Letters to the Editor
5161 E. Memorial Dr., Muncie IN 47302
Plans and Drawings
I really appreciate it when you publish plans for known great-flying models that are available also in kit form. They give me an opportunity to evaluate the designs and see how difficult they might be to build, whether from the plans or from the kits.
It says a lot for the designers, too, when they are willing to "give away their secrets" by publishing their plans in MA. (I just happened to revisit Jim Ryan's P-47, in the July issue.) Such generosity reinforces for me the club feeling of AMA.
And I especially like the perspective drawings of Tom Chipley. Not only do they illuminate the model designs, they also draw me into the articles and make me more interested in building the models. In fact, I'd love to see Tom's drawings listed on the contents page of the magazine—it would be the first place I'd turn to, to see if there was a model I'd like to build.
Donald Skiff Ann Arbor, Michigan
Dick Korda
I was saddened to read of Mr. Dick Korda's death in the February Model Aviation. Mr. Korda was one of my model aviation heros in my youth. I remember building a copy of his 1939 Wakefield winner when I was 10 or 11 years old (1940, 1941).
I would like to see a little additional background on Mr. Korda. Did his interest in aviation project into his schooling? College? Type of work? Was he married? Children? Are they interested in aviation? Model building? What aircraft did he fly? Own? I know that for a time he owned and flew a Schweizer 1-26 glider.
Einar Thomassen Belvidere, New Jersey
Lift From an Aircraft's Wing
In your March 2002 issue, you printed an article entitled "The Lift From an Aircraft's Wing." There have been many articles written over the years trying to explain to those who don't have degrees in aeronautical engineering how an aircraft wing generates lift. While this article is closer to the truth than most, it still contains inaccuracies.
The author, Gregory Romine, uses the simple, one-dimensional, laminar flow form of the Bernoulli equation for his explanation. This form of the equation has been around for generations and is often seen in elementary explanations of the Bernoulli effect. There are more complex forms of the equation which take into account three-dimensional effects, turbulent flow, and even temperature differences between the wing and the air. The solution to these equations is complex and usually beyond the mathematical ability of most modelers.
Where authors run into difficulty is when they try to use the simple form of the Bernoulli equation to explain the flow around a three-dimensional wing. The authors have usually heard such buzzwords as "downwash" (or air deflected downward, as in this article) and try to incorporate them into their explanations.
Romine has done a much better job in explaining lift than most authors have and came to the correct conclusion that a wing's lift can be completely described by the Bernoulli equation when the wing is at a zero angle of attack.
Where Romine erred was in his explanation of lift when a wing is at an angle of attack. A one-dimensional flow equation is not meant to solve or explain a three-dimensional problem; and, in the real world, the flow around a wing in flight is three-dimensional. To make a one-dimensional flow explanation fit a three-dimensional flow scenario, authors usually add in something like air being deflected downward to explain the lift.
Where does all of this leave the Bernoulli equation? You'll have to trust me on this unless you want to spend a lot of time and money on expensive computers and their programming. The advent of large and fast computers has made the solution of very complex aerodynamic problems much easier than it was when the simple form of the Bernoulli equation was first derived. By taking the three-dimensional Bernoulli equation and using an appropriate differential equation solution methodology on a big enough computer with enough run time you can solve the equation for the problem of a three-dimensional airplane in motion through the air.
Once you have the solution, you will see that not only does the pressure distribution on the airplane account for all the lift of the airplane, but the solution predicts the downwash and all other three-dimensional phenomena that Romine and others might describe. The Bernoulli equation is alive and well in the field of aeronautics and requires no additional help.
I do not expect this to be the last published article to try to show that the Bernoulli equation does not completely explain lift. The source of a modeler's lift is always of interest to the average modeler and most modelers know of the Bernoulli equation. To explain lift by simply stating that it is due to the Bernoulli effect doesn't satisfy most modelers who want more of an explanation and the explanation doesn't take up much column space.
Since authors of profit-making publications have to fill up their required column space, I expect many more articles trying to either debunk or amplify Bernoulli to be printed. I hope, however, that these articles will not appear in the pages of Model Aviation.
Model Aviation is a special situation in my opinion. It is the magazine of the AMA and, as such, should be dedicated to giving modelers as much correct information as possible and should not need to puff up articles to fill up space. This is especially true if incorrect or misleading information is used to puff up the articles.
Basil Cooper Bessemerville, Alabama
Having read the recent article "The Lift From an Aircraft's Wing" by Gregory Romine in the March 2002 Model Aviation magazine, I was very disappointed to see that he is promoting the erroneous concept that the air goes faster over the top of an airfoil because it is curved and therefore the air "must" travel faster over this longer path.
