Turning tendencies on jet engines?

[shdw]

I think you are correct about the net torque effect on the engine being zero, but it's not because the air comes out straight.

If you agree the net torque effect is zero than you'd be silly to think it not measurable (true, however, that it is not because of) by the air coming straight out of the jet. It would be analogous to agreeing lift can be measured by the air pushed down by the wing, but, at the same time, saying the net downwash does not match the net lift.

What you describe in both your case, and my analogy, is Newton's law. After the air has past through the system, the net effect on the air leaving that system (the engine or the wing) explains the net effect of the system. Otherwise the equal and opposite reaction Newton speaks of would not hold true.

I could certainly be missing something, but it seems to me you said the same thing Fish said, except you used different wording.

 

[inigo88]

No I think you're right, late night reading comprehension fail on my part. Sorry fish.

 

[shdw]

No I think you're right, late night reading comprehension fail on my part. Sorry fish.

Happens to the best of us. How's the engineering degree coming along?

Seagull recently recommended the Flightwise Volume 1 & 2 books to me. I purchased them and have nearly finished the first volume. If you're interested in the practical link in the theory/analytical side you're studying, I'd highly recommend these books. The authors goal is to provide the conceptual side that's often scarce in engineering books whose primary focus is on the analytical (the math) side of the spectrum.

Here is a page from the Preface to give you some background on the text directly from the Author:

Through my experience of teaching the aerodynamic principles of flight over a number of years, I have become very aware of the need for a book based on the above principles. Flightwise has therefore been written in the hope that it will fill this gape in aeronautical literature.

On one side of this gap there is the essential and mainly excellent range of aerodynamics text books, some targeted at engineering undergraduates, some at graduates, some of the broad coverage and some highly specialized. These books are almost invariably very heavily weighted towards the analytical aspects of the subject, and are hence inevitably packed full of mathematics, much of which is very advanced by any standards. Some also offer substantial textual explanations of the principles being dealt with, but these explanations may not appear to the reader to stand alone but rather to be an integral part of the related analytical procedure. Whilst this is no doubt intellectually justifiable, my experience of students is that they tend to gloss over any difficulties that they may have in understanding the principles properly, so long as they can master the tricks needed to produce the required answers. Such an approach is perhaps well geared to our existing higher education teaching and examination system. Sadly the student is under great pressure to pass examinations, where questions tend to be mainly mathematical in content since such are easier to set and to mark 'objectively' than questions designed to reveal the student's understanding of the material.

Two specific examples from my experience vividly illustrate the problem resulting from such an approach. At a lecture more than halfway through an advanced course being given to well qualified and motivated graduates, a gasp of exclamation spontaneously arose from almost the entire class, when for the first time they suddenly realised what a boundary layer was, and that it had physical significance for an aircraft. The subject of the course was Boundary Layers!

I added emphasis in the final paragraph.

 

[inigo88]

I'm still pluggin along, thanks for asking. I'm actually in Aerodynamics this quarter... finally! Getting little glimpses of the light at the end of the tunnel (even though I have over a year left). Finally purchased John D. Anderson, Jr's "Fundamentals of Aerodynamics" and the book is getting quite the workout from class. We're reviewing the conservation laws from fluid mechanics, specifically the Momentum Integral Equation, and lets just say it's not my favorite. Believe it or not I was never a math guy growing up and always much preferred the conceptual approach, so I will definitely have to check out Flightwise. Nowadays, this is what I'm doing.

(Anderson, John D, "Fundamentals of Aerodynamics," 6th edition, pp. 133 <-- since I think I remember you mentioning you have this book.)

This thread is a great example of why CFIs have to be able to explain the same thing in multiple different ways - there are just too many learning styles out there (although you wouldn't/shouldn't be going into THIS much detail with a student). I've got Momentum Integral and control volumes on the brain, and fish is talking about the exhaust coming out straight resulting in a zero net torque. His control volume is the air around the engine. I'm thinking all the opposite reaction forces are happening inside the engine already, so my control volume is limited to the engine itself. We're talking about the exact same thing using two different frames of reference. Whoops!

At a lecture more than halfway through an advanced course being given to well qualified and motivated graduates, a gasp of exclamation spontaneously arose from almost the entire class, when for the first time they suddenly realised what a boundary layer was, and that it had physical significance for an aircraft.

Ah c'mon people that's ridiculous!

 

[fish314]

I'm still pluggin along, thanks for asking. I'm actually in Aerodynamics this quarter... finally! Getting little glimpses of the light at the end of the tunnel (even though I have over a year left). Finally purchased John D. Anderson, Jr's "Fundamentals of Aerodynamics" and the book is getting quite the workout from class. We're reviewing the conservation laws from fluid mechanics, specifically the Momentum Integral Equation, and lets just say it's not my favorite. Believe it or not I was never a math guy growing up and always much preferred the conceptual approach, so I will definitely have to check out Flightwise. Nowadays, this is what I'm doing.

(Anderson, John D, "Fundamentals of Aerodynamics," 6th edition, pp. 133 <-- since I think I remember you mentioning you have this book.)

This thread is a great example of why CFIs have to be able to explain the same thing in multiple different ways - there are just too many learning styles out there (although you wouldn't/shouldn't be going into THIS much detail with a student). I've got Momentum Integral and control volumes on the brain, and fish is talking about the exhaust coming out straight resulting in a zero net torque. His control volume is the air around the engine. I'm thinking all the opposite reaction forces are happening inside the engine already, so my control volume is limited to the engine itself. We're talking about the exact same thing using two different frames of reference. Whoops!

