Had a DE ask my student why the arrow he was flying had a stabilator instead of an elevator "like cessna's have"? I know stabilators are create a little less drag and are better for maneverability, but not sure what he was looking for. Thinking it might have to do with stability as well. Anyone have any thoughts as to why piper used a stabilator instead of an elevator like on cessna's?
Elevator vs Stabilator
- Thread starter FOD
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Ok, think I might have figured it out. I’m thinking that it has to do with longitudinal stability. The aircraft in question was a piper arrow, which has a pretty forward C.G, making it really longitudinally stable. Adding a stabilator will give it more maneuverability to help decrease some of its natural stability. Let me know what you think?
meritflyer
Well-Known Member
It does have to do with longitudinal stability like you mentioned. The Cessna, being a high wing, has constant airflow from the trailing edge of the wing down to the stabilizer creating a significant amount of tail down force. Even as flaps are added the center of lift moves aft creating a nose down tendency. This is balanced by the air rushing over the wing and striking the elevator creating a dynamic balancing force or tail down force. Because of the additional down-flow air from the high wing, not as much control surface is needed to provide pitch stability.
Piper, being a low wing, has the stabilator with the servo on the back. When the relative rushes over the upper surface of the wing and is deflected downward off the trailing edge, it goes nowhere even near the stab which reduces its effectiveness overall. The servo on the back provides a dynamic balancing force so the airplane is not over-controlled by simple stabilator movements. When flaps are added to its configuration notice the nose of a Piper will drop much more than a Cessna. As the center of lift moves aft with flaps it creates a nose down moment. In a low wing there is no force to counter-act the nose down attitude where as the high wing has the air over the main wing striking the tail providing the tail down force or dynamic balancing to the aircraft longitudinal axis.
To sum it up, the Piper, due to airflow patterns off the wing and their effect on lpitch stability, needs a larger control surface to provide enough longitudinal stability. The Cessna has a much greater force acting on the tail with the high wing deflecting the wind off the trailing edge to the elevator which increases its effectiveness and ultimately results in less need for a larger control surface.
Clear as mud right?
Piper, being a low wing, has the stabilator with the servo on the back. When the relative rushes over the upper surface of the wing and is deflected downward off the trailing edge, it goes nowhere even near the stab which reduces its effectiveness overall. The servo on the back provides a dynamic balancing force so the airplane is not over-controlled by simple stabilator movements. When flaps are added to its configuration notice the nose of a Piper will drop much more than a Cessna. As the center of lift moves aft with flaps it creates a nose down moment. In a low wing there is no force to counter-act the nose down attitude where as the high wing has the air over the main wing striking the tail providing the tail down force or dynamic balancing to the aircraft longitudinal axis.
To sum it up, the Piper, due to airflow patterns off the wing and their effect on lpitch stability, needs a larger control surface to provide enough longitudinal stability. The Cessna has a much greater force acting on the tail with the high wing deflecting the wind off the trailing edge to the elevator which increases its effectiveness and ultimately results in less need for a larger control surface.
Clear as mud right?
moxiepilot
Well-Known Member
the only part of merritflyer's post that i find important & missing is the fact that the stabilator has an anti-servo tab which moves in the same direction of stabilator travel.
this tab prevents overcontrolling the pitch attitude about the lateral axis, since such a large surface is moving. if this were not present controlling the attitude would become cumbersome and possibly result in PIO.
this tab prevents overcontrolling the pitch attitude about the lateral axis, since such a large surface is moving. if this were not present controlling the attitude would become cumbersome and possibly result in PIO.
PanJet
Well-Known Member
meritflyer said:It does have to do with longitudinal stability like you mentioned. The Cessna, being a high wing, has constant airflow from the trailing edge of the wing down to the stabilizer creating a significant amount of tail down force. Even as flaps are added the center of lift moves aft creating a nose down tendency. This is balanced by the air rushing over the wing and striking the elevator creating a dynamic balancing force or tail down force. Because of the additional down-flow air from the high wing, not as much control surface is needed to provide pitch stability.
Piper, being a low wing, has the stabilator with the servo on the back. When the relative rushes over the upper surface of the wing and is deflected downward off the trailing edge, it goes nowhere even near the stab which reduces its effectiveness overall. The servo on the back provides a dynamic balancing force so the airplane is not over-controlled by simple stabilator movements. When flaps are added to its configuration notice the nose of a Piper will drop much more than a Cessna. As the center of lift moves aft with flaps it creates a nose down moment. In a low wing there is no force to counter-act the nose down attitude where as the high wing has the air over the main wing striking the tail providing the tail down force or dynamic balancing to the aircraft longitudinal axis.
To sum it up, the Piper, due to airflow patterns off the wing and their effect on lpitch stability, needs a larger control surface to provide enough longitudinal stability. The Cessna has a much greater force acting on the tail with the high wing deflecting the wind off the trailing edge to the elevator which increases its effectiveness and ultimately results in less need for a larger control surface.
Clear as mud right?
That's a great explanation I haven't heard before. It makes sense though, but then again, why are elevators overall more popular, even on low wing aircraft? The majority of the aircraft in the world are low wing, but most have elevators.
