If you know anything about aerodynamics.....
- Thread starter clayfenderstrat
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phoenix 23684
Well-Known Member
Being that I have an engineering degree I will have to agree with you. Is not weight, but a lift vector applied downward. However a lift vector is nothing more than a force and weight is a force as well F=MA. and since both use the same units, one can argue that a lift applied down = weight.
Weight is nothing more than a representation of how much force is applied by any mass, in this case on Earth.
My experience is that courses geared towards pilots often oversimplify concepts to the point that they are no longer correct. therefore the correct answer choice should have been a force applied downward.
A bigger problem with your test question is that he says the Bernoulli effect. Again simplifying a concept until it's no longer correct. Is not the Bernoulli effect, but rather a combination of the low pressure, or CP and Newton's third law on the elevator that causes the elevator to apply a downward force. Sure Bernoulli's effect is part of what causes the pressure differential, but it's that pressure differential combined with the 3rd law that results in the force.
A counter argument to your professor is that since Weight = Mass X Gravity, then what he is saying is that the weight of the stabilator alone counters the weight of the nose, i.e. a 10 lbs elevator keeps the airplane leveled, right!!!
Weight is nothing more than a representation of how much force is applied by any mass, in this case on Earth.
My experience is that courses geared towards pilots often oversimplify concepts to the point that they are no longer correct. therefore the correct answer choice should have been a force applied downward.
A bigger problem with your test question is that he says the Bernoulli effect. Again simplifying a concept until it's no longer correct. Is not the Bernoulli effect, but rather a combination of the low pressure, or CP and Newton's third law on the elevator that causes the elevator to apply a downward force. Sure Bernoulli's effect is part of what causes the pressure differential, but it's that pressure differential combined with the 3rd law that results in the force.
A counter argument to your professor is that since Weight = Mass X Gravity, then what he is saying is that the weight of the stabilator alone counters the weight of the nose, i.e. a 10 lbs elevator keeps the airplane leveled, right!!!
TXaviator
Well-Known Member
yet another example of garbage being taught to pilots that is flat out NOT TRUE.
the tail creates lift (because of bernoulli & action/reaction). the lift is acting toward the earth, and combines with the force of weight, for the sum total earthward pointing force vector.
period.
can we now move on to talking about shock cooling and running the "engine over square" ?
the tail creates lift (because of bernoulli & action/reaction). the lift is acting toward the earth, and combines with the force of weight, for the sum total earthward pointing force vector.
period.
can we now move on to talking about shock cooling and running the "engine over square" ?
drunkenbeagle
Gang Member
MikeD, is this another one to add to your list???
deek
New Member
I say good on you for bringing up the the question. You are in college after all, and one of the things you learn is how to support an argument, debate and negotiate. Unless the prof is a total d-bag, he should be excited that a student is really interested in the subject matter and wants to continue a discussion till they learn the answer.
Realms09
Well-Known Member
AS, I am not disagreeing with you but I wanted to add a distinction:
Stick and Rudder is a great book and completely outshines FAA material and, at least in this particular instance, the ideas put forth by the professor. There is a huge difference between simplifying correct material to make it practical for a wider audience and presenting fundamentally flawed information whose only virtue is that it happens to be simple. The former is what Langeweische does as well as what tgrayson, fish, and seagull do on this forum. Unfortunately, the PHAK generally falls into the latter category as do the inherited views of some pilots.
The great thing about a GOOD model in science, engineering, business, or flying is that it contains a lot of information in a small amount of mental space. I think that makes the recollection of which X causes which Y a lot easier. When models are BAD there is a lot more stuff to memorize because nothing links together and there are contradictions. The FAA presentation of lifting forces falls into that camp.
In summary I 100% agree that models should be USEFUL to the person learning them. The distinction here is they can be simple without being flat out wrong. Furthermore, they are more useful when they have a basis in truth.
Amen...and a very big Thank You to those guys (among others) for investing their time and efforts on this forum. Their wisdom and knowledge are outstanding, but it is there willingness to share same are what make them invaluable members here.
flyinguitar
Well-Known Member
I agree your wording is more correct from a physics point of view. (My wording implies that an aerodynamic force contributes to "weight", which is at best sloppy and at worst flat out incorrect. And you are correct, there will be a drag component as well.)
