Thrust Line

[Creases67]

Cant understand this .High wing mounted engines I assume produce a thrust line that lies higher than the CG.
So how does the nose pitch up on opening thrust ?

[bucket_and_spade]

Bit more specific info needed. Most, but by no means all, conventional airliners have under-wing engines in pods i.e. the thrust line is below the CG. This results in a pitch up moment on thrust application (delayed as the engines take some seconds to spool up). If you have a thrust line above the CG (like in many bizjet designs) I imagine you'd have the opposite effect i.e. a pitch down, all else being equal.

If you're talking about piston-engined aircraft this, I suppose, could be complicated slightly by propwash over the wing/tail giving slightly unexpected (at first glance) pitch characteristics.

Do you have a specific example? Hope the above isn't too simplistic - not sure of your experience...?

B&S

[Capt Pit Bull]

It may not, depends on the type.

But of course you get subsequent effect. A higher mounted engine might initially give you a pitch down with an increase in thrust, but then your speed will increase which will tend to make you pitch up.

pb

[barit1]

Perhaps Creases67 refers to a high-wing a/c (Il-76, C-17, Ba146) with underwing engines. In that case the thrust line is quite close to the CG.

[Creases67]

Apologies for not framing the question correctly. What I meant was an airplane like IL 76 or even the C17 have high wings,Underling engines and most of the weight concentrated well below the thrust line. Now, is it not convention that the thrust line needs to be below the CG datum for obvious and universally consistent power on and idle response

[barit1]

I've been told that the MD-11 - and to lesser extent the DC-10 - experience quite a pitch-up in the event of a #2 engine failure. Easy to see why.

[Creases67]

I'm not sure the pitch up due to engine failure on an MD 11( low wing,underslung engines)would be a consequence a higher than CG thrust line. However I am willing to be corrected.

[barit1]

Two under, one over.

[Creases67]

Agree. So which engine do you refer to when failed would produce the pitch up ?

[DBate]

Agree. So which engine do you refer to when failed would produce the pitch up ?

Engines 1 and 3 (i.e. the wing engines) would produce the pitch up.

MD11... with an ENG 2 failure (the center engine) during takeoff (and after V1 for this example) you will actually have to push the yoke and be very gentle during rotation to avoid a tailstrike as the pitch up tendency from the wing engines is not any longer 'countered' by the center engine.

With all engines operating the MD11 is pretty stable in pitch when it comes to power changes.

So all in all, barit1s statement is correct.

Cheers,
DBate

[Creases67]

Thanks for the inputs. However my query remains. Is there no convention on where the thrust line should lie ? My understanding is that below the CG it will produce a pitch up on opening thrust. So what is the rationale of design if different aircraft have different "natural " feel ? Taking the examples quoted by the posters above,an MD 11 in raw data conditions would behave quite opposite to say an A330. Is that not strange ? Thanks

[john_tullamarine]

Design is a compromise to make a lot of conflicting task and systems needs all fit and work reasonably well together.

Unless you have a list of all the relevant considerations which went into the design configuration definition exercise, there is no way you can infer much at all with any real confidence ... some Types have a higher thrust line than others is about the best you can do.

Those folk with a design background (and we have quite a number of aerodynamicists and stressmen who choose to dally on this site) can offer reasonable guesses as to what might have gone into the design for a given Type.

[barit1]

Perhaps an extreme example of a

high thrust line