Ever since I was a little kid, I've been wondering about this odd feature I saw on some passenger/cargo jets: the "third" engine in the vertical tail fin. These links will show you a couple of DC-10s with it:
http://www.airliners.net/open.file/1165290/M/
http://www.airliners.net/open.file/1166001/M/

It looks like it was designed to make the plane crash! Maybe it tilts up and maybe that's an optical illusion, but either way, there's no way its direction of thrust is in line with the vehicle's center of gravity; that line would clearly pass above the COG. Normally, that means you'd be creating torque to push down in the front and up in the back.

I wasn't sure that it's actually an engine, and if it was one, then, since it's about the size of the other two combined, I thought maybe the explanation had to do with a combined effect from working together with the other engines. But that idea can't work, because there's another jet plane that has only high-mounted, up-tilted engines, no lower and/or non-tilted or down-tiled ones for them to be working together with: the A-10. And there's no question that they are indeed engines, or even whether or not they're tilted up (although the geometric misalignment of thrust direction and COG would still be there without the tilting anyway, just because they're mounted so high). Here are some A-10 pictures:
http://www.a-10.org/photos/p.asp?Fil...a10%2D01%2Ejpg
http://www.a-10.org/photos/p.asp?Fil...2D10form%2Ejpg

Obviously, these things do fly, and, at least in the A-10's case, there's a definite, unquestionable tilt that indicates that the designers were deliberately going out of their way to increase this seemingly-off-kilter effect, not decrease it. So there's got to be some reason. On top of that, the Wikipedia article on the A-10 gives a reason, and it's exactly the opposite of what they look like they would do: it says the plane would otherwise tend to pitch down, and the engines were supposedly tilted like that to counter that, which means the tilt is meant to introduce an upward pitching effect.

How in the world can that be the case? And if Wikipedia is wrong on this, then still, how in the world can these configurations work at all and not pitch the plane into the ground rapidly?

If those planes were actually rocket ships in space, yeah they would be out of kilter and end up flying in circles. But in an atmosphere it's an okay arrangement because the wings are generating lift and so everything is everything is stable. Now maybe it might be a little more efficent to put the tail engine lower down and in the actual passenger compartment. Now some people might complain that this would actually be a safety hazard, but it would have the advantage of sucking up any snakes that happened to be on the plane.

Quieter operation, and easier control correction in an engine out situation.
But......no fun when an engine disintegrates and sends turbine sections through all three hydraulic systems. This doesn't happen every day.
Dan

Concerning the DC-10s (and MD-11s), the engine in the tail is the same size (in fact, the same model engine) as those on the wings. What makes it look larger is the intake ductwork, at the end of which the engine is mounted. A careful check of the tail engine will show that its axis is parallel to those of the wing engines. This combined with the short moment length compensates for any rotation when combined with the wing mounted engines being below the C.G. The duct has to be tilted to guide air from the intake to the engine due to the tapering of the aft of the fuselage.

The DC-10 was originally in competition with the Lockheed L-1011 TriStar for a medium haul widebody aircraft. At the time the best solution was using three engines. These engines were too heavy for a 727-type tail mount, and the tail engine was too big to mount inside the fuselage as with the 727. So two were hung on the wings and the third was integrated into the tail external to the fuselage.

To me at least the L-1011 was much more aesthetically pleasing that the rather Buck Rogerish DC-10 solution.

Here's a cross-section drawing of a DC-10 that illustrates the above.

__________________
A person's name, or a mark representing it, as signed personally or by deputy, as in subscribing a letter or other document.

Hey, that's a great marketing idea for reselling all those old 727s!

__________________
A person's name, or a mark representing it, as signed personally or by deputy, as in subscribing a letter or other document.

That one was rough...amazing any people survived.

With an airplane, the wings tend to roll over forward if left to the own devices. This is the reason for the horizontal tail. It helps the plan keep it's butt down. Also, the direction the nose of the plane is pointing is rarelt he same direction it's actually flying. This is called the angle of attack, though it more specifically refers to the angle of the wings againt the ground (assuming the ground is perfectly flat).

In the case of the A-10*, the engines are mounted in line with the X axis, but the nozzles are angle up to redirect the thrust. Part of this was intended to mask some of the heat from the exhaust for protection against IR guided missiles. The horizontal stabilizer is supposed to get in the way of a lock. Since the A-10 is also very front heavy (Canon, ammo, titatium armored bathtub around the cockpit) it needs a little help in keeping the butt down. The additional downforse of an upward facing exhaust will help it out there.

*The A-10 info here comes mostly from the fact sheet of the modles I built 20 years ago, and may not be completely accurate, but that is how I remembered it.

__________________
I'm not evil.
An evil person would do the things I think up.

bzzzzt

The angle of attack is the angle between the aircraft X axis ("where the nose is pointing") and the flight velocity vector (the direction it's flying) in a vertical plane indeed, agreed on that.

BUT your reference to a perfect flat ground is only correct when the direction of flight is also horizontal! In ascending or descending flight, it is a wrong way to define angle of attack. After all, when descending with a flight path of 30 degrees down with your nose pointing 28 degrees down over a flat ground, the ange of attack is 2 degrees and not -28 degrees!

