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Old Nov 28, 2012, 07:32 AM
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Iflyrc_vic's Avatar
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Fying Horizontal Stabilizer

I have just finished building a Gee Bee R3 Racer. I only bought the plane because it looks really cool (IMO). There have been many reports that this model (20cc 59 inch version) has numerous bad flight characteristics (ground handling, takeoff, snap tendencies, hot landings, etc). I have not flown the plane yet but I am very concerned about the maiden.

After checking all of the building issues pointed out by other modelers, I discovered I made a huge building error. The horizontal stabilizer has a slight curvature on one side and virtually flat on the other side, making it a "flying" stabilizer with some lift during flight. My problem is that I INSTALLED IT UPSIDE DOWN. THE FLAT SIDE IS ON THE TOP AND THE CURVED SIDE IS ON THE BOTTOM.

Although there is only a slight curvature on the upper surface, this stabilizer would be a lifting type stabilizer. The difference in the upper and lower surfaces are very minor which is why I did not notice it (dumb move in any case). I have been building and flying for over 20 years. This is a first for me and I am embarrassed to tell anyone about it. It will be an ugly job to remove the stabilizer and turn it over as I will have to remove one elevator half and cut out the stabilizer.

My thoughts are that, if I fly it as is, the tail will tend to drop as speed is increased. This may be offset by the 5 degrees of down thrust in the motor. In any case, some up elevator will need to be applied to keep the model flying level. This may add to the snap tendencies of the plane in turns and slow flight.

My question is, how will this affect the flight characteristics of the plane?
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Old Nov 28, 2012, 07:51 AM
Red Merle ALES
Curtis Suter's Avatar
United States, Mt, Helena
Joined Apr 2002
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The Tail Lifts Down

Quote:
Originally Posted by Iflyrc_vic View Post
After checking all of the building issues pointed out by other modelers, I discovered I made a huge building error. The horizontal stabilizer has a slight curvature on one side and virtually flat on the other side, making it a "flying" stabilizer with some lift during flight. My problem is that I INSTALLED IT UPSIDE DOWN. THE FLAT SIDE IS ON THE TOP AND THE CURVED SIDE IS ON THE BOTTOM.
I have only had one sip of coffee this morning but it sounds like to me that you installed it correctly!

Here's why:
What does the horizontal tail do? Why is it there? Well it's there to counteract the negative pitching moment of the main wing. When air flows over the main wing the leading edge of the wing will pitch down and this will occur at approximately the 25% mean aerodynamic chord (MAC) of the wing. So the horizontal tail is placed on the plane so that it lifts down which results in the leading edge of the main wing to pitch up. So the horizontal tail is either fully symmetrical or slightly cambered, with the most curvature on the bottom as mounted on the plane.

Google it for more info.

Let us know how the test flight goes, it's a beautiful model.

Curtis
Montana
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Old Nov 28, 2012, 10:01 AM
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That sounds aerodynamically correct. But it goes against what you would think when installing the stabilizer.

I will Google this to see what turns up.

Here is a closeup of the stabilizer. As you can see, there is very little difference in the curvature on the top and bottom. It is more apparent when you place a straight edge on the two surfaces.
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Old Nov 28, 2012, 10:26 AM
agnotology
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Joined Jan 2007
3,657 Posts
The stabilizer orientation will not have an affect on the airplane, other than possibly a millimetre of elevator trim.

Stabilizers on RC models typically have very small loads on them, particularly with near symmetrical wing airfoils. The load can be up or down. It depends on the stabilizer volume (size x distance from wing 1/4MAC to tail 1/4MAC), and the CG location (static margin).

A model with a small stab like this will usually be flown with a slight down load on the tail. Having the flatter side up will actually reduce the trim drag very slightly.

The flying problems this model might have are most likely from the elegant elliptical wing tips. The small size of the tips will almost guarantee that the stall starts out there and progresses inward. This will cause the tip stall/snap tendencies. Adding some washout to the wing by holding the tip past were you want it and re-shrinking the covering, would likely be a good idea.

It is always going to be an exciting model to fly because of the small vertical and horizontal stabilizers. The CG location will be fairly critical.

Kevin
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Old Nov 28, 2012, 11:51 AM
Ascended Master
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Palmdale, CA
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Some noteable planes have "upside-down" horizontals.. flattish on top, curved on the bottom... C-130. L-1011...
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Old Nov 28, 2012, 12:59 PM
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United States, UT, Salt Lake City
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Don't worry about it -
these fly just fine .
The weight of these models is the reqal problem - so you must pay attention to any rapid changes in attitude.
take off and landing have to be at flying speeds - that is- do not horse it off the ground and fly it to the ground in level attitude then gently reduce power as you ad a little up elevator
Trim adjustment will change with speed - accept it -
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Old Nov 28, 2012, 03:08 PM
B for Bruce
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The 'Wack, BC, Canada
Joined Oct 2002
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As Kevin says above you won't see any issue at all other than the initial trim setting for the elevator.

One aspect where it MAY work out to your benefit is in a pylon style steep banked turn. In that case the fixed portion of the stabilizer with the extra curavature on the lower side MIGHT provide some slight amount of gain by not allowing the airflow to separage so easily from the lower surface. But I want to emphasize the "MIGHT" in all this. And to see if you can detect the difference or not you'd need to build and install a second totally flat tail section. And in any event the effect would be minor at most.
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Old Nov 29, 2012, 01:03 PM
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I've seen upside down tail foils on free flights. If I remember right, one was Sal Taibi's Starduster X.
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Old Nov 29, 2012, 02:11 PM
B for Bruce
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HELM, I think you might be thinking about Taibi's Pacer old timer. The Starduster series all uses an upright airfoil.... that is it's arched side is pointed up so it's lifting upwards.

The Pacer was an oddity among free flight designs in its day and even now in that it used the lifting section with the arched side pointed down.
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Old Nov 29, 2012, 03:45 PM
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Quote:
Originally Posted by Sparky Paul View Post
Some noteable planes have "upside-down" horizontals.. flattish on top, curved on the bottom... C-130. L-1011...
Pretty much all of them in fact.
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Old Nov 29, 2012, 03:59 PM
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Quote:
Originally Posted by BMatthews View Post
HELM, I think you might be thinking about Taibi's Pacer old timer. The Starduster series all uses an upright airfoil.... that is it's arched side is pointed up so it's lifting upwards.

The Pacer was an oddity among free flight designs in its day and even now in that it used the lifting section with the arched side pointed down.
I dont know, I've seen it before somewhere on a pylon type free flight. Maybe it was the Witchdoctor.
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Old Nov 29, 2012, 05:42 PM
B for Bruce
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The 'Wack, BC, Canada
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Not unless the guy put it on upside down by accident or due to newbiness.

I'm sure you did see one but it just wasn't one of the better known designs. Or it was simply aa mistake or accident. I haven't seen a published design or anything at the flying fields in all my days other than the Pacer and a few other selected old timers and one specific Nordic style glider in Zaic's yearbook that have the lifting stab airfoils set up for lifting down.
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Old Nov 29, 2012, 06:56 PM
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On our pattern planes for years -in 1980 on- our std pattern model trim included getting balance and stab settings (all of our desins and kits included fullyadjutable horizontal stabs) finalized such that the elevators trailed even OR slightly down OR slightly up.
at proper balance, the shape of the tail group meant nothing.. the model was simply trimmed to have X amount of downforce - we built/ setup hundreds of these .
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Old Dec 15, 2012, 12:16 AM
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When it works out well, which I'm sure it will, you can brag about the custom design and how after long nights of testing, you concluded with the current configuration.
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