Why do...

[Zipper730]

While I know the wings on a missile are called fins, I'm going to use airplane terminology to grasp a bizarre question that I've had trouble grasping for years but never frankly had the guts to ask without looking like a total retard.

While I can remember concepts and figures and stats and stuff: I'm honestly curious why airplanes have huge gigantic wings and tiny little fins mounted only on the top-side, yet missiles have huge fins on the top and bottom sides, and often just as big as the wings are?

I grasp the basic concept of gravity -- it pulls down; I grasp lift, which pushes up: I understand that; I grasp that there's a center of gravity and a center of pressure which are deliberately out of alignment so the aircraft's g-load reduces after the pilot centers the stick, and that's why you either have a tail, a canard, or trailing edge pitch elevators/elevons.

I know the transverse g's taken by an airplane are less than the vertical loads, but I thought they were fairly substantial especially with certain maneuvers and I'm amazed there would be enough lift on that axis to hold it up (I'm also frankly amazed that even with heavy top-rudder inputs, the tail fin would be able to keep the plane aloft, and not roll the plane with the control forces being only applied on one side).

I actually do like to learn, so once all the laughter subsides, I could use something.

 

[pbehn]

A missile doesnt have to land, it is fired and travels at mach 2.5+ to where it is going and then explodes. Countering the effects of gravity are just accomodated by the steering of the fins. Most of the missile is either explosive, propellant or guidance system so once fired it doesnt need much lift and has huge thrust to generate it.

The bloodhound missiles which were ram jet powered did have more substantial wings.

As a clue to the relative forces, a Sidewinder missile's fuse activates and arms the missile after 5 seconds at plus 20g acceleration, the earths 1g is a minor force.

 

[pbehn]

duplicate

 

[XBe02Drvr]

Hi there Zipper,
You might find all this easier to understand if you divorce your thinking from "up" and "down" and recognize gravity as "toward the Earth's center" and lift as "perpendicular to the plane of the wings". Then you realize that lift and gravity are not always directly opposed to each other. Referring to that airplane in an 80 degree banked turn we mentioned in a previous thread, the major portion of the lift being generated is dedicated to snatching the plane (laterally from gravity's perspective, but nearly vertically from the aircraft's perspective) away from its former straight flight path. (Back stick = Positive G = "UP" from the pilot's perspective, regardless of where the earth is in all this.) Now our 80 degree banked airplane has only a tiny portion of its generated lift devoted to fighting the earth's pull, and unless the pilot is pulling a lot of G the anti-gravity component of all that lift is probably not enough to equal gravity, and the plane starts to settle earthward. At this point it is essentially ballistic, and will settle earthward at whatever rate the balance of.forces dictates.

The same applies to a missile in flight and an airplane in knife-edge flight (like Blue Angels/Thunderbirds "razor pass"). Both are essentially ballistic, though some airplanes in knife-edge can generate some lift by cocking the nose high (relative to gravity)(top rudder), and using their fuselage as an airfoil. Seldom does this amount to more than prolonging what is essentially a ballistic maneuver.
A Mach 3 missile is traveling at roughly the speed of a .30/30 bullet. Did you ever see a rifle bullet with wings? It's a ballistic object, and so is the missile. The bullet maintains stability by its rotation, the missile by its fins. But the missile needs to be steerable, the bullet doesn't: enter....WINGS! The missile's fins are quite capable of deflecting its alignment toward the target, but wings are necessary to generate the lift to pull the missile away from its straight-line inertial flightpath. Otherwise, it just tries to point its nose toward the target without achieving much change in the fltghtpath. Might work against a lumbering bomber, not so with an agile evading fighter. Modern high-G "dogfight" missiles need even more lift. (Biff, you up? What's the latest and greatest?)
Cheers,
Wes

 

[pbehn]

An aircraft in level flight must have a lift of 1g, aircraft can perform momentary turns of 12 or more. I have no idea what the maximum g turn a missile can perform and anyone posting such a figure obviously doesnt know (because those who do actually know dont post on forums). The post by XBe02Drvr is informative, when discussing the flight path of a shell or bullet consider that the trajectory is governed by gravity and air resistance slowing it. A missile has its own thrust so air resistance doesnt slow it down in the same manner.

 

[XBe02Drvr]

pbehn is rght. I didn't mention the effects of velocity decay.

Ballistics, Part 2: A bullet starts losing velocity as soon as it leaves the muzzle, so it's trajectory drops exponentially further below the line of sight the farther it travels. A missile, on the other hand, launches at the lowest velocity it will have in its flight, and accelerates from there until its motor burns out, then coasts to its target, usually a fairly short distance. As soon as its guidance system detects that it's dropping below its path to the target (as most do at the moment of launch), it commands a "turn" in the direction of the target. To us earthlings, this would correspond to our concept of "up", but to the missile its just a turn. The glorified '59 Cadillac tailfins that pass for wings on missiles provide the lift to overcome the missile's "downward" motion, and to follow whatever steering commands Guidance issues.
Now comes the real trick the missile has up it's sleeve: it has two sets of wings! If Guidance commands "right" or "left", then the "vertically" mounted wings generate the lift to steer in that direction. No need for banked turns and dividing your lift between turn performance and fending off gravity! Neat, huh?
(You out there, Biff? drgondog? pbehn? You got any corrections or updates for this antiquated explanation?)
Cheers,
Wes

 

[tomo pauk]

With such a big speeds, the missile's body should've been providing quite an amount of lift. We can recall that an F-15 landed with a part of it's wing missing due to the mid air collision.
Plus the F-14/MiG-29/Su-27 families of aircraft, where the fuselage was deliberately designed to provide the lift.