I'd have to say your explanation is the best I've ever heard, although I have heard that Piper used stabilitors more because it was the "new" and "in" thing back in the day, and it was something different than Cessna. I'm not sure as to the accuracy of that statement.
meritflyer
Well-Known Member
moxiepilot said:
I actually did state that, I accidently called it a servo.
moxiepilot
Well-Known Member
yeah you did. my bad :sitaware:
I_WANNA_BE_ATP
New Member
If you stop drinking from the water, you'd noticed that Mox 
moxiepilot
Well-Known Member
I_WANNA_BE_ATP said:If you stop drinking from the water, you'd noticed that Mox
get a real job, nurse boy!
drstrangelove
New Member
I'm not prepared to decipher merit's explanation, it's too early. I can't find any thing seriously wrong with his argument other than it's silly. Stabilator vs Stabilizer is mostly about two things, design philosophy and cost. I had a long conversation with a fellow student when I was working on my ratings about lift and whether or not a wing imparts momentum to the air mass causing it to move down. The assumption is that in the act of creating lift the wing pushes air down and calls on Newtons law stating for every action there is an equal and opposite reaction and pushes the plane up. Sounded good to me. However my fellow student said that it was Bernoulli that makes a plane fly, the low pressure over the top surface of the wing. She has a Masters in Aerodynamic Engineering from University of Michigan. I belive her.
The interacton of airflow high wing vs low wing can all be solved with either a stablilator or stabilizer, there are examples of both ways out there. The Cardinal uses a stabilator and a conventional tail Bonanza uses a stabilizer. Design philosophy, the lead engineer, says I want a stabi___ and the bean counters say a stabi_____ is cheaper and easier to build. The aero and mechanical engineers figure everything else out so it works.
Dont go spreading mis-information, while your explanation is not wrong it is not "the" reason. Too many newbies who become instructors and never really learn what's what see this kind of stuff and take it as gospel.
Here is an absolutely true statement about this subject. Stabilators and stabilizers provide a means of pitch control. There are advantages and dis-advantages to both, but the end result is identical, pull back houses get smaller, push forward, houses get bigger.
The interacton of airflow high wing vs low wing can all be solved with either a stablilator or stabilizer, there are examples of both ways out there. The Cardinal uses a stabilator and a conventional tail Bonanza uses a stabilizer. Design philosophy, the lead engineer, says I want a stabi___ and the bean counters say a stabi_____ is cheaper and easier to build. The aero and mechanical engineers figure everything else out so it works.
Dont go spreading mis-information, while your explanation is not wrong it is not "the" reason. Too many newbies who become instructors and never really learn what's what see this kind of stuff and take it as gospel.
Here is an absolutely true statement about this subject. Stabilators and stabilizers provide a means of pitch control. There are advantages and dis-advantages to both, but the end result is identical, pull back houses get smaller, push forward, houses get bigger.
drstrangelove
New Member
And another thing, man I'm grouchy this morning. A servo tab creates feel, or what is called the Stick Force per G gradient. Maybe that is what you were trying to say but it was unclear.
Stabilators have to be aerodynamically balanced around the aerodynamic center of the surface. If they weren't not even DE727UPS could push or pull the yoke on a cherokee. Since it's balanced it has little forcing it back to the slipstream position. The servo tab is essentially a small elevator on the trailing edge of the stabilator generating a centering force. It also doubles as a trim tab but that's another story.
Stabilators have to be aerodynamically balanced around the aerodynamic center of the surface. If they weren't not even DE727UPS could push or pull the yoke on a cherokee. Since it's balanced it has little forcing it back to the slipstream position. The servo tab is essentially a small elevator on the trailing edge of the stabilator generating a centering force. It also doubles as a trim tab but that's another story.
meritflyer
Well-Known Member
drstrangelove said:
Tell that to the aerodynamic engineer that works for Boeing. He lives next door to me. He actually read the initial post with me (trying to get the guy to be a JCer) which lead to our discussion and my reply. Secondly, I am sorry to say that it is both the Newtonian Theory of Inertia and Bernoulli's Principle that relate to creating lift. Ask your "friend" with the MS of Aerodynamics to explain the mathematical principle of lift and you'll hopefully understand that the two theories can not exist without eachother when discussing all of the pertinent elememts to of lift. Without eachother in the overall equation, they can actually be diproved through basic physical principles. NASA proved this a thousand times over and has several publications on such findings.
You must be trying to throw your weight around as the new guy. We were all there at one point or another.
Welcome to JC.
meritflyer
Well-Known Member
Dugie8 said:
All it comes down to is needing a larger moveable control surface to maintain pitch stability for different configurations based on airflow patterns.
poppa2trolls
New Member
This subject came up fairly often when I was an F-16 instructor, so I went to the folks at General Dynamics (now Lockheed Martin) to get their explanation. Basically, greater control surface area equals greater control. In our case, the specific design of the stabilators---two large, independent surfaces (left and right)---had a lot to do with supersonic attitude control. Haven't ever talked with a light aircraft engineer, but would suspect similar rationale... except for the supersonic stuff.
meritflyer
Well-Known Member
No problem. Like I said, all my ramblin' on about this or that all comes down to one basic principle which is the effectiveness and size of the control surface for specific aircraft and their configs.