Cubdriver

Gesres, air does not approach the horizontal stabilizer from anywhere near that angle. I repeat, because this is crucial: air flow behind the main wing does not approach the horizontal stabilizer from anywhere near that direction. Even the NASA video we have been talking about for 4 pages shows the correct angle. I hope those diagrams are something you cooked up on your pc because otherwise I would worry would is putting out such flawed material. Again, I am not trying to belittle you but you have to do better than that. Downwash behind the main wing raises the angle of flow of air approaching the tail plane A LOT, depending on aspect ratio and many other things.

gesres

The diagram is a schematic to show you a concept that you are missing; it leaves out unnecessary detail. The important point is that the *primary* influence on the tail AoA is the geometry of the aircraft and its orientation to the relative wind. Yes, the airflow will be somewhat altered by the presence of the remainder of the aircraft but it doesn't change the essential conclusion.



Now, I am sitting here looking at a chart in the printed report of the Tailplane Icing Program. It shows an almost linear relationship between the aircraft AoA and the tailplane AoA. What is shows is that the tailplane AoA becoming more positive as the AoA on the main wing is increased. Any other influence on the tailplane AoA is taken into account on this chart and it shows the fundamental relationship that I drew. This is figure 4 in the report, if you have it. The report also says how the tail AoA is computed:Notice what's missing? Elevator position. That leads us back to the original point. Will you admit that your definition of AoA is incorrect? If not, let's work on that. You can't understand the NASA report until you accept the definition that I've given above.

Once again, I will note that NASA and I agree on the physical phenomenon and the appropriate pilot response.

gesres

This appears to be it:

http://gltrs.grc.nasa.gov/reports/20...000-209908.pdf

It's a big report. I've had it for a couple of years and usually keep the copy in my car; I often peruse it a lunch. There is a great deal to be learned about airplanes in that report, so I highly recommend it.

I doubt it, I don't see its relevance to this topic. The CP's movement is normally irrelevant, since using the aerodynamic center concept makes the CP redundant. As for how far the CP moves, at zero lift, the CP is located at infinity and moves forward as the AoA increases, so there is potentially a large variation in its location. That's why the concept isn't used in stability calculations.

Cubdriver

Alright, but that diagram needs to be redone by adding a large downwash angle due to flow behind the main wing flying at a high angle of attack, as may be encountered at slow speeds on approach. Even during cruise flight there is a considerable downwash component, maybe a 5 degrees or so.


No I do not, and I am on the road all week. Will be interested in getting a copy.

That looks fine for a first approximation. How about providing some suggested hypothetical numbers for a slow speed case and a fast speed (cruise) case to make clear what you would expect on a real airplane. I could come up with something like this and I would like to, but I am away from all my books this week.

If I am wrong, I am wrong, no big ego here. I got where I am by pursuing a deeper understanding. The great thing about science is you find out the truth by conducting practical experiments. Empirical proof is the standard. Science is not for the weak-minded because it is generally hard work and long hours. There are a lot of knocks in the endeavor and that's why most people prefer not to get into it. I do, and I actually enjoy it. Even being found wrong carries multiple advantages in my view. I take pride in being among a fairly small number of people who care how aircraft really work, and apply it to engineering and flight science. The reason I came to APC is because there are others here like myself who are also very passionate about it. We ponder this stuff all day long. The more science-inclined pilots also have a unique ability to apply their experience to the theory they read about in the books. I like to have discussions with pilot-engineers more than any other group.

I also do not contradict the NASA recovery procedure. The reason I am into this discussion is that besides memorizing a canned response, it is much better if you understand what it is you are really doing by performing a particular maneuver. I do not think the teaching liturgy is presently very good on this topic.

Singlecoil

How about just assuming a T-tail like your X-plane screenshots? Just disregard the wing downwash acting on the tail for the sake of discussion.

gesres

Quote:

Originally Posted by Cubdriver

Alright, but that diagram needs to be redone by adding a large downwash angle due to flow behind the main wing flying at a high angle of attack, as may be encountered at slow speeds on approach.

Heavens, no. That defeats the purpose of the diagram. Right now, it communicates exactly what it needs to communicate. The downwash isn’t relevant to a conceptual understanding of what is going on here. Bringing in those interesting, but irrelevant details obscures the fundamental truth of this phenomenon. Achieving understanding is usually about removing detail, rather than including it.

To highlight my point about the validity of focusing on the aircraft AoA, here is an excerpt from the report, page 13:So, if you wanted to get accurate numerical changes in tail AoA, you would have to include the downwash, but since we’re just trying to see conceptually why pulling back on the yoke reduces the tail AoA, we don’t need to consider downwash to achieve that objective.

Hopefully we're in agreement now. Airplanes are more susceptible to tail stalls at fast speeds, because the horizontal stabilizer maintains an orientation to the relative wind that produces a more negative AoA. Pulling back on the yoke increases the negative lift coefficient on the horizontal stabilizer (without increasing the AoA by definition) and rotates the tail plane down, reorienting the horizontal stabilizer with respect to the relative wind so that it experiences a less negative AoA. This reduced AoA will unstall the horizontal stabilizer.

Cubdriver

Gesres, we are in agreement but I think it is far more complex behavior than perhaps either of us realizes. I am not trying to back peddle but I think there are a bunch of variables entering into the physics of tail plane behavior. There is main wing downwash, freestream angle, elevator deflection, air speed, ice complications, angle of attack of both the main wing and the tail, incidence angle, aeroelastic or oscilliatory effects, tail coupling length, tail height, power effects, quite a list of factors and these are just a few off the top of my head. As humans we all have a wish to think we know what's right and that what's right is simple. We do that because it is empowering to do so, but we have to be careful and work to the bottom.

The most inaccurate thing about my utterings in this discussion would be that I did not give enough weight to freestream air flow angle as it appears to the tail. This dominates any other effect, I am fairly well convinced at this point. You deserve credit for pointing this out and you were right. This is why the tail stall recovery procedure works as it does: by de-rotating the tail of the airplane back to a more level angle in relation to the freestream air approaching it, the angle of attack is reduced at the tail. This in turn recovers it from the stall.

But there were highly significant facts to be had the ideas I argued such as main-wing downwash has a large effect, and that elevator deflection angle is related to the angle of attack at the horizontal stabilizer. But I think these are not the strongest effects. I stick to my idea that the chord line (NASA I believe calls it absolute chord line) is drawn from the leading edge of an airfoil to the trailing edge, including any angular contribution added by an augmenting device such as an elevator, flaps, or ailerons, in this case an elevator. And that the angle of attack is made larger in the negative when an elevator is deflected as during an approach. These are correct thoughts. It just happens to turn out that the principle you were arguing for seems to dominate in the list of factors affecting it, and I agree.

I still have not read the paper copy NASA report but I will and if there is anything I feel is important I'll mention it later.

shdw

Wasn't this already covered? Wasn't the source used NASA?

Cubdriver

Well as Gesres pointed out it does not appear in the NASA formula for estimating tail plane angle of attack listed earlier by him. This is perplexing, but I have decided the report speaks better to the topic better than anyone else, and it is a fun read as well. You need to look at the 100+ pages of the report to grasp what's going on in a tail stall, and not all of that is light reading. The report covers testing on only one airplane although it is representative of other types to an extent. Imagine all the reports required to cover all FIKI-certified aircraft.

shdw

Not only that, the definition itself was given: