Ben Lanterman

I am having a lot of difficulty trying to "resist the urge to curse, flame, degrade, insult or embarrass someone in your post".

I wrote this letter to Fly RC this morning while I was still ranting and cursing. Has anyone else read the article by Jeff Rankin in April Fly RC magazine?? Here is what I wrote to them............

Good morning,

I enjoy your magazine but......... Isn’t anyone at Fly RC bothered by the total lack of science and accuracy put out by Jeff Raskin in the April issue? He is right only on one point - that the air molecules don't have to come together at the trailing edge of the airfoil. He spends 85 percent of the article talking about the concept that the popular explanation of lift in books have had this wrong for years (Note that this is not what is in any modern text book used by professionals in the field). Apparently just those read by Jeff.

Then he says, "The wing moves air down, and therefore the wing goes up for the same reason a rocket goes up when it sends gasses down". - To say that the downwash is the mechanism of lift is totally wrong - that is like saying the tail wags the dog!

Thousands of aero engineers have been hard at work for 100 years studying the pressures above and below the wing and getting PHD's based on the math and knowledge of how the differential pressures on the wing are the direct cause of lift on the wing. There are hundreds of studies about shaping the wing to optimize the wing shape to maximize the efficiency of the wing to make these pressures. Then along comes Jeff and throws the whole thing in the garbage can with his concept that we don't have to worry about that, it is just the reaction to the downwash, like a rocket exhaust, that causes the lift.

Look at rockets for a second -

Mass rocket x acceleration of rocket = Mass exhaust x acceleration of exhaust. But only because they are connected by F=ma. Its simple - Mass exhaust x acceleration of exhaust = Force of exhaust (which is reacted on the rocket nozzle as a force) That force on the nozzle is a Force on rocket = Mass rocket x acceleration of rocket.

He is saying that the wing moves air down, and therefore the wing goes up. But he ignores the basic lift mechanism of the wing. He looks at what happens AFTER the lift has been produced by the wing and says that is the lift mechanism.

Jeff's approach is like seeing a pile of elephant poop and saying that it is what causes the elephant to move. Not that it is the result of metabolic processes that allow the elephant to move, not the muscles and the "combustion" processes that convert sugars into energy in the cells of the body controlled by an elegant set of nerve cells.

Jeff is just, oh, so wrong! If you paid him for the article you just got swindled!!

The wing lift is produced as a result of the angle of attack of the wing causing flow variation in the air. Those variations result in a pressure field that is lower on top of the wing and higher on the lower surface of the wing. The differential in pressures is the direct cause of lift on the wing. The wing can be moved only by a Force, remember Force = mass x acceleration (remember that a force is needed to counter the gravity acceleration for level flight).

I have been an aeronautical engineer for 40 years and have been involved in numerous wind tunnel tests where pressure taps were put on the top of the wing to measure and analyze these pressure fields on the wing. The integral of those pressure fields is, not surprisingly to us, the total lift that is measured on the model by its internal strain gauge balances.

Next time you feel like publishing something approaching the science of aerodynamics how about running it by an aero engineer to check it for accuracy? It would only take a minute and stop the efforts of people like Jeff who are just wrong. Please include me on that list. I am one such person, there are many that are much better qualified than I am that would be more than happy to look over your articles before publishing.

Ben Lanterman
Aeronautical Engineer Purdue 1965
Aero Dept. at McDonnell Douglas working on F-4, F-15, F-18 and other projects.
Presently retired, Active RC modeler for 40+ years.

HighPlains

My Feedback: (1)

Not a Mac fan? Jef is the guy that "invented" the Macintosh operating system (I may have left out a few steps). Of course Xerox had this little research office in Palo Alto Ca that had already invented most of everything we use now in computers, but since Xerox made copiers, their management didn't think that object computing fit their corporate model. Anyway, the kids at Apple (a bite of knowledge or was that a byte?) got to see the future, and everything was as it should be until another kid did the future a little bit cheaper and by being a lot more clever outperformed blue and everyone else.

See, just like you need to know a little about aerodynamics to fly (quoting from his article), you need to know a little about computers to read email.

Keep the torches lit, Ben. Science!

