Quick question: horizontal stabilizer

[zlzgrom86]

I know that the vertical placement of a wing affects the stability of an aircraft do to the pendulum effect I think. But what about the horizontal stabilizer? The F-104 has a very high horizontal stabilizer, while a F-100 has a very low stabilizer, Hawker Hunter is semi-high, and F-16 is about the middle.

So what aeronautical principles govern the height of a horizontal stabilizer?

[Brandano]

if you look at the 104 you will see that it has a pretty evident anhedral on the main wing. this is to counteract the "dihedral" effect of the large fin and high mounted tail. It makes sense if you consider the distribution of aerodynamic area when looking at the plant projection of the plane at different roll angles. However the position of the stabilizer is usually dictated by the need of keeping it relatively clear of the wing wake.

[Hughbert]

Stability gained from the position of the wing is due to 'pendulum effect' - on a high wing, the CG is below the wing, so if the aircraft is banked by some disturbance, the mass will cause a restoring moment (turning force) - imagine a clock pendulum being pulled to on side and let go. The reverse is true of a low wing (imagine a pendulum upside down).

The horizontal stabiliser doesn't operate in the same way - put simply it supplies a force at the end of the tail to counter the pitching moments of the rest of the plane and make it fly straight. Since the force is acting vertically, it doesn't make that much of a difference (to the stabilising force) if the stabiliser is higher or lower on the tail.

What does make a difference is if the stabiliser is in unsteady airflow, such as the wake or 'downwash' Brandano described. Different shaped wings generate different amounts of downwash, so for some aircraft they need a higher tail to get it clear. Also the structural design of the aircraft needs to be considered - the F-104 and Hunter both had single jetpipes at their tails, which I expect the designers didn't want to put a stabiliser spar through.

[Texas Buzzard]

Those stabs we see way up high, as in some of the Airlines planes flying now are there for a purpose. The purpose is to get that stab in clean air so it is more effecient. Clean air or unturbulent air is good for a hor. stab.

On those planes with the "stab on the top" have a very turbulent air flow down at the base of the fuse while on to of the fin it's clean

Cosider some old trainers. The wing is high and the stab is down low on the bottom of the fuse....it is happy tho" because it , the stab, is out of the downwash of the wing.

What I have tried to write here is some reasons for stab placement TO GET OUT OF TURBULENCE OR DOWNWASH. Ideally one wants to avoid placing the hor. stab ib the downwash of the wing.

[JetPlaneFlyer]

'Pendulum stability' (i.e the gravitational pendulum centering effect) is pretty much dismissed in most modern aerodynamic texts. The accepted explanation of why high wing aircraft are more stable is due to the flow patern around the fuselage. Check out section 5.5 in this book: http://books.google.com/books?id=6-_iGbJHM-8C&pg=PA473&lpg=PA473&dq=mechanics+of+flight+dihedral&source=bl&ots=uD6g9TWauO&sig=cjRRS8_RHAnY1njz59vdWYvHx4A&hl=en&sa=X&oi=book_result&resnum=1&ct=result#PPA483,M1


Tail position is usually decided due to the factors already stated by others in the proceeding posts, plus a couple of others...
. 'structural' considerations
. Avoid wing wake
. Avoid fuselage turbulence
. Move tail out of ground effect during for landing/take off

However it's not practical to move the tail out of the wing downwash as was suggested because downwash in not restricted to the area immediately behind the wing but effects the whole mass of air to the rear of the aircraft both above and below the wing itself.

[Hughbert]

Pendulum stability may not be included in most aero text books but it's still mentioned in aero lectures :eek:

Although I haven't seen any qualitative methods which include it, more 'this exists'

[Brandano]

Personally I think that "pendulum stability" is dihedral under another name. Any aerodynamic surface above the COG will add to the dihedral effect, any under it will subtract from it. As far as the tail producing negative lift, this is an oversimplification, and usually is false. The tail produces less lift than the wing, but it rarely creates negative lift. You just have to see a plane coming in for a slow landing to realize that even with full up elevator the tail still has a positive AOA.

[JetPlaneFlyer]

Pendulum stability may not be included in most aero text books but it's still mentioned in aero lectures :eek:

Although I haven't seen any qualitative methods which include it, more 'this exists'

I guess that depends on the academic level at which the lecturs are taking place. it's quite possible that regular pilot training type lecturs still do refer to the 'gravitational' pendulum effect. However the subject has long been dropped from any authorative text and replaced by the 'fuselage effect' explanation as described in the book i gave the link to earlier. Some books and lecturs may actually still refer to the 'pendulum effect' but in name only, not in operating principal.

