I checked this out and saw some explanations on stability theory that looked questionable, despite a supposed "experiment" the author performed to prove his theory that the tail of a Cessna 172 lifts positively when the CG is near the aft limit. He provided no video of the streamers he used to "validate" his theory, nor even a description of the expected or observed behavior of the streamers.

I am a 4700 hour Airline Transport Pilot, Flight Instructor and aerobatic (have done full-scale aerobatic competition) pilot. I have also worked with professional engineers as a UAV test, demo and revenue pilot. In that job I made several recommendations for design improvements and all of the changes made had the expected results. The web site author's statement didn't make sense to me so I emailed the author the following:

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"Regarding:

Here’s an explicit example. I’ve actually done the following experiment:
I took a Cessna 172 Skyhawk and put a couple of large pilots in the front seats, with no luggage and no other passengers. That meant the center of mass was right at the front of the envelope, so the tail had to produce considerable negative lift in order to maintain equilibrium. There was lots and lots of angle of attack stability.
I took the same Skyhawk and put a small pilot in the front seat, a moderately large mad scientist in the back seat, and 120 pounds of luggage in the rear cargo area. That put the center of mass right at the rear of the envelope, so the tail had to produce considerable positive lift in order to maintain equilibrium. The airplane still had plenty of stability. (As far as the pilot could tell, it was just as stable as it ever was.)
The easiest way to determine whether the tail lift is positive or negative is to observe the direction of motion of the tip vortices, as discussed in section 3.12. To observe the vortices, I attached a streamer of yarn, about half a yard long, to each tip of the horizontal tail, at the trailing edge.[/i]


On what basis, beside observing tufts, did you conclude that the tail is lifting with the aft CG loading? My understanding is that the tail has a down-force under all normal flight conditions and loadings of the C-172. Now that you have challenged this supposition, I ask for further support of your conclusion. Do you have video of the streamers? Have you done a moment - neutral point analysis?
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The statement in bold (emphasis added) was the one I took exception to, and it turns out to be completely wrong. This may not seem like a big deal to those who don't have an accurate understanding of stability and control, but it reveals a severely flawed understanding of said principles, and one reading this author's explanation will also not gain an accurate understanding, so what's the point to read it?. If one is to take the trouble to wade through diagrams and explanation, one might as well get the truth for all his or her trouble. Stick to established texts, like the Pilot's Handbook of Aeronautical Knowledge if you want a good basic understanding of aerodynamics. There are many other good book as well.

His response was this:
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Hi --

You wrote:

> On what basis, beside observing tufts, did you conclude that the tail is
> lifting with the aft CG loading?

1) Theory let me to believe positive tail lift was possible, indeed
plausible.

2) The fact that the tail airfoil is symmetric (not cambered) suggests
that it is optimized to produce both positive and negative lift.

3) I did the experiment, as described.

4) There is a total lack of credible evidence to the contrary. A
thousand pieces of soft evidence do not outweigh one piece of hard
evidence. The opinions of people who don't know what they are talking
about do not count for anything.

> My understanding is that the tail has
> a down-force under all normal flight conditions and loadings of the
> C-172.

That's your opinion. That's not evidence.

> Now that you have challenged this supposition, I ask for further
> support of your conclusion. Do you have video of the streamers?

No, I don't have video.

If you think I'm lying, do the experiment yourself. Replication of
experimental results is absolutely central to the definition of what
_science_ is.

> Have you done a moment - neutral point analysis?

No. Have you?
*********************************


I figured he misunderstood me so I continued to ask for additional information (three times total) and he has only managed to insult me several times, and repeatedly insists that I perform the experiment myself, without offering ANY additional supporting information. I quoted two authoritative works on aerodynamics: The FAA's Pilot's Handbook of Aeronautical Knowledge and a private work: Flight Theory for Pilots. The latter work contains a great deal of math for a pilot's text and I found it to be quite excellent, though it is not inteded as a work on aircraft design. Both of these works are in clear and direct opposition to the web site author's "theory", and in agreement with each other. I could have quoted many others. His answer was essentially: the books are wrong, and "argument by appealing to authority isn't scientific". He calls my references "unsubstantiated opinion", who's authors haven't performed his experiment. He claims to have attempted to get the folks in Oklahoma City (the FAA) to "correct" the Pilot's Handbook of Aeronautical Knowledge", but to his dismay, they will not. Seems to me they were just showing good sense.

I already understand stability theory in a way that agrees with every aerodynamics text I have ever read (not to an engineer's level but more than adequate for a pilot). So I appealed to an aeronautical engineer, who ended up calling the web site author a clueless wanker. I even called Cessna Aircraft Corporation and got positive confirmation from a service engineer and several Flight Test Engineers, that the tail on the C-172, and by implication almost all aircraft except canards and a few aircraft that use computers to stabilize them, pushes DOWN with respect to the aircraft, when it is positively loaded. The service engineer invited the author to call him.

The web site author has not responded to the Cessna reference as of 10 Jan, AM. And refuses to answer additional emails or correct his website. His position shows a fundamentally flawed and potentially dangerous understanding of stability and control theory, making the entire section of his web site pertaining to stability and control of worse than no value. I hope he is re-evaluating his position, considering that he is in clear and direct opposition with the intelligent and well-trained people responsible for ensuring the aerodynamic integrity of Cessna's products. I hope he corrects his web site, since it is a disservice to the aviation community to provide grossly flawed aerodynamic information to people seeking a deeper undersanding of aerodynamics.