__________________
To the regular visitor of internet bulletin boards it is clear that it's an excellent idea your parents get to choose your real name.

And here I thought I went into enough pointless qualifying in my explanation to cover myself... D'oh.

Yeah, I was assuming level flight.

__________________
I'm not evil.
An evil person would do the things I think up.

Tog, look at the A-10 pictures I linked. Your description of how & why this would work is intuitive, but the opposite of the way the plane is designed. The end of the engine that points up is the front, not the back, so the exhaust is angled 9° down, not up.

OK, then why did they make wings that would create so much lift that they needed to be compensated for by engines that push the nose down? Does more lift decrease the stall speed and let the plane cruise slower but still stay in the air? (That would make sense for the A-10 at least, if not the passenger plane, but others have provided answers about the passenger plane's third engine simply balancing the other two, as I suspected all along before the issue of the A-10 came up.)

Like a previous poster noted, when the DC-10 and L-1011 were designed, it was determined that two engines weren't enough and four engines were too many for a plane of that size (based on engines that were available at the time). There are only so many ways you can arrange three jet engines on a plane. The DC-10 put the third engine above the fuselage and the L-1011 put the third engine in the rear of the fuselage like the Boeing 727.

Every design decision has trade-offs. For the DC-10, putting the engine over the fuselage has the advantage of a simplier and lighter installation. There is no need for "S" ducts to take in air above the fuselage (like the 727 and L-1011) and route the air to the engine. This is probably more efficient because you eliminate drag internal to the duct. It should also be quieter inside the cabin and have more useable interior space than the 727/L-1011 arrangement. However, putting the engine up there also has drawbacks. These include the possibility of causing pitch changes when you change power, the relative difficulty of working on engines mounted that high, and the need to build your vertical stabilizer strong enough to withstand the engine loads (heavier).

The photos you provided appear to show the center engine with the intake higher than the exhaust. Whether this is true or an optical illusion, you can be pretty confident that the engineers aligned the engine to reduce cruising drag and to reduce the effects of power changes on pitch. Engineers have a little trick they use to reduce the effects of power changes on pitch. On many designs where the thrust line is offset from the airplane centerline, they'll angle the engine. On a design like the DC-10, it would seem reasonable that an increase in thrust would cause a nose down moment, so the engine would be mounted with the engine exhaust higher than the intake. That way, an increase in thrust would cause a nose up moment to offset the nose down moment.

I don't know the reason why the center engine appears to be mounted with the exhaust lower than the intake. The wing mounted engines are below the aircraft centerline, so an increase in thrust there would likely cause a nose up moment. Perhaps they wanted the center engine's nose down forces to offset the nose up forces from the wing mounted engines. Perhaps they wanted any sudden increase in thrust at low speed (such as during a missed approach) to push the nose down, reducing the chances of a stall. Regardless, the horizontal tail surfaces on the DC-10 are pretty big to handle offset thrust.

Typical airplanes actually have the horizontal tail generating negative lift, or downforce. The reason is stability. When you disturb the airplane in the pitch axis, it tends to return to equilibrium. For instance, if you cause the airplane to pitch up, the horizontal tail generates less downforce, causing the nose to drop. Makes it easy to fly and control.

The downside of this is extra drag. Lift comes with what's called "induced drag". Since you're generating lift in two contradictory directions, you get more drag.

You can fly unstable airplanes, especially with the help of computers, but most are stable.

__________________
Cum catapultae proscribeantur tum soli proscripti catapultas habeant.

The engines I'm wondering about on the A-10 and DC-10 look like they're positioned (and, in the A-10's case, angled 9°) to have the opposite effect: tail pushing up instead of tail pushing down...

Yep, Those pilots are a credit to the industry. Never quit. Keep flying the aircraft. Think. And be creative if you have to.
And it illustrates the power of prayer.
I still maintain that you are safer in the air than on the roads.
Best regards, Dan

Don't forget Dennis Fitch, who was a passenger on that flight.
Funny, that word never appears in Capt. Al Haynes' talk about the accident. Plus it certainly didn't help those 111 people who died as a result of what happened.

__________________
A person's name, or a mark representing it, as signed personally or by deputy, as in subscribing a letter or other document.

If they quit, they die. That's pretty good motivation.

Along similar lines, sometimes you hear someone credit a pilot for steering to avoid ground obstacles, like houses, trailer parks and gas tanks. They say the pilot was concerned for those on the ground and steered to an empty area. Now I don't want to discredit any pilots, but the reason they steer for an empty area is to give themselves the best chance of survival. They may give those on the ground some thought, but it's their own butt they are really trying to save.

True, but I have to add a nuance wheerre pilots of crashing jet fighters stayed longer in their very ill bird (burning, tumbling, what have you) in order to steer away from cities/people/etc and guide the plane to open field, the sea, and the like.

I'm talking about cases where the pilot already knows he'll have to eject at one time or another, and quite soon indeed. There are also pilots staying with their craft and landing it when ejection was already justified but staying in the craft was not that deadly yet, but as ejection isn't a real pleasure, they tend to try other options first .

__________________
To the regular visitor of internet bulletin boards it is clear that it's an excellent idea your parents get to choose your real name.