Cubdriver

Actually I confess finding one error in my analysis. The "elev" figures are elevator deflection values, not tail plane angle of attack values as I said above. Still, I think the inference can be made that higher elevator deflection values corresponds to higher angles of attack on the tail plane and the argument still holds. It's getting late here, thanks for the discussion.

shdw

How and why?

You presented those pictures as proof. Claiming them to show the "-15 degree tail AOA." Only to find that it was elevator deflection, where -15 degrees refers to an upward deflection. A condition we know is necessary for a slower flight, higher main wing AOA, configuration.



Cub, you should try crunching the numbers, ignoring thrust. Then put in a variety of thrust levels. Make it easy and apply the thrust line through the CG. I am, unfortunately, out of currency/practice with calculus and basic physics. You are not. A simple number crunch would provide plenty of information.

Information you won't get on x-plane. That is, you don't see the process taken to arrive at the result like you will here.

Cubdriver

That's good news, because NASA says point blank in their video they DO NOT fully understand the phenomenon; and they are the most advanced authority on it at this time.

Thank you, and I agree about X-Plane. It is a versatile program for doing many things flight simulation at a very low cost. You can design your own aircraft, test aircraft, argue with internet users, practice approaches and so on. FAA and NASA are also patrons of the software. FAA has an approved version for loggable flight time and NASA is known to use it for preliminary vehicle design. I also know of several ABET universities who teach aerospace design with it although the university I attended the professors usually wrote their own simulation software.

I already fessed up on this. Refresh me again how many aerodynamic analyses you have provided on these forums over the years?

And they are not there. However, I stand by my analysis as a casual item even with the absence of explicit tail plane angle of attack output data. I can show through hand and software analysis that the values would be close to what I claimed. Angle of attack and angle of elevator deflection are closely related. In fact, your claim that the tail plane loading in the cruise flight screen grab, actually it is not cruise flight as the flaps are down, is similar to tail plane loading in the slow speed screen-grab, is not very impressive. At 15 degrees deflection the elevator hits the stop and cannot go to its full potential. That does not imply that it couldn't do more. For all we know a larger tail plane would take the main wing right to stall. In the slow speed screen-grab alpha is shown as about 8 degrees. So there is 2-4 more degrees more to go before main wing stall. Most wings stall between 10-12 degrees. I agree however it would be much better if X-Plane provided tail plane angle of attack data. As far as I know it does not in version 8, the one I am using here. Also, I will try and find the time later today to generate some cruise flight screen-grabs for more exciting discussions.

joepilot

It is clear that some people here are using terms interchangeably that are not interchangeable.

Tailplane AOA and tailplane load are not the same animal.

Some of the arguments here seem to be saying that because you have to use almost full elevator for slow flight, that this means that the tailplane is delivering more downward force than at high speed when tailplane AOA is low. This is clearly not a valid conclusion.

Joe

Cubdriver

Thanks Joe, I concur. The lift equation would have it that as the airplane slows, AOA compensates and goes up. Same load; different speed.

I am going to do some more screen grabs that show the loading is similar. You can already see it is the case from the two already posted. AOA is the difference- slow means the tail plane is at a high angle of attack.

Here's the screen grabs in cruise configuration.

gesres

That would be an incorrect inference. The elevator deflection angle alone has nothing to do with the AoA on the horizontal stabilizer. If you read the printed report of the NASA research, you will see that they use the conventional definition of AoA...the difference between the relative wind and the chordline of the unaugmented airfoil. The latter meaning no flap deflection. If the orientation of the horizontal stabilizer with respect to the relative wind does not change, flipping the elevator back and forth doesn't change the AoA. This is in contrast to the absolute AoA, where the AoA is measured with respect to the zero-lift line of the airfoil, which would change with elevator deflection. Definitions are important when trying to understand this sort of technical report.

But, since we know in reality the airplane will rotate, up elevator causes the tailplane to rotate downards, reducing the negative AoA. What this means is that pulling back on the yoke lowers the AoA of the tailplane, just as NASA says, which is why this is the proper stall recovery. This also explains why a faster speed makes a tail stall more likely. Since the tail moves up at faster speeds, the negative AoA increases.

This part of the report is very clear and certain if you understand what they mean by AoA.

larso387

That's a good video!

Cubdriver

It would? What then, is the elevator for if not to change the angle of attack on the horizontal stabilizer? I am open to hearing how difficult it is to come up with accurate computations on this due so many influences such as main wing downwash, prop wash, trim setting, and so on, and this is why I used X-Plane to attempt it. But it is basic flying theory that lift is adjusted by means of angle of attack which is adjusted by means of control surface deflections. We need to get this straight before arguing about (discussing, reviewing) the semantics and definitions of specific terms.


Ok.

Of course it does. The air changes flow direction in response to the obstacles it "sees" or encounters. You can't throw all the physics out. An elevator changes the angle of attack of the horizontal stabilizer by moving the chord line, just as a wing does when the flaps are lowered. Imagine a chord line connecting the leading edge of the horizontal stabilizer with the trailing edge of the elevator, this line contains the effective chord when the elevator is deflected. The chord line we started with that was located in the stabilizer minus the horizontal stabilizer becomes null at that point.


I'm with you although I can't remember anyone defining that way, but fine as long as we are clear.

Ok, stop here for a second. Lets try to agree on some body-fixed coordinates. The longitudinal axis of the airplane is a zero reference, say, at the floor. You and I are sitting on seats in the airplane. When we look up to the ceiling of the airplane, our eyes form a positive angle with the floor. Having defined that, let's continue...

Yes, if you mean the angle becomes larger in the negative. As an absolute figure disregarding polarity, an absolute value with no plus or minus assigned to it, it also grows larger as we pull back the yoke. We shall define this as an increase in the negative direction.

My statement from the beginning was simply that it is counterintuitive to increase the angle of attack on a non-flying surface in order to make it fly again. Very few out there would argue against this simple point. It is basic experience for all wing stalls and by extension tail stalls. You are still not showing me how the angle of attack is being reduced by pulling the stick back. If you are claiming that increasing the angle of attack the horizontal stabilizer has makes it resume flying after it stalls, I am all ears. But this would be a very novel theory to say the least.

Forget positive and negative for a moment. Ask this: does the absolute value (no tricks here, same term used in fifth grade math class) of the angle of attack on the horizontal stabilizer grow larger in cruise or smaller? If we can agree on this then we can argue whether to define this value- whatever number under the sun it turns out to be- as a negative number or a positive on, which is only a matter of agreeing on which way is positive and which way we shall define as negative. I am not trying to be patronizing but if we can start from points of basic agreement on things like this I think we can get somewhere on the more convoluted tail-stall issue.

gesres

It changes the lift coefficient by changing the camber. AoA remains the same. By definition.

Your misunderstanding on this issue depends on the fact that you are using a definition of chordline which is not used by the aerodynamic community; rather, you're using one that FAA pilot literature uses and it's simply an analogy to show how flaps work. Let me say again: the chordline being used is the one defined by drawing a line from the leading edge to the trailing edge of the unaugmented airfoil, and it remains the same regardless of any flap deflection. It's a definition, it doesn't have to make sense. But still, it does. Much of the aerodynamic literature is geared towards exploring how variables affect various aerodynamic issues, and keeping AoA and flap angle as separate variables makes it easier to compare one set of data against another. Take a look at the lift slope data in your Shevell book; for flap deflections, the lift slope curve just shifts up and slightly left. This shows the large increment in lift coefficient that takes place at every given AoA. They also show a slight reduction in AoA of the stall for a flapped airfoil, which we know is true. If AoA were based on the chordline you imagine, the stall AoA would be much larger than for the plain airfoil.

If you accept my definition for the moment (I can inundate you with evidence later), you will see that the stated tail stall recovery technique suddenly makes sense, whereas it doesn't using your definition. Look at the following diagram and see how the AoA on the tail is much larger when the airplane is moving faster:

shdw

I have not read the NASA write up, only watched the video. Do you have a link by chance?

Anyways, the real question I have is what about CP. Does NASA mention it's roll in shifting closer to the CG with a high AOA? Or is it merely for the angle of incidence, one I can't imagine being over a couple/few degrees, or is it?