spring. Slop in the pivot is simply unacceptible. So rather than getting fancier and putting bushings in the V-mount (which certainly can be done) we are looking at staying simple and using pull-spring or pull-pull with a conventional elevator for the next plane design.

My beef with hinged elevator arrangements has been that once a person is pulling substantial up elevator - say 10 degrees - then the drag is much higher and the (relatively) low drag bucket is rather narrow. So efficiency in tight turns suffers. This is even with a foil such as the HT-22 which is purpose-designed for the application. With more moderate deflections on the order of 5 degrees or less the HT-22 is just fine. I haven't run the numbers to see how far it stays good, but it is not good by 10 degrees.

What I'm doing different is biasing the elevator response to favor up elevator deflection over down elevator deflection. So I'm expecting the drag for a substantial down elevator push to be somewhat higher than it would be for the HT-22. But I only know of one instance in a typical flight where I briefly push a strong down elevator. That is only momentary, at the top of the launch. The impact of the increased drag will be indetectable. On the other hand, speaking for myself I quite frequently use large up elevator deflections. Actually nearly every time I core a thermal! So I think that tradeoff is well justified.

A very slight increase in drag with elevator compensated full down flaps is actually a bonus from my point of view. I expect it to be there, very slightly, but I haven't run the numbers yet to prove it.

I also haven't run the straight line flight numbers for launch speeds yet. But things are looking quite good so far.

I'm thinking I can probably finish the development tonight so I might get it posted tomorrow. Stabilizer airfoils are so easy compared to everything else... But as usual it will be done when I think it is done.

Gerald
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Old May 05, 2010, 02:27 PM
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threcixty's Avatar
Falcon, Colorado
Joined Oct 2004
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Gerald,

I am excitedly awaiting your new stab foil! Being biased towards an elevator type arangement, I can offer some encouragement based on experience.

The past couple of planes I did were flying stabs. They work fine, and definately seem to have more authority in landing configs and slower speeds. That being said, the elevator equipped planes matched the response and authority in every other flight phase.

From a production standpoint, a fixed stab is easier to produce, and takes a few steps out of the finishing of the airplane. Stab mounts can be moulded into the fuse and jigged for bolts instead of having to line up and glue a Vmount on. It is also two less pieces to mould for the Vmount.

Another argument for a flying tail is that decalage (sp?) will always be spot on, where an elevator arangement might not, especially when moving cg or adding ballast. From what I have seen, the difference is not noticable, even with a few degrees of elevator "trim" off of the original airfoil. I think it is probably like your Edge stab airfoil that is slightly cambered. It does notably give up any performance to a non cambered stab.

Jim
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Old May 05, 2010, 02:39 PM
G_T
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Jim,

Yes, ease of manufacture is one of the things I've been considering. I was intending to be working on a fuse plug this week but I've been delayed a few days. Having the stab mount be part of the fuse is a bonus.

Agree on longitudinal dihedral (decolage) as well. The only place being off hurts is if it is off in the direction where up elevator trim is the default condition, and then large up elevator is required in a tight turn. Drag is higher in that condition.

Another consideration is that I have examined a number of spring-V mounts that various persons have made. Many of them I consider to be overly bulky but at the same time underbuilt for stiffness. This can lead to problems of having to use overly long pivot pins or run the risk of the stab separating from the fuselage. It is pretty straightforward to make a good one but I don't think people are performing stiffness tests when they maverick their own versions. Without the testing it is only a guess as to whether it meets structural requirements or not. I've heard of stabs separating and from some mounts I've seen I understand why that might be possible. The only time I have had a stab separate is from a midair and it actually saved the stab.

Something that just bolts on seems easier for everyone to reproduce reliably based on experience.

Gerald
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Old May 05, 2010, 03:17 PM
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Cary, NC
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Excellent timing sir - I was just making templates for the ht22, but didn't cut any foam yet. I'll hold until you publish it.

M.
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Old May 05, 2010, 07:05 PM
Walter Roos
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Canton, Ga.
Joined Aug 2004
3,191 Posts
Thanks Gerald

I would like to give you my order now for about six units, you can forward my order on to Phil so I will get them soonest.

Thanks That would be order # 0001


walt

PS I have an order for 2 Tail sets from Richard, stranded in never-never land due the the Volcano ash thing.
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Old May 05, 2010, 08:42 PM
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Falcon, Colorado
Joined Oct 2004
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Just wondering... Your flying tail airfoil is biased towards "up" elevator. I tend to fly my planes with a fairly rearward cg. Is there any advantage towards a lifting type stab? Would a stab foil that produces positive lift help relieve trim forces in different speed flight modes? If it did, would that be a good or bad thing?

Maybe the next new trend is "elevatorless" stabs! Less flex on launch, and all is cured with some reverse diff.

