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Stabalizers and elevators - 10/14/2007 9:16:05 AM   
David Ingham


 

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Status: offline 		I have been re-reading Model Aircraft Aerodynamics by Martin Simons, 1978, because I have a copy. It has some nice data and tells much about what was common practice, but sometimes draws wrong conclusions.
He concludes that there is no aerodynamic advantage to combining the stabilizer and elevator into a stabilator, because he misses the way these work together and just considers the effect of the elevator. If the critical thing is how quickly the turn can be stopped and started, then, coming out of the turn is harder than going into it because, without the tail, the airplane is unstable. In this case the best is the conventional hinged elevator, because, coming out of a turn, the stabilizer is already trying to resist the turn and the elevator cambers it in the right direction. If how tightly it can turn at constant rate is critical, then the elevator is fighting the stabilizer, so turning the whole stabilator is better.
The treatment of laminar vs. turbulent boundary layers is better. The data for a Göttingen 801 airfoil shows a large decrease in lift and increase in drag for Reynolds numbers below somewhere between 20,000 and 100,00, for laminar flow, compared to turbulent boundary layer. The laminar boundary layer remains separated, while, if turbulence is initiated early, the turbulent layer re-attaches after a short "bubble". So, for some angles of attack, within this range of Re, a turbelator or lumpy paper covering, greatly improves performance, compared with a perfect reproduction of this airfoil. Below around 30,000, this airfoils is hopelessly thick and cambered. He presents this nicely, with a comment something to the effect that some modelers may be taking great pains to drastically reduce performance.
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RE: Stabalizers and elevators - 10/14/2007 3:38:26 PM   
da Rock



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Status: offline 		It really is a good book, and was the best aerodynamics for models book for years. It came out in the late 70's and Eppler and others hadn't shown up yet.

You owe it to yourself to make the effort to find, "R/C MODEL AIRCRAFT DESIGN" by Andy Lennon. It's from the publisher of Model Airplane News and is in a couple of the better LHSs around here. He covers just about everything. But surprisingly doesn't write about stabilators. Or at least doesn't do a comparison to the detail that Simons did. I've also seen it on Amazon, I think.

(in reply to David Ingham)
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RE: Stabalizers and elevators - 10/15/2007 12:31:12 AM   
BMatthews



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Status: offline 		I'm not sure I'd quite go along with a blanket claim that the stabilizer is resisting what the elevator is trying to do. You can't really treat the two surfaces as separate entities. What you have is a total unit that alters both camber and angle of attack at the same time versus a full moving stabilator that can only change its angle of attack.

For small deflections the stabilator will have a lower drag than the separate surfaces. However there's going to be some point at which the camber aids the lift required to direct the tail of the aircraft where a separate stabilizer and elevator will have less drag than the all moving stabilator. At some other point the drags to lift may cross again but since it's related to so many variables you'd have to analyse specific situations.

This is totally aside from structural stiffness and strength issues of course. Strictly lift and lift to drag required to move the tail around.


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Free Flighters go that extra mile........

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RE: Stabalizers and elevators - 10/16/2007 8:31:00 PM   
bobmac010



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From: Poughkeepsie, NY, USA
Status: offline 		In Stabilator vs. Elevator, the stabilator provides more maneuverability because of the larger moving surface while having less drag.

Elevators and a horozontal stab are a good economical setup, and are self neutralizing, (the airflow makes the turn surface return to neutral).
Stabilators need to have the pivot point near the center of lift so the forces required to move the enormous surface are low.
There are conditions where the center of pressure on the stabilator moves forward of the pivot point, and will not return to neutral by itself without the assistance of a moveable tab called, (odly enough), a "servo".

Horozontal stabs are generally stronger, lighter and easier to setup compared to stabilators.
There are many full size aircraft that use stabilators for maneuverability, like the F15, F18, and yes, even the Piper Arrow.
In radio control, you would need to have a much stronger servo to move the stabilator enough to have the same effect as a horozontal stab and elevator.

Is one better than the other? I think that in model aircraft it is just personal preference, unless you are building a scale model and require a specific setup.
Some of the 3D planes have such a small horozontal stab and large elevator that they almost act as a stabilator with leading edge slats.

I guess the main thing to remember here is - Pull back, plane goes up - Push forward, plane goes down.

Bob

(in reply to BMatthews)
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RE: Stabalizers and elevators - 10/16/2007 8:59:24 PM   
BMatthews



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quote:

ORIGINAL: bobmac010

In Stabilator vs. Elevator, the stabilator provides more maneuverability because of the larger moving surface while having less drag......



I'm afraid that this is also a bit of urban legend but to some extent has a grain of truth.

Both systems are easily capable of pushing a wing to the point of stalling in a loop or very tight turn if they have enough throw. So one is not really more manueverable than the other if you look at it that way.

While it doesn't seem like a stabilator is moving much the angle of attack change overall is roughly the same to get teh same elevator response. But the movement of the elevator needed to give the stabilizer/elevator combo the same angle of attack change is more noticable.

