If you know anything about aerodynamics.....

[drunkenbeagle]

In a 60 degree (2G) bank, the load factor doubles. Agree? The aircraft isn't actually any heavier, but for all practical purposes, the wing IS supportng twice the weight as in straight & level flight.

The wing in a 2G bank has two accelerations acting upon it. Gravity is one. The turn is the other. Acceleration, as we know from kindergarten, is the rate of change of a vector quantity (velocity in this case). It is the horizontal component of lift that is increasing the load factor. The turn is an acceleration, because it is changing direction. Centripetal force is a better term than load factor. And it is not directly related to bank, it is equal to the mass of the aircraft times velocity squared divided by the radius of the turn. (F = mv^2/r).

That is why it is called "load factor" and not weight. I blame the stupid US system for defining the pound as both the unit for force and for mass. Weight is by definition the force caused by acceleration due to gravity. The term can not be used to describe any other forces.

 

[tgrayson]

The fact that the HStab is an airfoil makes the entire argument. Otherwise the engineers would have just put a heavy piece of lead back there.

Actually, lead would work fine to keep the aircraft in trim, but you'd be limited to one AoA unless you had a way to slide the lead weight back and forth. The downside is that the weight would move the CG behind the Aerodynamic Center, causing the aircraft to become unstable.

 

[Minuteman]

Good thing gravity is a constant.

Well, technically speaking. (allow me to indulge in some of that "negative tranfer of learning"...)

The "apparent gravity" can vary for several reasons: its Latitude, Altitude, Speed (centrifugal), and Direction of Travel (Coriolis effect) in "level, unaccelerated flight."

Gravitational force varies with the inverse square of the distance between masses. The Earth is an ellipsoid, so gravity will vary with Latitude, as well as Altitude. At FL350, the gravitational force will vary by about 0.53% from Equator (0.9940 g) to Pole (0.9993 g).

There's the centrifugal force that is a result of flying around the Earth. At FL350 and M=0.85, you're averaging 827 ft/s. That's good for lopping off about 0.0010 g. Because the radius of the earth is less at the pole, the reudction is slightly more, but not appreciably.

And the Coriolis effect, which has the greatest influence when the aircraft is traveling at the equator, will be good for removing another 0.0036g under these conditions (flying east).

For the numbers I used (FL350, M=0.85, Latitude=0°, Track=090°), the apparent weight of an airplane could be 1.07% less than the same mass-times-g on the ground.

For a 600,000 lb. 747, that is 6,400 lbs the wings don't have to carry, which could translate into a similar percentage-reduction in drag and fuel burn.

(Hurrr!)

 

[CoffeeIcePapers]

Figure 5-10 in the PHAK shows a downward aerodynamic force at the tail.

 

[CFIT99]

EVERYONE HAS VERY GOOD POINTS...some are still missing it.

THERE'S THE RIGHT WAY
THE WRONG WAY
AND THE FAA WAY

Learn it how they want you to learn it then move on.

I believe the professor has a point but ultimately is wrong

 

[sdfcvoh]

I should have added "unmovable" and "in every attitude of flight..."

But I wanted to give you a shot at my response.

Actually, lead would work fine to keep the aircraft in trim, but you'd be limited to one AoA unless you had a way to slide the lead weight back and forth. The downside is that the weight would move the CG behind the Aerodynamic Center, causing the aircraft to become unstable.

 

[CoffeeIcePapers]

EVERYONE HAS VERY GOOD POINTS...some are still missing it.

THERE'S THE RIGHT WAY
THE WRONG WAY
AND THE FAA WAY

Learn it how they want you to learn it then move on.

I believe the professor has a point but ultimately is wrong

The FAA disagrees with the prof.

PHAK 4-14

Compensation for this nose heaviness is provided by setting the horizontal stabilizer at a slightly negative AOA. The downward force provided holds the tail down.

The slide and the question are both confusing and incorrect, IMO.

 

[clayfenderstrat]

Thanks for all the replies. I am working on a follow-up to send back to him. I really do respect him and the class, and I don't think he realizes that the question is simply worded awkwardly. I'll post my follow-up and his response ASAP.

 

[falconvalley]

You know how to make an impression on your professor? Find a way to show that if a wing was upside down it would, in fact, get close to Earth, very fast. Lift counter-acts gravity, but doesn't know what gravity is and cannot tell when it isn't applied upward. Good examples of this are how lift vectors change during turns and graveyard spirals. Come on, professor! You don't even need to be an engineer for this, you just need private pilot knowledge!