No worries Inigo. I suffered through those same courses... but 15-16 years ago. I HAVE to speak in conceptual generalizations only. My math and theoretical background on this stuff is so rusty that when I try to look at Navier-Stokes equations and the like, you can hear my brain squeaking! I used to be good at that stuff, but it's been a very very long time. So don't look to my posts if you're looking for too much in the way of theoretical rigor.

 

[Autothrust Blue]

Ah c'mon people that's ridiculous!

Even us dumb pilots know that!

 

[inigo88]

Thanks fish314, at least you have the 15 year excuse. I'm in the classes now and I still don't know what the heck is going on (I just pretend to ).

 

[shdw]

Thanks fish314, at least you have the 15 year excuse. I'm in the classes now and I still don't know what the heck is going on (I just pretend to ).

I'm telling you bro, that book will clear up a whole lot. And for a smart guy like you it will be like bathroom reading. I got both volumes for 50 bucks on Amazon. PM me an e-mail and I'll send you a few pages if you're interested.

 

[fish314]

Thanks fish314, at least you have the 15 year excuse. I'm in the classes now and I still don't know what the heck is going on (I just pretend to ).

Don't worry inigo.... someday you won't remember any of that expensive edjumacation either. Beer... in large quantities... helps!

 

[Blackhawk]

The ATR goes bananas when you try flying with rudder.

I would beg to differ. Granted, not much rudder is needed, but FAs could tell when I was flying and appreciated the fact that I would keep the ball centered instead of letting the airplane fishtail in the turns (this was a -72). They could really feel it in the back when a pilot would fly with his feet on the floor.

Now transitioning to a turbofan I will never forget my first V1 cut. It was myself and another turboprop red neck. The instructor hit the freeze button and exclaimed, "What the %^#* are you turbo trash pilots doing? This is a jet. There is no need to flail around at a V1 cut. Fly the airplane to 700' and then run the QRH!"

 

[fish314]

I would beg to differ. Granted, not much rudder is needed, but FAs could tell when I was flying and appreciated the fact that I would keep the ball centered instead of letting the airplane fishtail in the turns (this was a -72). They could really feel it in the back when a pilot would fly with his feet on the floor.

Now transitioning to a turbofan I will never forget my first V1 cut. It was myself and another turboprop red neck. The instructor hit the freeze button and exclaimed, "What the %^#* are you turbo trash pilots doing? This is a jet. There is no need to flail around at a V1 cut. Fly the airplane to 700' and then run the QRH!"

Hey Blackhawk, was this an aileron airplane, or did it have spoilers (or some combination of both)? Because if it was ailerons, and in particular aileron ONLY, while there probably isn't any yaw from the engines there would still definitely be adverse yaw from the ailerons, at least while they are deflected. Now, if it's spoilers only, there isn't really any adverse yaw from those. If it's some combination of the two, then it probably depends on whether the spoiler effect overpowers the aileron effect or vice-versa.

As for the asymmetric engine problem, my jet (KC-135, basically a 707) definitely requires rudder when you lose an engine. But in that case, the yaw is easy to identify: 2 engines on 1 wing operating, and only 1 on the other.

 

[Itchy]

I'm still pluggin along, thanks for asking. I'm actually in Aerodynamics this quarter... finally! Getting little glimpses of the light at the end of the tunnel (even though I have over a year left). Finally purchased John D. Anderson, Jr's "Fundamentals of Aerodynamics" and the book is getting quite the workout from class. We're reviewing the conservation laws from fluid mechanics, specifically the Momentum Integral Equation, and lets just say it's not my favorite. Believe it or not I was never a math guy growing up and always much preferred the conceptual approach, so I will definitely have to check out Flightwise. Nowadays, this is what I'm doing.

OMG. Graduated in 86, and about a year ago, stumbled across a lot of my old notes. I used to eat this stuff up, and now it might as well be Arabic. Went to the mushy side of my brain at least 2 decades ago.

 

[Cessnaflyer]

I would beg to differ. Granted, not much rudder is needed, but FAs could tell when I was flying and appreciated the fact that I would keep the ball centered instead of letting the airplane fishtail in the turns (this was a -72). They could really feel it in the back when a pilot would fly with his feet on the floor.

Our YD always kicks off for some reason and that is why it goes bananas.

 

[inigo88]

OMG. Graduated in 86, and about a year ago, stumbled across a lot of my old notes. I used to eat this stuff up, and now it might as well be Arabic. Went to the mushy side of my brain at least 2 decades ago.

It does kinda look like Arabic huh!

 

[Blackhawk]

Hey Blackhawk, was this an aileron airplane, or did it have spoilers (or some combination of both)? Because if it was ailerons, and in particular aileron ONLY, while there probably isn't any yaw from the engines there would still definitely be adverse yaw from the ailerons, at least while they are deflected. Now, if it's spoilers only, there isn't really any adverse yaw from those. If it's some combination of the two, then it probably depends on whether the spoiler effect overpowers the aileron effect or vice-versa.

As for the asymmetric engine problem, my jet (KC-135, basically a 707) definitely requires rudder when you lose an engine. But in that case, the yaw is easy to identify: 2 engines on 1 wing operating, and only 1 on the other.

These had spoilers but they where not always that effective. Mind you, we are not talking "stepping on the rudders", but very light pressure to keep the ball centered.

 

[Blackhawk]

Our YD always kicks off for some reason and that is why it goes bananas.

It takes very light pressure- almost thinking rudder- on the 72.