Stabilators will actually stall at a lower angle of attack change than the traditional stabilizer/elevator. All else being equal an airfoil with higher camber will stall later. So a conventional setup can often generate the same pitch force while producing less drag to do so.

However for smaller travel angles and minor trim changes you're right and the stabilator will have somewhat less drag for a number of reasons. But for higher angles of deflection the crossover where the conventional setup is actually better is soon reached.


_____________________________

Bruce-
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Free Flighters go that extra mile........

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RE: Stabalizers and elevators - 10/16/2007 10:14:46 PM   
Red B.


 

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quote:

ORIGINAL: bobmac010
...There are many full size aircraft that use stabilators for maneuverability, like the F15, F18, and yes, even the Piper Arrow...

Stabilators are used on transonic and subsonic aircraft in order to ensure that the tail surface never has much camber on it. High camber promotes the formation of shock waves that render the control surface ineffective. At subsonic speed there is no real advantage to using a stabilator.

/Red B.

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/Red B.

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RE: Stabalizers and elevators - 10/17/2007 5:12:05 PM   
crasherboy


 

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Status: offline 		IMHO it seems that the stabilator would produce the less drag[no hinge line to get in the way] and the quickest response. The full size Cesnas use the regular stabilizer and elavator ,I thnk the reason being they are less touchy[takes a bit of muscle for a stall] whereas a stabilator if set up for performance could get a novice pilot into trouble. A full stall 50' from the ground would be big trouble!

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RE: Stabalizers and elevators - 10/18/2007 12:53:42 AM   
Ed_Moorman



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Status: offline 		You get a lot of drag from the gap in the stabilator-fuselage joint. The elevator hinge line can be sealed with tape or same color covering.

The force on a stabilator depends on where the pivot point is. Obviously, if the pivot is up near the leading edge, there would be a lot of force on the controls. I tried a stabilator on a plane back in the 1970s. Sawed the stab off and added a stabilator. Flew the same. It was not overly sensitive.

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R/C Report Magazine, Fun Aerobatics Column

(in reply to crasherboy)
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RE: Stabalizers and elevators - 10/18/2007 1:28:21 AM   
bobmac010



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Status: offline 		Bruce,

In simple terms, I think you nailed it as far as forces, but the drag or efficiency really depends on the application.

My explanation was for "normal" flight.

In extreme maneuvers like 3D, the elevator/stab combo is the best because of the "undercambre" that is created when elevator is moved.
An undercambre airfoil is a high drag airfoil, but it is also high-lift, and can withstand higher angles of attack before entering a complete stall.

If you notice, a Cessna's elevator is about 15% to 20% of the total horozontal tail surface, (depending on the model).
On the other hand, a Katana's elevator is about 65% of the total horozontal tail surface.
The small stationary stab of the Katana acts as a leading edge slat much like you find in a large airliner, or cargo plane's main wing, (creating an undercambre airfoil). This enables the wing to create much more lift while being able to obtain higher angles of attack without completely stalling. This also creates a very large amount of drag, slowing the airliner/cargo plane for landing.
A stabilator cannot produce the same forces at lower airspeed because it will stall at extreme angles of incidence. In those conditions you are relying entirely on deflection instead of deflection and dynamic lift as in the elevator/stab setup.
In a model, this can be disasterous, because you cannot "feel" the stabilator about to stall. Stalling the stabilator can cause the tail of the plane to sink, creating a nose-high condition that in a moderately powered model will create a full stall. On an approach it would spell disater

Crash:
The Cessna's setup is two fold. One is to keep the cost down, the other is the fact that you can stall a stabilator.
This can be fatal to a novice.
Being a "touchy" aircraft has a LOT more to do with overall design instead of just the tail configuration.

A stabilator in a highwing aircraft with flaps, under the right conditions, can induce negative "feel" pushing the controls toward and past neutral , giving a pilot reverse feedback through the controls because the forces on the stabilator may be behind of the pivot point of the stabilator.
Most private planes with stabilators are either low-winged aircraft, or T-tails. This keeps the tail section out of the wash of the main wing preventing the adverse effects of applying flaps.

As Bruce said, both config's are capable of creating a full stall, regardless of velocity, (unless you are talking about a canaard - a completely different scenario).

If you have a touchy aircraft, it can be from several different sources.
CG too far aft.
Short moment arm from CG/CP/CL to tail control surfaces.
LARGE control surfaces that are aerodynamically balanced, (so the forces required to move them are small) - Full Scale ONLY.
Poor design.

Red B:
In subsonic aircraft there is no real reason to use stabilators much in the same way that electric cars have not become mainstream yet.
In "normal" flight, the stabilator is more efficient, but should not be used in certain configurations. It can get pilots into trouble when they use flaps, spoilers, etc.

Also, if you are piloting a sonic or trans-sonic aircraft, I would hope that you have had specific training in its flight characteristics before being put in the front seat, and it is most likely not a model plane.