Tall Paul

I enjoy reading Raskin's versions of aerodynamics.
His planes (the ANABAT) fly, so I guess he must have something right.. as if it takes much to get a sloper to work.
His explanations of "why planes fly" are the amusing parts when he writes.
Completely out of his field, which is computers.
It's a different approach.
I'd be more surprised if the magazine editors were up to critiqueing his stuff though.
They'd be more impressed by the reputation than the words, and presume the words matched the reputation.

Ben Lanterman

HighPlains - actually this is being written on the fastest G5 mac that money can buy. Six hard drives, tons of ram, more power (insert Tim the Tool Man's grunt here). I am a fan of his work regards to computing and have had a version of nearly every Apple computer since the Apple I. The first MacIntosh was a marvel and I had one at home and at work. Great stuff.

He then assumes that since he is a good computer guy that anything he comes up with on aerodynamics is right too, apparently without reading any accurate literature on the subject. He must read the pulp fiction type of presentation that was and still is wrong. He should know better, it is bad science. If he would do some work on the subject before spouting in print he would be more credible.

Agreed Paul - actually we know that anyone can make an airplane fly and not know a thing about aerodynamics! I see it happen everytime a kid throws a paper airplane and most of the first decade of airplane designs (the Wrights excluded - at least they tried to understand). Heck I made dozens of nice flying airplanes as a kid and didn't know anything except a rough idea that aerodynamics was like what Jeff is expounding about. I have since found that I was wrong.

I think there must be some ego trip that come in to play in Jeff's writings. Big computer science guy knows more than all the stupid aero types messing around with things they don't know. He comes along and saves all of us from the wrong concepts that we have been laboring under for all these years.

It would be like me telling him that his concepts about computers are wrong since we all know all you have to do is flip the switch on a box and the computer works. Obviously I know that nothing happens in the box. Oh yes, I need to spend a lot of time talking about the misunderstanding that early scientists had about electricity flowing from positive to negative.

I wish he would stop the nonsense. Want to bet he never does enough work on the subject to know enough to knowingly retract what he wrote. Ego tooooo big.

rmh

Boy -I hope you got all of that out of your system.
You don't agree with Raskin - so be it .
I don't think he really cares.wether or not his ideas pass the criteria others deem important.
He gets the results he wants , operating in his own fashion.
I don't think he was trying to tread on anyone's toes - he just wrote his opinions.
To be a bit pointy fingered - why should modelers be concerned with data which primarily applies to vehicles travelling in far higher size/ speed weight envelopes?
This stuff is all for fun and relaxation.
Ask your heart doctor wether or not this approach is better .
Ben -I gave up worrying about other's opinions years back.
You may change them -but at what cost to you?

onewasp

Ben,

Unfortunately xxxholes are xxxholes regardless of net worth or perceived excellence in other fields.

Unfortunately the world is filled with them. They seem to thrive on the controversy they create and become legends in their own minds.

Even IF they have reached 'excellence' in one field they really identify themselves by asserting themselves (legend in their own minds) in a field in which they have little or nothing to offer.

xxxhole------about the only word in the language without an adequate synonym.

He has thoroughly defined himself-------forevermore.

My thoughts.

Ben Lanterman

Agreed with everyone even Dick, surprisingly ......... except - there is a fundamental desire in most engineers and modellers to want to understand what they are doing. In your writing you indicate that. Understanding aerodynamics even in a simplified version still should be right. I think the average modeller is entitled the best efforts to present why his airplane flies. For those of us who overpower our monsters it is not that important but there is a whole bunch of guys who fly gliders in competition that take this stuff to heart. A lot of guys wonder why a flat plat wing works, why camber, how flaps work etc.. It doesn't have to be presented in terms of vortex theory and the Euler equations of motion but....... it shouldn't be dumbed down to the extent that Jeff does. The full scale aerodynamics isn't that much different from model sized aerodynamics, there is just a Reynold's number effect, certainly the basics are still valid.

It is a shame Jeff does it. Last month he wrote an excellent article on perspective and how it influences what you see as you try to fly the model.