However, we have to work within the proper context, which is not a physics classroom, but introductory flight training. The goal is to develop students' intuition for what the main forces on the aircraft are, and what the relationship between them is in different regimes of flight. Simplification is therefore necessary. Having a simple set of forces that have simple relationships between them (lift = weight and thrust = drag in unaccelerated flight, lift > weight in turns with lift/weight = load factor, etc....) will actually help a student in the cockpit with the basic flight maneuvers they are trying to learn. Trying to think about subtleties such as the drag component of the negative lift produced by the horizontal stabilizer is, in my opinion, distracting and beyond the scope of what pilots need to know.
And I can't resist commenting on the whole Newton/Bernoulli issue. (For the record, I agree with fish134 and tgrayson's comments.) I always get a chuckle from how entrenched people's positions are. Basically, to discuss lift rigorously, you need a lot of mathematics called partial differential equations. I know there are a lot of highly educated people on this forum, but for those who haven't taken a lot of math let me give you an example of what a common progression of math classes would be:
calculus -> multivariable calculus -> differential equations -> partial differential equations
Basically, it's graduate-level mathematics. If you haven't gone that far (and I have not), that doesn't mean you can't talk about lift at all. But don't think that you're somehow the only one who knows the truth.
(FYI, I majored in physics, and although I never studied aerodynamics, I feel I developed a good intuition for logical and faulty physical arguments.)
drunkenbeagle
Gang Member
The context here was a University-level classroom, not primary flight training. I would hope a University-level class would manage to get this right. Honestly, the physics is high school level stuff (no calculus required for the statics-mechanics part of it anyway).
The FAA BTW does make a distinction about lift, particularly in the Rotorcraft Flying Handbook. Deflection of air downward is treated using Newton's 3rd law there, not bernoulli's law.
The fact that airfoils DO deflect air downward is a very real thing. Among other things, downwash from the wing increases the the action of the horizontal stabilizer (at least on non-T-tail A/C).
The major flaw with the "four forces" model is that it fails to define the proper inertial frame of reference. The four forces may have components acting in any direction - Gliders for instance would not work without gravity having a forward component RELATIVE TO THE AIRCAFT.
Polar742
All the responsibility none of the authority
It could have been primary flight training at the University. Could have been some PPL level stuff.
At said university, they don't do the whole in depth aero deal until down the road a bit. Second 2 years if I remember right.
tgrayson
New Member
Yes, it is, but that doesn't mean that it contributes to lift. The airfoil deflects the air using the pressure distribution around the airfoil. All the airfoil "sees" is the pressure distribution because that is the only way an object has to interact with a fluid. Bernoulli's law will explain the pressure distribution and the law is derived using Newon's laws of motion. It is simply an expression relating the exchange of potential and kinetic energy in an airflow.
These are secondary effects and, in general, are undesirable. The downwash over the horizontal stabilizer decrease longitudinal stability of the aircraft.
The primary flaw is using FAA materials for reference on a scientific subject; that's where you're getting this "Newton and Bernoulli" stuff. If you're an engineer, I suggest getting an engineering textbook.
texas_pilot
Well-Known Member
I wish I was alive in the 60's, too many people just roll over and take it to avoid conflict these days. Just because it isn't perfect doesn't mean you shouldn't verbally fight for a necessary and correct change. Especially on an intellectual level, stupidity and misinformation has run this world for far too long.
In the late 60's I effectively applied a 'scorched earth' reply with an administrator- it took him almost 4 years and a blatant lie, but he got me finally- not once but twice.
This makes very good sense, tho
"clayfenderstrat: Sounds good, as moxie said try to avoid direct claims of him being wrong. While he is wrong, those are typically taken as ego attacks and will not help your cause."
TheresaPilot
Well-Known Member
Just a quick shout out to the SOSU grads reading this thread...
"Well, actually......"
"Well, actually......"
B767Driver
New Member
In my opinion, the FAA is responsible for the overwhelming majority of pilots believing in gross inaccuracies in the topic of lift production. I don't really blame people for not knowing the truth...they weren't taught properly from the "gospel". But it is worth individual exploration for the truth...and is quite an interesting journey. It was for me.
tgrayson
New Member
Agreed, but I do blame people for being absolutely certain that the FAA material is the ultimate authority on the subject and being dismissive of anything the engineering community has to say.
gcman6
New Member
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I watched this video today and around 50 sec. in the AF pilot describes the force on the horizontal stabilizer as a downward facing lift component, for what it's worth. It's a good video too.
I watched this video today and around 50 sec. in the AF pilot describes the force on the horizontal stabilizer as a downward facing lift component, for what it's worth. It's a good video too.
seagull
Well-Known Member
FBW allows things like that in commercial transports as well. The other choice for tactical aircraft is the canard, but you get into problems there if you don't stay subsonic.