Ed:
With all due respect, please see my explanation above:
Is one better than the other? I think that in model aircraft it is just personal preference, unless you are building a scale model and require a specific setup.

Bob

(in reply to Ed_Moorman)
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RE: Stabalizers and elevators - 10/18/2007 12:51:42 PM   
da Rock



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Status: offline 		A couple of things need to be said here.
A wing creates it's maximum lift in total disregard to what kind of structure is behind it controlling it's pitch. And it stalls at whatever AOA it stalls at, also in total disregard to the horizontal tail's design. If the wing has slats or flaps, then they are part of the wing system and are considered, but whatever is aft on the fuselage doesn't change what the wing does, only how the entire airplane affects what the pilot will need to ask the wing to do.

The stab can't enable the wing to create more or less lift or hold any greater angle of attack before stalling.

Also, stabilators are rigged to produce forces as required by the specifications of the design envelope. All horizontal tails will stall at extreme angles of incidence and if those angles are expected in the design specs, whatever the horizontal tail design is, it's rigged to respond to keep the airplane in the design specs. Flexible camber systems like stab/elevator horizontal tails effectively change their incidence when the elevator deflects. They basically change the airfoil shape, and along with that goes their pitch. The way aerodynamic designers deal with greater required AOA from stabilators is to simply increase the available deflection As long at the stabilator deflects enough, it won't stall. In fact, the conventional tail has a limit imposed by the fixed stab (although some have trimmable ones), whereas the stabilator is limited only by the mechanics of it's pitch control system. So in fact, the stabilator system can be a lot more adaptable to envelope pushing conditions.

BTW, a very large number of model gliders are designed with stabilators and they are definitely subsonic. There are also a number of fullscale gliders that use it. And a number of private aircraft use them. They are often lighter to build, having no requirement for hinging along the span and such. They have been found quite useful by many. And the designers know that there aren't any particularly specific problems with them that occur when flaps or spoilers are used. And modelers who fly gliders have proven that over and over.

So don't worry about there being any problems specific to stabilators that aren't solved by having sufficient deflection and appropriate controls.

They work for cutting edge supersonic aircraft and for our relatively small gliders. And about everything in between. Basically, by designers who know how to employ them.


(in reply to bobmac010)
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RE: Stabalizers and elevators - 10/22/2007 11:59:30 PM   
David Ingham


 

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Status: offline 		Thanks for the book suggestion, da Rock. Maybe I will get it, when I have more time to read.
Since I posted I have found more invalid assumptions. In Appendix A, he assumes the same angle of attack for climb and glide of a free flight model, (resulting in the optimal angle being 55 degrees). This may often be a good angle, but the logic makes no sense.

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RE: Stabalizers and elevators - 10/23/2007 2:57:14 AM   
bobmac010



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Status: offline 		Dave,

The best climb angle is about 60% cruise speed at full throttle. This gets you higher, quicker than any other angle, unless your thrust to weight ratio is greater than 1.
You cannot put a fixed angle on the best climb rate. Put a full scale Cessna at 55 degrees for long and it will end up back at the recycling plant, but put a F-15 at that angle, and it will go until it is out of sight.
Best climb angle is also determined by the density of the air, so the same plane with the same load may climb better at a shallower angle in the summer than in the winter.

While not having read that book, if the author is stating that the optimal angle for climb of a specific model in indoor flight is 55 degrees, he may be absolutely correct. If he is making a general statement about all models, I would regard anything it that book as highly as if Snoopy told me personally.

Oh, and remember that logic is the art of going wrong with confidence.

Bob

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RE: Stabalizers and elevators - 10/23/2007 3:33:59 AM   
da Rock



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quote:

ORIGINAL: David Ingham

Thanks for the book suggestion, da Rock. Maybe I will get it, when I have more time to read.
Since I posted I have found more invalid assumptions. In Appendix A, he assumes the same angle of attack for climb and glide of a free flight model, (resulting in the optimal angle being 55 degrees). This may often be a good angle, but the logic makes no sense.



? Appendix A ?
My copy is a hardbound and appears to be 2nd edition or later as mention is made to the first publication in 1978. It has chapters 1-12 and Appendix 1, 2, and 3. And there is a section in Appendix 1 that deals with the best angle of climb. But it doesn't mention glide or for that matter, AOA of either. Also his chapter 4 covers the same topic, best angle of climb, but actually doesn't mention AOA, just the climb angle. The same chapter has a section on the three glide strategies for minimum sink, best L/D, and how to cover the greatest distance. I really haven't found where he assumes an AOA for anything. I bought that book when my primary concern was R/C gliders and hit it pretty hard for info on glide strategy. I don't see any of his discussion that matches glide and climb and especially AOAs.

BTW, he does mention that he received information etc from Professors Eppler and Wortmann.

(in reply to David Ingham)
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RE: Stabalizers and elevators - 10/23/2007 2:39:11 PM   
da Rock



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quote:

ORIGINAL: David Ingham

I have been re-reading Model Aircraft Aerodynamics by Martin Simons, 1978, bec