With what he has said about aerodynamics - it isn't just an opinion that might be as good as the next guy's - it is just totally wrong. Then to use the forum of a monthly column to continue spouting it is nonsense. It is insulting to all of the many thousands of us that spent 4 years in college (many more) and worked in the industry.

Actually it is bad for the heart to keep it inside, venting is better.

I would encourage everyone that disagrees with him to write the editor, Tom Atwood, at

[email protected]

Johng

My Feedback: (4)

I haven't seen the Raskin article, but I don't think Ben is making stuff up.

So, we are talking about the science of flight. There is no room for opinion, though Dick mentions the word twice. It does not apply here. If the guy was saying what Ben relates, he shouldn't be published, and Ben should be complaining. The force of lift is as much fact as the force of gravity. It's not my opinion that I weigh this much, , it's a physical fact. Same with the way lift is generated. It would be opinion as to whether I look good weighing this much, and opinion can come into play with the assessment of flying qualities, but not the physics of it. There's understanding - or there's ignorance. But opinion counts for nothing with regards to the facts.

I see another firestorm about the downwash chicken, the bernoulli egg, and if poultry flies, and as soon as that's started I'm gone.

Ben Lanterman

Chuckle, that was funny John. Get the article, it is worth reading.

I would put the article text on here but would probably get in trouble.

My friend here in St. Charles, MO is Dave Evans, he was the head of the whole aerodynamics department of the St. Louis part of Boeing. We did the fighters and a few other things at this location, used to be McDonnell Douglas. Now, mind you, there is no higher level to get and still be in the aero dept, he was the head honcho (whatever that means) big weenie, etc... He is retired now and we fly RC models together. A nice guy.

He said he started to read the article and saw immediately where it was going and thought it not worth finishing, that it was wrong.

I was just a misguided mid level aero engineer, but guys, Dave was the head of the whole thing and a heck of a good engineer, non better in St. Louis in aerodynamics. He didn't like the article. What does that tell you??

Not opinion, just sound engineering evaluation. Opinion is OK in religion, girls, steaks, things you eat that go crunch, beers, whiskeys, wether or not 3D is flying and things that scare you. Opinion isn't a part of being precisely accurate in a discussion of physics of which aero is just a subset.

Granted the modelling press is there to entertain and educate and mercy knows Fly RC is a very good magazine. But we need to hold the modeling press to a high standard of accuracy since a beginner interested in models and why they fly tend to believe what a learned person like Jeff says. Why teach the beginner wrong or incomplete stuff. It is only a matter of a few words to do it right. A well educated modeler just might enjoy the hobby a little more knowing how and why things work.

Totally different subject, Dick, did those little square hardwood sticks really hold the motor? I had to look twice to see them. I understand that everything clamped together tightly makes a rigid structure but I sure would have the urge to skin the sides of the box with quarter inch ply. That motor must be a beautifully smooth running machine. If I were you I would have needed to explore the flight characteristics of the airplane a year or two in order to write the article :-)

rmh

I have never had a problem using this technique.
To be blunt - I consider most fuselage setups (arfs ) to be grossly overweight - not too strong- just too heavy.
And - why would you use slabs of plywood up front ?
ultimately you have to pull on the longerons -so why not design a pylon or mounts -which will not twist and fasten directly to the longerons .
My Bucker was built in 1989 -and has flown with Tartan twin- G62 (not a good plan) ZDZ 80 singles -twins and a ZDZ60 - total aircraft flying weight as ranged from 16 lbs 9 ozs to 18 lbs -- 1790 squares .
there is one piece of quarter inch ply in it -the engine mount plate -
Also it flew at the TOC before I got it back from Steve. ( 2nd place 1990)
structurally still tight as a drum .
I do have a few years time in grade at designing structures - -commercial (high tonnage presses) machinery as well as models airyplanes -
I broke some stuff figuring out how far I could go but that is what I wanted to see.

Ben Lanterman

I don't have a "feel" for the big airplanes sturcturally. What I meant was to enclose the squares with small pieces of ply which would make a ply box with the square wook in each corner. It would resist torsonial loads better. Could put one lightening hole in the center of each one without much loss. I guess I tend to think that the shaking and power involved is really going to tear apart the front of the airplane but again I simply don't have as good a feel for what is required as you do.