Read the paragraph on 'High Wing Stability' here on the Jan 2008 page of the old Aerodynamics index site: http://web.archive.org/web/20080109035821/selair.selkirk.bc.ca/aerodynamics1/Stability/Page5.html

Steve

[vintage1]

Well I am so glad that the thousands of parachutists and hang glider people will be delighted to know that without a fuselage or a tail, their contraptions will never be pitch stable and will kill them instantly. :D

The condition of pitch stability is that the differential of nose up moment is positive with respect to dive angle: You can do that either directly using a pendulum moment, or indirectly using the implied speed increase plus decalage to pitch the nose up automatically. Nature doesn't care..

[Hughbert]

I guess that depends on the academic level at which the lecturs are taking place

How does 3rd Year of a Masters in Aerospace Engineering at the University of Bath count as academic level? :p just kidding.

Our stability module this year only focused on pitch stability, so I don't know any methods for the other aixs but the lecturer briefly touched on the other axis, mentioning briefly both pendulum and fuselage effect. I find pendulum easier to explain :D

[JetPlaneFlyer]

'pendulum' i.e. vertical CG position does i believe still play a part (though it does appear to have been purged from all the texts i've come accross) but the key issue is that it's not related directly to gravity but to the CG position relative to the vertical centre of drag and it's only effective due to sideslip. (this is why i kept mentioning 'gravity pendulum effect'). An aircraft unlike a pendulum does not have a fixed 'pivot' about which it's forced to rotate.

[Brandano]

Think of "pendulum stability" as "how big a portion of the wing is on the side of the lower wing relative to the CG". That's why I am saying that pendulum stability is a dihedral effect with another name. Just makes the lower wing bigger through another mean.

[Hughbert]

That's true, it's a simple explanation of a complex system, which is never going to be accurate. Even if (as I suspect) it's largely or partially negligible on full scale aircraft it's a useful and easy visualisation, whereas flow patterns over wings/vertical centre of pressure are not.

And it's not entirely negligible on RC aircraft, at least smaller ones. I've had a foamy be made more or less stable by the vertical position of the battery - since I haven't change the wing location it has to the CG position affecting the stability.

Anyway, that's off topic :D

[mnowell129]

The tail produces less lift than the wing, but it rarely creates negative lift.
Tails produce negative lift quite often, especially in general/commercial aviation.
You just have to see a plane coming in for a slow landing to realize that even with full up elevator the tail still has a positive AOA.
Positive AOA with respect to what, the ground? This statement doesn't consider the significant downwash from the wing putting the tail at a very significant negative angle of attack, and therefore negative lift.

[Thomas B]

Those stabs we see way up high, as in some of the Airlines planes flying now are there for a purpose. The purpose is to get that stab in clean air so it is more effecient. Clean air or unturbulent air is good for a hor. stab.

On those planes with the "stab on the top" have a very turbulent air flow down at the base of the fuse while on to of the fin it's clean

Cosider some old trainers. The wing is high and the stab is down low on the bottom of the fuse....it is happy tho" because it , the stab, is out of the downwash of the wing.

What I have tried to write here is some reasons for stab placement TO GET OUT OF TURBULENCE OR DOWNWASH. Ideally one wants to avoid placing the hor. stab ib the downwash of the wing.

Some aircraft have other issues with high mounted stabs....at certain angles of attack and sometimes when stalled, the T-tail still ends up blanketed by the wing and causes issues, up to and including crashes.

You do want the horizontal tail out of the way in cruise, if possible, but you still want the aircraft to perform properly at higher AOA and when stalled.

[Bax]

It's not just for aerodynamic reasons an airplane may have a "T" tail......

Some aircraft have "T" tails because the engines are in the way...witness the DC-9, B727, BAC-111, and so on. Many executive-type jets are the same way.

In some aircraft, there's a "T" tail because that's how they wanted to style the aircraft...witness Piper's "T"-tail Arrow, Seminole, Tomahawk, and Lance. Beech used a "T" tail on the Duchess, which also received criticism as a styling move that really didn't do anything for performance.

[Brandano]

The F104 is a classical example of a plane where the T tail wasn't chosen for aerodynamic reasons (and is indeed susceptible to the deep stall mentioned earlier). On the 104 the tail was mounted on top of the fin to move mass away from the centerline, and mitigate the effect of inertial coupling. Another reason to use a T tail, especially in gliders, is because they are less susceptible to damage when landing in tall grass. A cross tail will work as well, but causes more drag.