Jim
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Old May 06, 2010, 12:47 PM
G_T
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Jim,

The shape of the airfoil within reasonable limits has no effect on trim or flying characteristics except with respect to drag. Ditto the location of the hingeline if there is one. The slope of the lift vs. AOA curve is determined primarily by the profile of the stabilizer not by its airfoil. To be technically correct I should throw a small caveat in there that flow separation and bubbles can alter this slightly in some cases. I'm also not talking about what happens near or after tail stall.

My full flying tails have negative camber because of two things.

First, one pushes up elevator faster and more abruptly than down elevator with the one exception of the pushover at the top of the launch. Therefore it is desirable to have up elevator transients handled better, in preference to down elevator transients. This favors negative camber.

Second, the airflow seen by the stab is a function of the turn radius. The tighter the turn, the more relatively curved the airstream is from the point of view of the stab (and the wing for that matter). This curvature is sweeping upwards. That makes the airfoils behave as if they have more camber than they really do. So starting with negative camber biases the airfoil in the direction that prevents it from getting too much effective positive camber and getting the low drag bucket off of the range where the stab will be flying.

For a hinged elevator arrangement, we get none of the above, except for the tilted and curved airstream in a turn. The tilt is what bites a hinged elevator. The airfoils for this purpose are positive camber in an attempt to get the nose of the airfoil to line up better with the incident airstream. This helps keep the flow attached and reduces drag. The difference is substantial.

BTW, even a fairly rearwards CG won't alter the lift the stab has to generate by much at all. So from that point of view the camber of the stab is essentially irrelevant.

I mentioned hingelines above... There is a trend in Europe to put hingelines at 50% or behind on elevators and rudders (when they have one). This is sound structurally, but rather poor aerodynamically.

When we apply an elevator or rudder control, we are making a shift in the zero lift line of the vertical or horizontal. The smaller percentage chord the control surface, the greater the required angle of deflection to get the same shift in zero lift line. Now the greater the angle of deflection, the more the control surface behaves as an airbrake and the less it behaves as a camber adjuster. So relatively narrow rudders or elevators require greater deflection for the same pitch or yaw adjustment to the model, and create a fair bit more drag when doing it. The stronger the maneuver, the greater the drag disparity.

Given narrow ineffective draggy rudders, is it any surprise that many forego these controls all together?

http://www.rcgroups.com/forums/showp...1&postcount=22

Edit: On the picture, the axis labeled alpha is the degrees of shift of the zero lift line.

Gerald
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Old May 06, 2010, 12:56 PM
G_T
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By the way, this freak of nature airfoil is done. I'll get it posted late tonight if I get the chance. I need to come up with a name for it!

For those doing moulded rather than bagged construction, it should most likely be done in the -3 degree configuration. You'll see why when you see the foil. That is unless one makes fully milled cores.

Although this foil is a pretty typical 5% thick, it is less volumous than a typical stab foil so core weight is less of an issue. It is also a fair bit pointier at the front than most.

Gerald
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Old May 06, 2010, 02:56 PM
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Falcon, Colorado
Joined Oct 2004
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Gerald,

The airbrake thing makes sense to me. The curved airflow does not. Could you recommend an article or animation that would help me wrap my brain around that concept?

I think you had a brilliant name: Freak!

Thanks again for your hard work.

Jim
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Old May 06, 2010, 03:31 PM
G_T
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Try thinking of it this way.

Pick a point in space that the plane is going to pass by. That point is motionless with respect to the air. Now the plane passes by this point while in a turn. Because the plane is in a turn, it has a positive pitching rate (that's what the up elevator does). So as time progresses the plane and the horizontal stabilizer will be rotating in the increasing pitch direction. This is equivalent to saying as the plane moves in space the stabilizer will be rotating in the increasing pitch direction. So, by the time the tail end of the stabilizer has reached the point, the pitch of the stabilizer has increased compared to when the front of the stabilizer reached the point.

We can achieve the same flow effect, approximately, by adding a positive circular camber to the existing camber of the stabilizer. From the point of view of that one point of air, the effect is the same. ... Not quite the same as for the air mass as a whole but that is another story ...

So, from the point of view of that point in air, the plane is spinning in pitch. When the plane completes one full turn it has spun the stabilizer through 360 degrees in pitch.

From the point of view of the plane, the point in the air has spun through 360 degrees in the other direction, by the time the plane gets back to that point.

I don't know if this has helped or not...

Gerald
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Old May 06, 2010, 05:02 PM
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C Robinson's Avatar
Oregon, USA
Joined Nov 2006
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Fringe

Quote:
Originally Posted by threcixty View Post
Gerald,

I think you had a brilliant name: Freak!

Jim
How about a Blend of Freak and Hinge or "Fringe"

Fringe: Near the Edge of a given Zone

Craig
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Old May 06, 2010, 05:29 PM
Just fly it!
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Cody, WY
Joined Nov 2007
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Gerald, what are you thoughts in respect to downwash off the wing. During turning the wing is operating at a higher Cl which dramatically increases the downwash off the wing. Also, the wake roll-up can double the downwash angle around the stab. How is this effecting your design?

Adam
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