I would probably hang the engine on 1/4 inch aluminum plate welded into full length aluminum longerons, and then wonder why it didn't fly as well as yours.

How did you tie the engine mount plate into the longeron structure? Does the front of the model twist any when the throttle is changed quickly? I have been to Joe Nall but for safety never got up close enough to the running motors to really see what is happening and I can't tell much when they are sitting still with the cowl on and not running.

The biggest motor I ever used was a Quadra 38cc gas on a World Engines Robin Hood. 100 inch wingspan. The sides of the airplane were 1/4 inch ply from the nose to aft of the wing TE but I don't remember what the firewall was. One or two layers of 1/4 inch at least - Of course I glassed all of the inside joints. I could have driven stakes with the fuselage. No wonder it didn't have much vertical performance :-) Takeoff was a really looong process.

Dick, you do know that if you need to get rid of any of those old airplanes that are just taking up space and/or if you want to let me see the structure first hand and investigate it for, say, 10 years or so, you can send them to -

Ben Lanterman
3432 Covington Parkway
St. Charles, MO 63301

Not going to work is it..... well I'll keep trying... catch you in a weak moment......

Tall Paul

The firewall on my Byron's 1/4 scale CAP 21 was 1/4" ply, epoxied and glassed to the -styrofoam- fuselage.. the fuselage was made exactly the same way a picnic cooler would be. 1/2'" (more or less) foam cylinder.
More than sufficiently strong for the task, with a US Engines 35 cc motor.
I have a SIG Big Bingo kit, which is a liteply forest! Just about everything is 1/8" liteply.. seriously overbuilt in my estimation.
If I build it, there's gonna be a LOT of balsa where the ply is... Like the trailing edge sheeting..Ply???? The center plates for the horizontal and vertical are liteply plates, full outline.. ????
Heavy isn't better.

LouW

I haven’t seen such religious fervor and denial outside of a “Holy Roller� revival. To get the credential thing out of the way, I am a graduate aeronautical engineer (Georgia Tech ’52) with many decades of experience in aircraft development and flight test. I am also a professional pilot and flight instructor. I have spent countless hours reducing and plotting data from manometer boards, oscillographs, tapes, etc. and interpreting the results. I have no question at all regarding the validity of the wing’s pressure field being the source of lift, and I have great respect for the aerodynamic principles that I have used throughout the years.

However, as a lift-producing wing causes an asymmetrical pressure field with it’s passage, the certain, and consistent result is the deflection of a mass of air downward. The mass acceleration is always proportional to the lift being produced. While it isn’t much use in design, and is largely ignored, it is a fact all the same.

Reality is that the pressure field concept and the momentum concept are equally valid (though obviously not equally useful) ways of looking at the same phenomenon.

rmh

Oh -the 100 on the standoffs -
the standoffs were straight grained rock hard maple -end grain ,drilled 1/4" and pulled tight using 1/4 20 (overkill)thrubolts.
This wood is extremely resistant to crushing loads .
You are right on the triangle supports -I do use these on my own stuff (this plane was for a customer) but use 5mm thru bolts to tie structure together.
I will look for pictures of motor support to longeron intersections.
found a decent one - the boxes are light ply - with triangle stock in each corner -which is attached to each of four longerons on latest models .
The open framed fuselage had spruce diagonals added after picture was shot -
the model 11 lbs at 1280 squares 40 cc engine - incredible power to weight setup.
My newest stuff is lighter - really .
The performance of my electrics has convinced me that earlier concepts were simply too heavy and underpowered .
For me .
As you know - it takes far more power to fly aerobatics at lower speeds .

Ben Lanterman

Lou, my complaint is that although downwash and pressure are all parts of the whole of what happens when wings lift and are generally equal in energies (unless the is some loss due to unforsee stuff happening), that "lift" - the thing that actually supports the wing like I can hold a wing up in the air with my hand - is the pressure field.

It can't be anything else. To keep the mass up in the air requires a force on it. That force is not downwash. for example I hold the wing up in the air with my hand and eventually have to go to the bathroom. It is all part of the process of holding the wing in the air but no one would say going to the bathroom is what is holding the wing in the air.

