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Old Apr 30, 2013, 11:23 PM
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sean.belardo's Avatar
United States, DE, Newark
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Help!
Inverted horizontal stab? Yes, No?

RCGroups,

I've recently become a hand launch addict newbie. I've been flying an old XP5 for a month or two and have recently received a beautiful HJZ-Model Predator-II kit from a friend.

But as a recent aerospace engineering graduate, I have to ask, "why oh why do the build threads of this bird show to mount the Horizontal stab camber side up???"

The build thread on the Predator II

(http://www.rcgroups.com/forums/showt...1002174&page=3)

shows that the cambered side of the airfoil facing upwards. This doesn't sound right to me although I've seen both camber side up and camber side down on DLG models.

To my knowledge, unless the CG is behind the center of pressure (like on fighters that are dynamically unstable and use flight computers), the horizontal stab should be producing negative lift through most of the flight regime for a dynamically stable plane, especially during launch and thermaling when there is a need for pitching upward (up-elevator). So if given a cambered horizontal stab, it should be inverted to produce negative lift with the least amount of drag.

There may not be a noticeable drag benefit with either camber side down or up, but what are your guy's opinions? It seems people seem to mount these flat bottom cambered horizontal stabs where ever it seems easier to mount (Flat bottom mounted to a flat pylon).


I've attached a photo of examples and how I would rank some tail configurations (Neglecting vortex interactions at the stab mounting pylon)

Name: Tail orientation.jpg
Views: 313
Size: 40.2 KB
Description:

D: (Least efficient configuration)
Cons
-Inefficient at producing negative lift
-Increased down wash effects

A:
Cons
-Inefficient at producing negative lift
Pros
-Decreased down wash effects

C:
Cons
-Increased down wash effects
Pros
-Efficient at producing negative lift

B: (More Efficient configuration)
Pros
-Efficient at producing negative lift
-Decreased down wash effects

Please list your opinions. Is there a benefit to have the horizontal stab camber side up? I think the only thing this would do is place the CG further aft when compared to an inverted stab if the Decalage angle was kept constant.
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Old Apr 30, 2013, 11:34 PM
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United States, CA, Tehachapi
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It's a topic that's been discussed quite a bit, actually. Several DLGs have it cambered up, several have it cambered down. Here's a link to an explanation for why it would be cambered in a "lifting upward" configuration: http://www.rcgroups.com/forums/showp...postcount=1284
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Old May 01, 2013, 12:35 AM
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Hmm, interesting points. I guess, when in a thermal, the plane would be at higher angles of attack relative to the airflow making a dropped Horiz stab's LE more desirable. Does anyone know what kinds of AOA DLGs experience when in a thermal? I'm guessing 5-10 maybe, haha.

I'm leaning more towards a flat side down orientation now. Any other opinions from anyone? Has anyone tried leading edge flaps on a horizontal tail? LMAO!
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Old May 01, 2013, 07:29 AM
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Sean:

I am an aeronautical engineer of the REALLY OLD school (my class was the last class at my University to use slide rules - if you know what that is).

Like you, I would have thought the horizontal tail would normally go flat side up, to provide the classical down-load to balance the moment of the wing blah blah blah. However, I receieved an after-market tail for a very good glider, along with the instructions to mount it flat side down. I trusted the supplier (Oleg Golovodov) and it worked fine. It wound up being at very near the zero-lift AOA most of the time anyway.

What I do now: When building a DLG, I put the tail on the top, not bottom, for the aerodynamic reason that it is further away from the weeds and stuff that cause it to take damage. I put the flat side next to the mount so that shaping is mimized. I sand and shim to achieve a decalage angle of from zero to 1 degree trailing edge up (with zero elevator deflection). That's my starting point for the first flight.

Guys with better computer skills than mine can run all sorts of programs and evaluate the different options that you propose. I hope they do. All my procedure does is it gets my airplane into the air for the trimming phase.

By the way: I prefer all-flying tails, but the various v-mounts available are too bulky for my tastes - too much drag.

Yours, Greg
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Old May 01, 2013, 07:56 AM
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Lift coefficient on a DLG horizontal should be nearly zero nearly all of the time. When it's NOT nearly zero however, stab performance can really show through. Most tails aren't bad in a low Cl configuration. Unfortunately they don't show how bad they are until you really need them... pulling hard in one last turn in turbulent air low over a tree line. In my opinion, horizontal stab performance makes or breaks a DLG.

Horizontal tails = secret sauce. Take a bad tail and put it on an otherwise good airplane and you have a bad airplane. The reverse is not true, meaning there's more opportunity to hinder performance with tail selection than enhance it.

@Greg, We can analyze our tiny tails forever but you still have to find something that has good all around flight performance... ie real world testing still teaches us alot.
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Old May 01, 2013, 08:32 AM
G_T
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Greg, you can make your own v-mount that isn't all that bulky if you want. It is one instance where I think the home made mounts are far superior to the commercial ones.

All - the discussion on this thread seems aimed primarily at hinged horizontal tails. Things are slightly different for full flying.
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Old May 01, 2013, 09:23 AM
Thermal, where art thou?
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FF/Conventional Horizontal

@All,

How does a full-flying stab fit into this discussion - do the same rules apply as with a stab/elevator combination? I have planes with FF HT08 and find nothing to fault with their behavior, but maybe I'm just in a comfort zone.

