tgrayson
New Member
Cl is linear with AoA for conventional airfoils within the non-stalling range of values.
Nice try.
Pitch. Power. Trim.
- Thread starter Approach Control
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Cl is linear with AoA for conventional airfoils within the non-stalling range of values.
Nice try.
Blackhawk
Well-Known Member
Exactly... but not after that. Lift decreases dramatically beyond the critical angle of attack. So it is pointless to teach something that is incorrect for the sake of simplification. The equation is not L=1/2p*s*AOA*V2, it is L=1/2p*s*Cl*V2. It is written that way for a reason. The only thing you do when you teach something incorrect the first time is ingrain it in someone's brain and it has to be "untaught" later on. Frankly, if you said something like what you originally wrote on a CFI checkride you would probably bust.Cl is linear with AoA for conventional airfoils within the non-stalling range of values.
Nice try.
tgrayson
New Member
If you think you're making a point, you're not.
Teaching that the throttle is the primary control for airspeed and the yoke is primary for altitude isn't a simplification, it's wrong, and will lead to wrong control inputs in some situations. The argument that it's a "simplification", and therefore a benefit to the student, is to me an excuse for an instructor not to expend the effort to achieve the correct understanding. In other words, it benefits the instructor, rather than the student.
Blackhawk
Well-Known Member
Funny... I don't remember writing anything about throttle, yoke, or any of that. We were talking about your statement that
"Consider that your lift is proportional to your AOA and airspeed."
I was the one arguing that you should not simplify for the student by writing such nonsense, so I'm not sure where you get off on accusing me of trying to simplify things for a student. To me, you are making an excuse instead of expending the effort to achieve correct understanding. The statement that lift is proportional to AOA is a half truth, and only serves to teach a student something that is not always true and could lead a pilot into trouble.
tgrayson
New Member
No, you're using that a red herring. You're arguing in bad faith and I won't participate.
Blackhawk
Well-Known Member
While it is easy to think about AOA as pitch attitude, it is actually based upon relative wind and the chord line of an airfoil and has nothing to do with your pitch attitude. I can stall an airplane pointed straight down going 120 MPH. As a matter of fact, at the bottom of a loop my nose and wings are pointed down, I am going at least 120 MPH and I am riding the edge of a stall.
I have flown helicopters, jets, turbo props and "small" airplanes. The concept of flying seat of the pants applies to all. You learn your pitch and power settings, and confirm what you see outside with the instruments. There have been a number of crashes where pilots of large jets flew them into the ocean on a clear day or night, just because the instruments where not telling the truth. In the two I can think of off hand static ports where taped up for an aircraft wash. Absolutely insane that pilots would ignore what they see outside and fumble with instruments that are not telling them the truth.
Blackhawk
Well-Known Member
Not at all. I think much of what you wrote has helped the OP. I guess you can read minds, however. My point is that you wrote something that was incorrect and should not be read by a student pilot without being corrected. Lift does NOT=1/2p*s*AOA*V2, and for a student pilot to be taught this is not only wrong, but very dangerous.
Douglas
Old School KSUX
Another thing you should know is that you have stumbled into an on going debate between CFIs and you should know that we all agree on the same aerodynamic principles, it is just the phrasing and the route taken to achieve the same thing, that is debated.
What was said, works only because the airplane I fly has positive static and dynamic stability.
http://en.wikipedia.org/wiki/Longitudinal_static_stability
http://en.wikipedia.org/wiki/Aircraft_dynamic_modes
I did not mention it because, the airplanes I fly do not have a large amount of excess power, which makes ascending not as practical, which means we don't do it. Gravity works for you a lot better when going down.
The thing with this method is your nose will not be steady in the descent (or climb) initially. This where positive dynamic stability comes in, the airplane is oscillating, hunting for equilibrium. After power is returned for the level off, it will hunt again.
I did it a lot more when I was still the one flying the airplane and didn't need to do as many maneuvers as possible in an hour.
Also, I might be over reading your post but you may hear your initial instructor say, "don't fly the airplane with the trim wheel". This means, the elevator is the primary control surface, the trim tab is a secondary control surface.
jrh
Well-Known Member
I'm not sure how you're envisioning trim, but I'll try to make it simpler.
First, think about how a plane is balanced in flight. The center of gravity is forward of the center of lift. The tail is an upside down wing, creating a downward force (upside down lift) that balances out the pitching moment from the center of gravity.
The tail has to produce a specific amount of force to keep the plane balanced, in other words, to keep it from changing pitch.
For any given airspeed, the tail has a corresponding angle of attack that produces the needed force. For instance, increase speed and the AoA must decrease.
What this means is that *without* a trim tab, the pilot would have to constantly be pushing or pulling on the yoke, holding the tail in a particular position (AoA) to keep the plane balanced.
The trim tab is simply a small tab that moves in the opposite direction as the control surface. When the pilot rolls the trim wheel in the cockpit, the tab moves. If it moves down, air hits it and forces the entire control surface up. Thus, the tab holds the control surface in position through aerodynamic balance, rather than wear out the pilot's arms.
Does this answer your scenario?
