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Triebwerk
Topic Author
Posts: 91
Joined: Tue Sep 02, 2008 12:29 am

### If The No. 2 Engine On A DC-10 Fires Up...

Which way does the nose tilt? (Assume the plane is moving well below rotation speed.

For some reason, I feel as if the plane should tilt backward if the tail-mounted engine revs up. My physics teacher from last year disagrees, saying--due to COG implications--the nose should actually tilt forward. What are your thoughts?

goboeing
Posts: 2566
Joined: Mon Jun 05, 2000 5:31 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

The thing actually appears to be mounted with the front higher than the back so as to shoot the exhaust flow slightly down below the horizon.

On the ground, that would push the nose down.

Starlionblue
Posts: 19816
Joined: Fri Feb 27, 2004 9:54 pm

### RE: If The No. 2 Engine On A DC-10 Fires Up...

AFAIK Your physics teacher is correct. The engine is well above the "mass centerline" and more thrust on #2 will push the nose down. Conversely, more thrust on #1 & #3 will push the nose up.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo

acNDTTech
Posts: 173
Joined: Fri Jul 25, 2008 7:15 pm

### RE: If The No. 2 Engine On A DC-10 Fires Up...

But remember, the elevators will counter act the thrust.....there are some laws that you just can't break, and the laws of physics are some of them.

speedracer1407
Posts: 330
Joined: Fri Dec 24, 2004 4:19 pm

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting Triebwerk (Thread starter):For some reason, I feel as if the plane should tilt backward if the tail-mounted engine revs up.

You may be intuiting the rear-end "squat" that happens on most cars under acceleration. Excepting certain suspension geometries that reduce or eliminate this effect, in pretty much any wheel-driven vehicle with a single front and rear axle, weight transfers rearward during acceleration and compresses the rear suspension. I'm no engineer, but I suspect that this is helped by the fact that the point of thrust, the tire contact patch, is below the CoG.

A jet engine acts on the air directly behind the exhaust nozzle. To exaggerate the nose-down effect a DC-10's #2 engine might have at high power settings, imagine that the tail fin is 100 feet tall and the engine is mounted at the very top. Now you have 50-60 some thousand pounds of force acting on a 100-foot-long lever, doing it's best to grind the nose right into the ground.
Dassault Mercure: the plane that has Boeing and Airbus shaking in their boots.

zappbrannigan
Posts: 231
Joined: Wed Oct 01, 2008 7:41 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

Spooling up that particular engine would create a nose-down pitching moment (note: this doesn't actually mean you'd notice any pitch on the ground - but the force would be there). This doesn't have anything to do with the fact the engine is slightly tilted (although this would increase the pitching force slightly). The above post explains it well.

This is shown in some tail-mounted twin-jets which are "self-flaring" - when the thrust is removed during the flare, this creates a pitch-up moment. This is the reverse of what you're talking about.

This is also the reason most light single-engine aircraft are designed with the thrust line below the drag line - so if the engine fails, the aircraft pitches down (which is what you want).

 Quoting AcNDTTech (Reply 3):But remember, the elevators will counter act the thrust

The authority of the elevators is dependant on the relative airflow over them. If there's no airflow, they do nothing. This question related to spooling up #2 well below Vr - or even stationary.

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting Speedracer1407 (Reply 4):I'm no engineer, but I suspect that this is helped by the fact that the point of thrust, the tire contact patch, is below the CoG.

It's not just helped by it, it's most of the reason for it to occur. A secondary effect (on rear wheel drive cars) is the countertorque from the wheels, but that's a lot smaller. If you do a moment balance around the CG you see the forward force from the tires applied below the CG, which causes a "nose" up pitching moment. The counter to this torque is weight shift on to the rear tires, which compresses the rear suspension and causes the squat.

The same thing is going on with the trijet, only the geometry is "upside down"...the engine is pushing above the CG, causing a nose-down moment. This is countered by weight shift on to the nose gear (at low speeds) or increased horizontal stab download (at high speed).

Tom.

fxra
Posts: 600
Joined: Fri Jul 16, 1999 1:03 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

Theory in practice: At a former employer we were operating a 2 engine ferry on an MD-11 with the number 2 inop. Part of the procedure for 2 engine ferry flights on an MD-11 is for the center main gear to be retracted at all times. The crew and/or loadmasters did not observe the CG limits for the center main gear retracted, and the aircraft departed with the CG in the aft section of the normal operating envelope (outside the center main retracted envelope). However, on taxi out to the runway (in ANC, not a short drive for us) the crew had no issues with the aircraft being light on the nose or even a report of lots of strut showing in the gate. Once in position on the runway, the crew advanced the 1 and 3 throttles for departure, and the airplane promptly nosed up and sat right down, much to the surprise of the Alaska Air crew next in line for departure. The crew pulled back the power and the nose settled right back onto the ground. The airplane taxied off the runway and returned to the ramp, where i think the crew swiftly changed pants.