 

[WestIndian425]

Nice thread!! I wish I had seen it sooner. Ultimately it boils down to what the professor wants. I'm glad you had the gall to stand up to him, but if he still wants to stick to his answer, then that's fine. Once you get your A and move on, that's important.

He who is convinced against his will (puts on sunglasses...) is of the same opinion still (walks away...).

 

[towhook]

Your teacher is wrong. Ask him if you redesigned the shape of the horizontal stab, but kept the weight the same, would the aircraft act the same? Of course not.

Weight = mass x acceleration due to gravity... very different from lift.

 

[stesta5074]

The word "weight" means something very specific. It doesn't in any way mean lift in a direction that happens to point to the ground. Weight is mass times acceleration DUE TO GRAVITY. That's it. No other way to say it. Toying around with the direction of the vector doesn't change the cause of the force.

In addition "lift" is a term used by laymen to describe the force caused by the difference in pressures due to speed (Bernoulli) and the equal and opposite reaction of moving air particles (Newton). It doesn't mean it has to "lift" up or down or sideways, even though we commonly refer to it in the upward direction. The most appropriate answer is lift, even though it is probably not the best way to describe what is going on. Weight is incorrect, regardless of what his lecture slides say.

There is a convention in aeronautics my roomate tells me about stating this force is, in fact, called weight. I can see the definition of lift being adjusted but I cannot accept a convention that says weight is a force not due to gravity. I suspect scientists will argue about this much like the "is pluto a planet" debates.

 

[drunkenbeagle]

I suspect scientists will argue about this much like the "is pluto a planet" debates.

No, we will not. Nothing to argue about. Weight is the normal force required to support an object against the gravitational force. Period. That is the definition of weight. If you are describing something else, it isn't weight you are talking about.

And to be totally accurate - lift is not totally from bernoulli with an aircraft, deflection of air from higher AoA is some of it too.

Teaching bad science will piss off many of us though.

 

[jergar999]

As your professor to explain tail stalls in icing conditions if his theory holds true.

 

[tgrayson]

And to be totally accurate - lift is not totally from bernoulli with an aircraft, deflection of air from higher AoA is some of it too...Teaching bad science will piss off many of us though.

And the above statement is good science? Like, from an aeronautical engineering textbook? Or rather, is it something from FAA sources, like the wretched Pilot's Handbook of Aeronautical Knowledge?

 

[seagull]

And to be totally accurate - lift is not totally from bernoulli with an aircraft, deflection of air from higher AoA is some of it too.

Teaching bad science will piss off many of us though.

And the above statement is good science? Like, from an aeronautical engineering textbook? Or rather, is it something from FAA sources, like the wretched Pilot's Handbook of Aeronautical Knowledge?

Obviously the beagle is meaning that statement to apply only in reference to high altitude hypersonic flight, right?

 

[at1024]

Or rather, is it something from FAA sources, like the wretched Pilot's Handbook of Aeronautical Knowledge?

:rotfl: But the FAA is always right

 

[sdfcvoh]

:rotfl: But the FAA is always right

You PASS!

 

[flyinguitar]

This was shown on the slide in lecture regarding the four forces acting on an aircraft, and is shown in the text book as well. So again ANY downward force is considered weight.​

This whole issue boils down to definitions. I imagine that if you look back at the lecture slide and the textbook, you will probably see that they clearly define weight as being the sum of all forces, whatever their origin, acting on the airplane in the direction of the ground. Likewise, they probably define lift as being the sum of all forces, whatever their origin, acting on the airplane in the direction of the sky. These terms need to be defined this way in order for the statement "lift=weight in unaccelerated flight" to be true. (If we consider pure lift, i.e. the total aerodynamic force produced by the wing, it will be greater in unaccelerated flight than pure weight, i.e. the actual mass of the airplane x g.)

In essence these composite forces are being named for their biggest contributor, and your professor is taking advantage of that to write a tricky question.

You are correct, of course, that the aerodynamic forces produced by the horizontal stabilizer are not "weight" in the gravitational sense.

If I were your teacher, I would have worded the problem the following way:

"Consider an airplane in straight-and-level flight. The aerodynamic force produced by the horizontal stabilizer contributes to which of the four fundamental forces acting on the airplane?"

Same question, more right answers.

 

[Realms09]

What is the background of your professor?