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Old May 01, 2013, 09:30 AM
Oleg Golovidov
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Quote:
Originally Posted by sean.belardo View Post
But as a recent aerospace engineering graduate...

To my knowledge, unless the CG is behind the center of pressure (like on fighters that are dynamically unstable and use flight computers), the horizontal stab should be producing negative lift through most of the flight regime for a dynamically stable plane, especially during launch and thermaling when there is a need for pitching upward (up-elevator). So if given a cambered horizontal stab, it should be inverted to produce negative lift with the least amount of drag.
Sorry for nit picking, but this is a pet peeve of mine, and since you are an aerospace graduate, you deserve to be nit picked for this
Stability does not require CG being forward of the center of pressure (CP) of the wing. It requires CG being forward of the aerodynamic focus (AF) of the entire aircraft and not just the wing. And you must know that CP and AF are not one and the same, and even more so, wing CP and aircraft AF are not one and the same. Depending on the lift coeff (CL) and the moment coeff Cm, the wing CP may travel a lot, and at slow speeds the horizontal tail of a stable aircraft may well be producing positive lift. This is also true when in turns at slow speed! Think about it. The elevator is deflected UP, but the tail is actually required to produce positive (UP) force. So the airfoil is really not in the optimum configuration at that moment. A down turned airfoil nose (positive camber) is just a small help in this particular regime. As others have mentioned, the lift coefficients of the tail are normally very small and any camber has nearly zero effect on the total drag in level flight.
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Old May 01, 2013, 10:07 AM
G_T
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BTW, fighters that fly transonic or faster experience CP shift (or change in Cm behavior if you will). Unstable at one speed may well be stable at another. This was one of the problems of early supersonic tests IIRC - how to get adequate stability and control at both sides of Mach 1. It gets even more interesting at even higher speeds. Welcome to rocket science.

PS - If I get the chance tonight, perhaps I'll post a little analysis for a rocket CP shift behavior as a function of mach number and alpha. OT though, just for fun.
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Old May 01, 2013, 10:08 AM
G_T
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Quote:
Originally Posted by BavarianCharles View Post
@All,

How does a full-flying stab fit into this discussion - do the same rules apply as with a stab/elevator combination? I have planes with FF HT08 and find nothing to fault with their behavior, but maybe I'm just in a comfort zone.

No hinge, therefore no kink on the top side with up elevator deflection. So, no bubble to ameliorate.
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Old May 01, 2013, 04:34 PM
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Sorry, it was late when I posted this. By "Aerodynamic Focus" do you mean Neutral Point (AKA the Aerodynamic Center of the entire Aircraft) ? If so, I agree :-)

I'm thinking of mounting the tail camber side up as the instructions say. Then maybe experiment with mounting the stab inverted later on.


Quote:
Originally Posted by olgol View Post
Sorry for nit picking, but this is a pet peeve of mine, and since you are an aerospace graduate, you deserve to be nit picked for this
Stability does not require CG being forward of the center of pressure (CP) of the wing. It requires CG being forward of the aerodynamic focus (AF) of the entire aircraft and not just the wing. And you must know that CP and AF are not one and the same, and even more so, wing CP and aircraft AF are not one and the same. Depending on the lift coeff (CL) and the moment coeff Cm, the wing CP may travel a lot, and at slow speeds the horizontal tail of a stable aircraft may well be producing positive lift. This is also true when in turns at slow speed! Think about it. The elevator is deflected UP, but the tail is actually required to produce positive (UP) force. So the airfoil is really not in the optimum configuration at that moment. A down turned airfoil nose (positive camber) is just a small help in this particular regime. As others have mentioned, the lift coefficients of the tail are normally very small and any camber has nearly zero effect on the total drag in level flight.
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Old May 01, 2013, 05:07 PM
G_T
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OT, but CP vs mach number for a small rocket.

Gerald
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Old May 01, 2013, 07:47 PM
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Quote:
Originally Posted by sean.belardo View Post
I'm thinking of mounting the tail camber side up as the instructions say. Then maybe experiment with mounting the stab inverted later on.
Pedantic question here: doesn't "camber" really refer to the difference in the curvature of the upper and lower surfaces of the airfoil? So saying "camber side up" is an ambiguous statement. Not trying to nitpick, I was actually having difficulty determining which side you were suggesting would be mounted up. I believe what you're saying you'll try is mounting convex side up - so that the tail's natural lift would be in the upward direction. Correct? Thanks,

-John

P.S. You have exactly the same introductory DLG sequence I had! Used XP-5, to a new Predator 2. If you get an Akcent-2 next and then start building your own, the mirror will be complete. ;-) Needless to say I approve of your selections.
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Old May 01, 2013, 08:57 PM
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BTW, congrats on getting the P2. I spent a long time debating what my next DLG should be and when a deal came along for a P2 I jumped on it. I haven't regretted it yet. The finish quality is immaculate and it flies like a dream. By far the best performing and best handling DLG I've owned so far. Unfortunately they aren't available anymore
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Old May 02, 2013, 03:16 AM
Chesty
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Why not just make the tails symmetric in relation to camber ?
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