Had either the the center main gear been extended or power being supplied from the number 2 engine, there likely would have been no issue for take off. I don't think they've done a engine ferry since that attempt. As a result, we changed the number 2 (winter in ANC without a hanger) and then ferried the airplane, non pressurized at 10,000 ft form ANC to ATL to repair the pressure dome (again, third time on said tail number its been sat down).
Visualize Whirled Peas

DocLightning
Posts: 21779
Joined: Wed Nov 16, 2005 8:51 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting AcNDTTech (Reply 3):But remember, the elevators will counter act the thrust.....there are some laws that you just can't break, and the laws of physics are some of them.

Unless you're Michael Jordan.

 Quoting FXRA (Reply 7):art of the procedure for 2 engine ferry flights on an MD-11 is for the center main gear to be retracted at all times.

Why is that?

So is your story why the thrust line on the #2 engine on the DC-10/MD-11 points downwards? To help overcome the tendency of the two engines on the wings to nose the plane up?
-Doc Lightning-

"The sky calls to us. If we do not destroy ourselves, we will one day venture to the stars."
-Carl Sagan

wilco737
Posts: 7275
Joined: Sun Jun 20, 2004 12:21 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting DocLightning (Reply 8):So is your story why the thrust line on the #2 engine on the DC-10/MD-11 points downwards? To help overcome the tendency of the two engines on the wings to nose the plane up?

Yes. It is slightly tilted. If you apply thrust, the MD11 is stable in the pitch. No pitch up or pitch down momentum.
During an eng#2 failure while still on the runway the nose like to come up as well. So you need to push the yoke a bit.

WILCO737 (MD11F)

fxra
Posts: 600
Joined: Fri Jul 16, 1999 1:03 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting DocLightning (Reply 8):Why is that? So is your story why the thrust line on the #2 engine on the DC-10/MD-11 points downwards? To help overcome the tendency of the two engines on the wings to nose the plane up?

I'm guessing, though never officially explained, to reduce drag and possibly the load on the hydraulic systems for gear retraction. Since the max take off wt on 2 engines is well below the structural weight requiring the center main, it seems logical you'd want less stuff hanging off the airplane.
Visualize Whirled Peas

CosmicCruiser
Posts: 2397
Joined: Tue Feb 22, 2005 3:01 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

If a wing engine fails during a high gross
weight takeoff, a greater control wheel force is
required to achieve normal rotation rates. Failure
of the #2 engine may result in an early/
over rotation.

just to confirm

acNDTTech
Posts: 173
Joined: Fri Jul 25, 2008 7:15 pm

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting Zappbrannigan (Reply 5):This question related to spooling up #2 well below Vr - or even stationary.

I wasn't looking at it that way, I thought they were talking about the plane in flight......then I noticed, "well below rotation speed."

 Quoting DocLightning (Reply 8):Unless you're Michael Jordan

Yes, but that's only because of his tongue.

Viscount724
Posts: 19316
Joined: Thu Oct 12, 2006 7:32 pm

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting FXRA (Reply 10):Quoting DocLightning (Reply 8): Why is that? So is your story why the thrust line on the #2 engine on the DC-10/MD-11 points downwards? To help overcome the tendency of the two engines on the wings to nose the plane up? I'm guessing, though never officially explained, to reduce drag and possibly the load on the hydraulic systems for gear retraction. Since the max take off wt on 2 engines is well below the structural weight requiring the center main, it seems logical you'd want less stuff hanging off the airplane

NTSB report on that ANC MD-11F (World Airways) incident in 2004.
http://www.ntsb.gov/ntsb/brief2.asp?...020X01664&ntsbno=ANC05IA002&akey=1

Northwest727
Posts: 379
Joined: Sat Jul 30, 2005 10:38 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

Pretty much like everyone else said, the nose would pitch down. As I am currently in a college-physics class, we just covered center of gravity, forces, etc. about a month ago.

Why it was designed like that, I don't know, I'll leave that one up to the engineers and smart people (who have already explained it)

 Quoting DocLightning (Reply 8): Unless you're Michael Jordan.

What ever happened to LeBron James?

[Edited 2008-11-12 18:39:10]

Triebwerk
Topic Author
Posts: 91
Joined: Tue Sep 02, 2008 12:29 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

Thanks for your in-depth responses! I e-mailed this thread to my physics teacher; I'm sure he'll enjoy reading the responses. (Considering you proved him right.

tdscanuck
Posts: 8572
Joined: Wed Jan 11, 2006 7:25 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

 Quoting Northwest727 (Reply 14):Why it was designed like that, I don't know, I'll leave that one up to the engineers and smart people (who have already explained it)

It's not really a design choice...if you have the engine up above the CG, the only way to not have it produce a nose-down pitching moment is to tip the back upwards to provide a countering moment...that means the engine is actually pushing partly down, increasing the load on the wings, increasing drag, etc., etc. It also means you need a curved inlet because your engine won't be parallel to the local airflow.

The S-duct style (727, L-1011, etc.) gets round this issue by having the thrust line be really close to the CG, but you inherit some other problems with it. Once you've made your design choice to put the third engine in the tail, you're basically stuck with the nose-down pitching moment.

Tom.

Northwest727
Posts: 379
Joined: Sat Jul 30, 2005 10:38 am

### RE: If The No. 2 Engine On A DC-10 Fires Up...

Now that you explain it, it makes much more sense. Thanks.

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