The air does go faster over the top. In fact it goes even faster than would be explained by the "longer path" theory. My disagreement with this article is not about the use of Bernoulli's equation to explain lift. Rather, it is with using the "longer path" theory to explain why the air goes faster. This simply isn't true and unfortunately this error has been and continues to be promoted by many sources.
The Glenn Learning Technologies Project within the NASA Web site has excellent interactive displays where they discuss this and other flawed theories as well as correct theories of flight. The address is www.grc.nasa.gov/WWW/K-12/airplane/short.html. There is a huge amount of information on this site which is intended for K-12th grade education. I highly recommend it to anyone interested in aerodynamic theory presented in an easy to understand way.
Other than his error above, I found Mr. Romine's article interesting and give him credit for presenting Bernoulli's equation clearly and with interesting experiments. It is my hope that Model Aviation continues to publish these types of articles.
Clin Lashway Albuquerque, New Mexico
The March 2002 issue of Model Aviation just arrived, containing the article, "The Lift From an Aircraft's Wing." It is good to try once again to educate us all on that subject. With that in mind, it seems that the author should have discussed the real origin of lift, circulation, and the attendant starting vortices, bound vortices, trailing vortices, and circulatory flow around an airfoil.
The author could have used any of several text books on aerodynamics to avoid presenting a misleading idea to those who may be impressionable and not already a student of aerodynamics. He could have gone on to show the experimental and theoretical work by Prandtl and Tietjens in 1931 or other authorities referred to in Volume II of Aerodynamic Theory by William Frederic Durand in 1963.
This is in response to an article titled "The Lift From an Aircraft's Wing" in the March 2002 issue of MODEL AVIATION.
In explaining the lift of a wing by Bernoulli's equation, the article stated "If some of the molecules pass over the wing and return to the same initial vicinity after the wing passes, they must travel a little farther in the same amount of time than the molecules that pass under the wing because of the curvature of the upper part of the wing."
Presumably this description is from the point of view of an observer who is stationary relative to the undisturbed air and to an observer whom the Bernoulli's equation is written for. It is similar to saying that, from the point of view of an observer who is stationary relative to the wing, two molecules in the airstream that are originally next to each other and later separated by the leading edge to take different paths have to converge from the trailing edge and become neighbors again.
There is no compelling reason for this to happen. Both statements are false and the argument breaks down. The point is, there is nothing wrong with Bernoulli's equation, but there is a big hole in the argument that says the air above the wing should move faster than that below.
The article also describes a second theory which attributes the lift to the angle of attack, i.e., the effect of the air being deflected downward so that an upward force on the wing is developed due to Newton's third law of motion. This theory is not satisfactory either because one should allow the air to accelerate not just vertically, but also horizontally, and the two give no compelling reason that the air has to be deflected downward.
For example, it is conceivable that airflow below the wing increases its speed while passing under the wing such that it incurs no downward velocity at the end.
Note 1: Here again, there is nothing wrong with Newton's third law, but there is a hole in the argument that says the air should be deflected downward by a wing with a positive angle of attack.
The author went on to state that Many of the people who take part in the controversy take a side: pro-Bernoulli or pro-angle of attack. However, an aircraft wing can generate lift by both processes simultaneously, and the two are not mutually exclusive.
We are studying a system with just the air and the wing and want to know the force between them. A valid theory should be able to explain the lift by considering either the air pressure on the wing surface or the overall motion of the air, since they are just two aspects of the same phenomenon. There is no point in attributing the lift to two different causes.
In depicting the controversy as a situation where people take one of two sides and come down to the middle to declare that both explanations could be correct, the article misses the real issue. Before, we had one bad theory purporting to explain a good phenomenon. Now we have two bad theories doing the same to a good phenomenon.
I learned Newton's three laws of motion and Bernoulli's equation in high school and got a Ph.D. in physics later. I read about and saw many demonstrations similar to those described in the article. I also read other articles on this topic in the literature. But I don't know how the lift is generated by a wing. I am happy to say that a few facts that most people in the hobby already know, e.g., higher angle of attack increases both the lift and the drag until the wing stalls, and that an uncambered or flat-bottom wing is better for low-speed flying.
As for the theory of the lift from a wing, I think it is more complicated than this and will leave it to the experts in the departments of aeronautical engineering.
Note 2: I am aware of the theory that explains the lift in terms of vortices. I think it is correct, but it is more complicated, and I am not an expert on that.
Transcribed from original scans by AI. Minor OCR errors may remain.
