View Full Version : Pitch up with power


Dick Whittingham
13th May 2003, 03:12
Here's another JAR question!

Low thrust line, and with added thrust the aircraft pitches nose up. But does it pitch up because the thrust line is below the C of G or because it is below the aircraft drag line?

This has taken up most of one day's worth of argument at Bristol, and I'm still not sure that we have an answer.

Dick W



LEM
13th May 2003, 03:50
In my opinion, for both factors, but above all because of the CG.
Regards

Notso Fantastic
13th May 2003, 08:14
I think it's a question of not looking too deeply for purely academic problems. Newton's Laws? If you apply a force to an object, it will move about it's C of G unless otherwise restricted? So a low thrust line increase will act around the C of G, but it certainly could be argued the centre of drag will be a factor. Does anyone really know where the centre of drag is? Is it important to anyone (within reason)?

bookworm
13th May 2003, 16:37
It depends in part on the reaction to the increase of thrust.

In the very short term, the aircraft accelerates. That means that, in the frame of reference of the aircraft, the "balancing force" is an inertial force at the centre of mass.

As the aircraft's speed increases, the drag increases, and the balancing force is the increase in drag at the centre of drag.

Alternatively, if the aircraft maintains the same speed and climbs, the balancing force is a greater aftward component of weight, at the centre of mass.

So both the centre of mass and the centre of drag ("drag line") may be relevant.

Notso Fantastic
13th May 2003, 18:58
Worm, you're looking for academic problems! Who says you are accelerating? You might be going around with barely a speed change, but pitching up. The question is why does it pitch up ? Answer - because the thrust line is low and below the c of g. The force isn't applied at the centre of mass, it's below it. There are other balancing forces to compensate (hopefully)- ie the pilot frantically pushing the stick forward to apply an upload on the tail.
I've done advanced physics, but this makes no sense:
"Alternatively, if the aircraft maintains the same speed and climbs, the balancing force is a greater aftward component of weight, at the centre of mass."
The morale again is: don't look too deeply for academic problems!

bookworm
13th May 2003, 19:49
I've done advanced physics, but this makes no sense:
"Alternatively, if the aircraft maintains the same speed and climbs, the balancing force is a greater aftward component of weight, at the centre of mass."

Look at difference in the equilibrium of forces before and after the thrust increase. If the aircraft has maintained the same speed and is climbing after the thrust increase, the major differences between before and after are:

1) The difference between thrust before and after due to the thrust increase
2) The difference in aftward component of weight before and after due to the change in flight path

The differential forces will be of approximately equal magnitude. If those two forces are offset so as to produce a couple, there will be a pitching tendency. in that case, the centre of drag is irrelevant.

It's not a question of looking for questions, the answer lies in providing answers. How's your morale now? :)

Checkboard
13th May 2003, 19:54
In the case you are talking about, Dick (say, a high wing Cessna), it is due more to the thrust/drag couple. Acceleration affects are only transitory - once the acceleration is compplete, the aircraft still wants to pitch up, until you have re-set the trim.

If you want to have a really fun discussion, draw a picture of a Piper Warrior (wing, and most of the airframe below the engine's thrust line), ask people to draw in the thrust and drag forces (i.e. thrust above drag) ... and then ask them why it still pitches up under increased thrust. I did three years instructing on Warrors, and this was always a fun question to ask!

Notso Fantastic for one who has done advanced physics, I would think that you wouldn't need a gentle reminder that any change in velocity (e.g. a pitch up without a speed change) is an acceleration ;)

Genghis the Engineer
13th May 2003, 20:02
When you apply a force to an aircraft it'll act initially about the CG. If the force vector doesn't go through the CG, then you have a moment about the CG, this will cause rotation of the aircraft about some point near to the CG - aerodynamic effects will generally stop it being spot on the CG.

So, at a first approximation, an aircraft with low thrustline and high CG will pitch up with power, and an aircraft with high thrustline and low CG will pitch down with power.

However, this isn't absolutely universal, because the horizontal stabiliser will usually to some extent be in the propwash. Hence increased power not only increases thrust, but increases local airspeed over some or all of the horizontal stabiliser (or for that matter over the centre section of the wings). So sometimes you'll see unexpected effects for that reason.

But, as a simplistic (i.e. JAR exam) explanation, I'd say CG will probably suffice as the answer.

G

N.B. Look at the engine mounting angle too. A low engine can still have a high thrustline and vice-versa - it's where the thrust vector passes relative to the vertical CG that counts, not the actual engine position.

Notso Fantastic
13th May 2003, 21:00
The morale is getting very low! Forgotten what the moral is!
All I know is if you go around on a 737, particularly a -200, you'd better remember to start trimming the elevators nose down for at least 5 seconds otherwise as the engines spool up, you're going to be pushing mighty strong! Was the question really intended to go any deeper than this?

LEM
13th May 2003, 21:20
and to add something I remind the 737 classic engines are tilted about 5 degrees up, to add more intake clearence to the already poor one from the ground and also to get a small vector lifting the airplane on takeoff!

Dick Whittingham
13th May 2003, 23:23
I,m disappearing for a week, so this is an interim thought. If you have an aircraft with a low thrust line that passes through the CG and a high drag line, then in steady flight there is a nose up couple from thrust/drag. In steady flight this is exactly balanced by a nose down couple, which might be lift/weight, the tail or both or either. If you then increase thrust there will be no additional coulple about the CG, but surely there will be a larger thrust/drag couple now affecting the aircraft, and if this is not balanced the nose will rise.

The reason I brought this up is that the JAR question offered both thrust line lower than the CG and lower than the drag line as options, and we suspect the question is invalid. If it is invalid, someone may have failed his exam on a duff question, and we need to straighten out the question bank.

Of course, natural longitudinal stability will bring the nose up as speed increases, but that doesn't figure in this question.

Much appreciate your help, back in seven days.

Dick

Keith.Williams.
14th May 2003, 02:52
The fundamental problem with this string (and almost every other string that deals with JAR examination questions) is that we do not know the exact wording of the original question. If for example it had included something like "what causes an aircraft to pitch up immediately after increasing thrust", or "pitch up as airspeed increases following an increase in thrust", then it would probably have been possible to select the correct answer.

If the pitch up is an immediate effect then it is probably caused by the thrust line being below the C of G, rather than it being below the drag line. To test this theory we might imagine how the effect would change if we raised the thrust line to coincide with the C of G.

If we assume that the aircraft is correctly trimmed prior to the test, the nose up moments will be balanced by the nose down moments. With the thrust line below the C of G, increasing thrust will increase the nose up moments. This will cause the nose to pitch up. The changing tailplane angle of attack may (or may not) eventually rebalance the pitching moments, such that the aircraft stabilises at a new higher pitch attitude.

If we now raise the thrust line so that it passes through the C of G, retrim the aircraft, then repeat the test, increasing the thrust will cause no immediate change in nose up moments. The aircraft pitch will therefore (initially) remain unchanged.

We can take these tests a little bit further by rearranging things so that the drag is above the thrust and the thrust is above the C of G. If we retrim the aircraft then increase thrust, the immediate effect will be a nose down pitch. This is because the nose down pitching moment caused by tailpalne trim force and increased thrust, will be greater than the nose up moment caused by drag.

Finally if we rearange things one more time so that thrust is above drag and drag is above C of G, then retrim and increase thrust, we will again get nose down pitching.

So the realtive heights of thrust and drag will not affect the immedaiate result. But the aircraft will pitch nose up if thrust is lower than C of G.

Getting back to the original scenario, if the increased thrust then causes airspeed to increase, the increasing drag above the C of G will cause more pitch up. The changing airflow over the tailplane and various other factor will of course also contribute to (and may even reverse) the overall effect. But if they are not included in the question, they should not be considerd.

The reall worry here is whether or not the examiners have a good enough grasp of their subjects to understand the possible interpretations of their questions. Much of the feedback from students suggests that they do not. This sounds like a case of an examiner experiencing a sudden flash of inspiration and thinking "That's really clever of me. Let's see if the students can get it".