 

[pbehn]

XBe

I have no special knowledge I am just taking part in a discussion.

Here is a pic of a Sidewinder.

The rear fins are fixed with "rollerons" to prevent rotation. The front fins provide the guidance to the target. They are symmetrical I suspect because there is no "down" as far as a missile is concerned, it simply doesnt fly long enough or slow enough for lift against gravity to be a consideration.. If it arms its fuse by accelerating at 20g then it can go vertically at a minimum of 19g. Those fins travelling at more than mach 2 would have no problem at all providing enough force to keep the thing in the air, it can follow and destroy aircraft doing evasive manouvers.

 

[MiTasol]

Hi Zipper

I think this is the answer you are seeking

Going back to early aviation, nearly all aircraft had the auxiliary landing gear at the back (hence the name tail dragger's) so it was not practical to have a vertical stabiliser and control surface below the fuselage. The vertical down stabilisers structure would have to be strong, heavy and rigid to minimise damage and the potential for damage which could jam the control surface is too great.

On nose wheel aircraft the tail must come down close to the ground during rotation to allow the aircraft to reach the correct angle of attack to become airborne so having a vertical stabiliser and rudder below the fuselage is again not practical. To increase this rotation clearance the Airbus 330/340 series aircraft have a complicated system that rotates the landing gear bogie so that only the rear wheels of each 4 wheel bogie take the weight during rotation (thus increasing the ground clearance of the aircraft's belly considerably) and even then they often have tail strikes (meaning the rear fuselage strikes the ground and the aircraft requires heavy maintenance prior to further flight).

 

[Graeme]

I guess there were a few machines that dared to be different with vertical-down stabilisers.
The most stunning in my opinion was the Super Crusader...

 

[pbehn]

The Do 335 had a cruciform arrangement, but the propeller behind meant things would be different anyway.

 

[MiTasol]

I guess there were a few machines that dared to be different with vertical-down stabilisers. The most stunning in my opinion was the Super Crusader...View attachment 373642

Yes but you must remember that the lower fins were connected to the landing gear system so that they rotated to horizontal to provide ground clearance as the gear went down.

 

[MiTasol]

Agreed and the Do 335 also had a very tall landing gear to provide clearance during rotation.

I suspect that it had a few tail-strikes in its relatively short life. You will also note the proposed version with the rear piston engine replaced by a jet engine has no ventral fin and rudder.
View: https://www.youtube.com/watch?v=jK8ydLY5QHQ


Tail strikes tend to be more common during takeoff when the aircraft is heavy and landing gear fully loaded and there are many youtube videos of this occurring

AIRCRAFT NEAR MISS AND UNUSUAL EVENT COMPILATION #1 includes two landing tail strikes.

YouTube is a search result for tail strike during take off

 

[Graeme]

Yes but you must remember that the lower fins were connected to the landing gear system so that they rotated to horizontal to provide ground clearance as the gear went down.

Yes mate, I understand that and I'm not disputing what you said earlier about the undercart making life difficult for ventral stabiliser designers, I'm just thinking of aircraft that tried it.

XP-56 was another....

 

[Old Wizard]

 

[MiTasol]

Yes mate, I understand that and I'm not disputing what you said earlier about the undercart making life difficult for ventral stabiliser designers, I'm just thinking of aircraft that tried it.

XP-56 was another....

View attachment 373671

No sweat mate. I was not sure you knew that those surfaces moved for TO-Ldg.

I guess part of the XP-56 ventral also moved for TO&L because ground clearance for rotation and flare is zero but admit I have never been interested it that machine so I am only guessing. My vague memory of the shape had no upper vertical stabiliser and rudder so I must be confusing it with some other oddball

 

[Graeme]

ground clearance for rotation and flare is zero

It seems to just lift off without raising the nose?

 

[Graeme]

And should that ventral stabiliser get in the road of take-off - just sit on it....

 

[Zipper730]

Yes but you must remember that the lower fins were connected to the landing gear system so that they rotated to horizontal to provide ground clearance as the gear went down.

I thought the fins went down when the wings were lowered. The YF-12 had a system where the fins came down when the gear went up...

 

[XBe02Drvr]

I thought the fins went down when the wings were lowered. The YF-12 had a system where the fins came down when the gear went up...

Look at the picture. You thought wrong. Wing is down and ventral fins are still flat. Phantom did it better with downward sloped horizontal? stabilators and an active stability augmentation system. This also fixed to a certain extent the pitch-up problem inherited from its predecessor, the Voodoo (though I did see one heart-stopping over-rotation that I thought was going to result in a crash and burn right in front of me).
Cheers,
Wes