Don't think of it as two separate actions. It appears as though you're thinking of it as, "first I change the trim, then a little while later I change the power."
Trimming an aircraft is just a natural consequence of a change in some other area. Let's say you want to climb. You could raise the pitch of the aircraft and leave the power the same. As the airspeed decreases (like rolling up a hill in a car) the tail has less air flow over it. That means the position of the elevator must change in order to keep the force constant. You can roll the trim wheel, deflect the trim tab to a greater extent, which would reposition the elevator to the new position needed.
The thing to remember is that this is all a fluid, dynamic activity. The airspeed doesn't instantly jump from 350 to 250 knots. It slowly changes. As the airspeed changes, the trim wheel is rolled to compensate for whatever the airspeed is at the moment.
Pitch attitude and AoA are completely independent of each other. The only reason a lot of pilots see a correlation is because the two will be the same when cruising in level flight, where most of us spend the majority of our time.
You want to know how to control AoA? It's strictly and purely from the physical position of the yoke. Move the yoke back and AoA increases. Move the yoke forward and AoA decreases. The reason for this is because you're changing the position of the tail. Simple as that.
This method described by Douglas is precisely how I teach it. In the real world, I've found it to be the simplest, smoothest way to get the job done.
And yes, it will work for climbs as well.
Remember, power from the engine is what ultimately keeps us in the air.
With zero power the aircraft is a glider and has nothing more than the power from gravity to move it through the air (like a car rolling down a hill). Add a little power and the plane won't sink quite so much. Add a little more power and it will be able to maintain level flight. Add a little more and it will start climbing a little. Add a lot of power and it will climb a lot.
Now you need to get in a plane and try these ideas out. Taking it from theory to the real world will be a really enjoyable experience for you I think.
jrh
Well-Known Member
Blackhawk
Well-Known Member
Wrong in an application sense as well. If an airfoil is stalled and you attempt to increase the lift on that airfoil by increasing the AOA, you will be in for a surprise- such as trying to lift a wing that is dropping in a power on stall with aileron.
It is dangerous because it teaches a student the false information that increasing their AOA should increase lift. Are you saying this is NOT dangerous?? If a pilot is in a stall and the ground is rushing up, they should just pull back harder on the yoke and increase the AOA to increase lift and stop the descent?? Not that there has EVER been an accident where this has happened.
MidlifeFlyer
Well-Known Member
Yeah, and
Both positions are equally ridiculous.
I teach pitch for airspeed and power for altitude because I think it's a much more effective way of moving the fledgling pilot from 2 dimensions to 3. But the folks who learn the other way will us the throttle to add power to return to a proper flying airspeed while they use pitch to maintain an appropriate glide path.
tgrayson
New Member
No, they won't, because they're cramming that throttle in. I do this all the time.
That won't work in the low/slow scenario.
tgrayson
New Member
Aircraft don't generally crash due to a lack of lift, they crash due to too much drag. Even after the stall, you will generally have lift=weight or pretty close to it; some of the weight is supported by drag. With a sufficient amount of power, you could fly with both wings stalled, if you could maintain directional control.
Pilots, in general, shouldn't concern themselves with lift, because the natural stability of the airplane will ensure that lift=weight. Mostly what they need is more thrust and less drag.
Blackhawk
Well-Known Member
Yes, induced drag increases with an increase in the Cl. But it does not necessarily follow that the decrease in lift will still be = to weight unless, perhaps, in a 1G stall. As far as sufficient power... well, it would also be nice to have a wishing well that really worked. When you regularly fly Cessan 140s and 152s in DAs above 6000', excess power is not there; more thrust is not there.
Not worry about lift??? Probably why we have pilots crash in high DAs each year. They don't worry about that old pesky 1/2p in the lift equation. Lift is over rated.
Pilots should most certainly know about lift and the things they can alter in the lift equation.An exercise I do to demonstrate all this to spin students is a deep power on stall where I can induce reverse command- the aircraft will roll and yaw away from the stick deflection due to the increase in AOA on the wing with deflected aileron having a decrease in lift and an increase in drag (relative to the opposite wing).
jrh
Well-Known Member
Wrong in an application sense as well. If an airfoil is stalled and you attempt to increase the lift on that airfoil by increasing the AOA, you will be in for a surprise- such as trying to lift a wing that is dropping in a power on stall with aileron.
It is dangerous because it teaches a student the false information that increasing their AOA should increase lift. Are you saying this is NOT dangerous?? If a pilot is in a stall and the ground is rushing up, they should just pull back harder on the yoke and increase the AOA to increase lift and stop the descent?? Not that there has EVER been an accident where this has happened.
Not many pilots are pondering the lift equation when recovering from a stall. They're just going through physical motions as taught to them previously.
In all honesty, I teach the lift equation using AoA instead of Cl because AoA seems easier to visualize and remember than Cl for most people. However, I always preface it by saying this equation only works up to the point of stalling. That gets most people through their private license just fine.
When talking about stalls, I make sure they understand a few key points:
Stalls come from exceeding the critical AoA, regardless of speed or pitch attitude
When a wing stalls there is a sharp increase in drag and decrease in lift
For most people, I don't go in to the math behind it, but I haven't noticed any major problems. If they understand those two concepts they can handle spins and inadvertent stalls fine.
Blackhawk
Well-Known Member
Again, why teach them something that is incorrect or half true?? I have not had ANY problems with student pilots understanding Cl. I don't expect pilots to ponder the lift equation as they enter a stall, but understanding the actual equation can make them understand the influence it has on performance. They suddenly understand why DA is important (1/2p). They understand that on most airplanes they primarily have two ways of changing Cl- primarily through pitch, but also through flaps on some airplanes (may also change S).
It also makes them understand why they should not try to use ailerons to level the wings in a stall prior to lowering the angle of attack- doing so can have the opposite of the desired effect. I can go on. Again, I see no reason to teach something that is half true in the interest of... well, I'm not sure why someone would.
Approach Control
New Member
Harai Goshi, Kosoto Gari, etal.; all [without exception] require control & manipulation of the opponent through some kind of "grab" technique. And, that's the problem with it.
We can talk later about MA. Right now, this thread is starting to heat up a bit and I don't want it to wander.
Yes, thank you. I got my stand-in metaphors mixed. When I was thinking of "zero," I unintentionally mixed it with the stall speed when I should have related the value [ratio] to 1, instead. My bad.
My idea was that the actual landing phase of what he was talking about would translate to the lift component of KE [or, the upward component of the flare] being equal to gravity at the moment of touchdown. If PE, is height above the ground, this would cancel PE at the moment of touchdown resulting in a value of 1, which leaves on the the forward component of KE remaining during the roll-out.
Did I correct the problem?
FIRST:
SECOND:
THIRD:
Gentlemen:
Thank you both for the sincerely felt engagement on the merits of the issue.
The original question was: would I benefit as a student pilot by obtaining from my flight instructor those values that allow me to manage the aircraft I train in efficiently, through the take-off, climb, cruise, descent, approach and landing phases of flight; so that I don't waste precious dual-time continually going over the mundane basics of trying to remember the proper control inputs in order to "put" the aircraft into a particular flight attitude at will when asked by my instructor to do so. I thought that this would be an efficient way to move-on with my actual flight instruction and I thought it would show some initiative on my part as student wanting to do my best.
Taken from Kershner, 6th edition:
Where; L = Lift in lbs, Cl = Coefficient of lift, S = Wing area, (Rho/2) = are Density [not pressure] and V2 = True Airspeed [true velocity]; you would get:
Lift = Cl*S(Rho/2)*V2.
So, that pretty much settles it, unless both of you are able to redress Kreshner.
Kreshner goes on to say that: "A plot of Cl -vs- AOA for a GA airfoil shows that Cl increases in a straight line with an increase in AOA until the stalling angle is reached, at which point the Cl drops off rapidly (fig. 2-8)."
Ok, so Kreshner makes it clear that given an NACA airfoil type, Cl tapers as AOA increases and he uses a 23012 type airfoil. However, what Kreshner also shows is that when you change the airfoil type to one that has a smaller Cl [like those used in most jets], that required AOA is significantly smaller before reaching the stalling angle. He uses an airfoil of type NACA 0006, to demonstrate that.
Now, I'm just the start-up student here, but does this not fail to address the point being made, that relative to the original question of Pitch/Power/Trim, that AOA is in fact relative to Lift - for only the purposes of those angles below the stall angle - given that I'm talking about flying aircraft that range between the C-172 and the SJ30-2?
Don't stone me for asking the question - I'm just trying to learn here. Help me understand why the proportionality [limited in this regard] is not appropriate. You guys are the experts, help me to understadn this.
Thanks.
Approach Control
New Member
Is that because that even though your nose if physically pointed "down," your actual flight path scribes a higher AOA as you pull the stick back into your stomach near the bottom of the loop?
Ergo: High AOA + Relative High Airspeed = Very Close to Vstall?
Understood, makes sense. Thanks.
jrh
Well-Known Member
Hmmmmm.
You're right.
I'm going to revise the way I explain the lift equation.
Let me explain why I have always taught it the way I did though...
Ok, maybe it's not so much a matter of understanding Cl (you're right, they can understand it) as much as it is I find AoA a lot easier to visualize and apply in the real world. Most of the time I bring up the lift equation because of a discussion I'm having with a student in relation to a real world scenario, such as when debriefing a flight, rather than a dry, academic lecture type of setting.
Cl is a number derived in a wind tunnel based on many different factors. Aside from this equation, it's not used much in everyday flying. It's not tangible. AoA, on the other hand, is something I can see out the window and feel in the stick. It's one measurement of one thing. It's something pilots refer to all the time, in many phases of flight. Pilots are very aware that if the critical AoA is exceeded, the plane will stall. That's why I find it more memorable and useful to use AoA in the lift equation.
Hey, I never said anything about changing the density part! I have density in the equation the same as you
Ok, you have a very good point about how Cl deals with various wing shapes, flap designs, etc. I've generally talked about those factors outside the context of the lift equation, but you're right, it would be more precise to include those things all at one time.