OK - really extreme - but it's the same thing. The lift production is a time dependent process. You can pick one molecule as it undergoes it's ride across the wing and early in the time history it is involved in pressure fields and might even be one that is hitting the wing. Time moves on and it finds itself at the trailing edge and moving down. There might be a little force at that going but not much. A little later it is going down with the downwash.. At that time there is nothing to push on. It is downwash but not lift.

That distinction while small to some is important to the exactness of the science. We can be that exact so why not? There is no reason to compromise on the subject. Since I did it for a living I do approach it with religious zeal, I hate for someone to tell me that what I have done all these years and when their statement is bogus based on bad science.

Dick, Rock hard mapel is indeed pretty structurally strong stuff. I am amazed at your the structures in the photos. They are certainly designed to fly and not crash. I keep thinking about the test stands that I had shaken apart when I was younger and had no one to advise me. .049's would shake apart pine mounts that were just glued together. I have a Charger (rudder only from the '60's) that has more wood in the nose than your big monsters!!

rmh

designing for a crash is futile.

The heavier you make it the more apt it is to crash.
I am a firm believer in the English race car approach- If something breaks - lighten the adjacent parts.
We just test flew a 8 oz 3mm Depron model at 300 squares - this one is "over the edge on ability to handle any wind.
lots of CF bracing but still, just too much flexing.
It looks like about 275 squares is the limit for 3mm Depron monoplane -for aggressive flying.
The model in the centre is 300 squares-one on the left is 270
little one 225- blue one is 1/4" fanfoald - this is light and much stiffer -but homely
We end up giving away/ selling cheap, the "learning curves".
Good trainers for the local friends --
I just bought a Aircraft World wind yer own motor - to see what they have discovered .
so far their $19.95 motor produces best power to weight of ANY of the rewinds we tried .
I want to do a 250 sq inch 3mm setup that is 5 ozs AUW.
This should be a "sweet spot" for that thickness foam.
We just keep re sighting the gun till we hit the bullseye-

DanSavage

My Feedback: (5)

What is the pressure differential between the top and bottom of a lift-producing wing?

Dan

LouW

The fact that both of us has made a living doing aero engineering stuff is in no way diminished by acknowledging the momentum explanation of lift. It is merely another way of looking at the same thing. A lover and a gynecologist look at a woman in vastly different ways. Which is the more valid? I suppose it depends on whether you are trying to address a medical problem or seeking a wife.

Siefring

Ben, You are also being non-exact.

Generally equal in energies- what tha heck does that mean.

Let me put on my physicist hat and mostly ignore the philosophical question of what holds up the airplane. I’m sure you know this but….

If an airplane lifts against gravity there must be an upward force greater than gravity. Pressure is defined as force-per-area. If you integrate the pressure over the top and bottom of the wing, subtract the downward pressure (on the top surface) from the upward pressure (on the bottom surface) you get the force. This must be so, because in physics we have defined the forces and pressure to make it so. Philosophically, does pressure hold the airplane up? You can say that, but it is not the whole picture.


We’ll it is, but that is also not the whole picture. The only way to get lift on an airplane is to push air (gas, fluid) down. The force pushing air down is equal to the force pushing the plane up. This is Newton’s Third Law. Philosophically, does the force used to create downwash hold the airplane up? You can say that, but it is not the whole picture.

It has been a while since I did any fluid dynamics, but here goes. When we treat air as a fluid (lets just say perfect, inviscous, incompressible) flowing over a wing we make some other definitions/assumptions. We often break-up our discussion of fluid properties into those in the direction of the flow and those perpendicular to the flow. The wing is rigid and the fluid cannot flow into the surface. The flow is always parallel to the surface and the “directed� flow does not apply any force to the surface. Instead force is applied to the surface through relationship between flow velocity and perpendicular pressure (a la Bernoulli and thermodynamic energy conservation). So the pressure fields ARE the only forces acting on the wing [8D].

Now doing the standard bad hand waving, in the places where flow is squeezed the directed flow speeds up (the fluid does not like being compressed) and the perpendicular pressure drops. Thus, the plane lifts because of the Bernoulli effect. Of course, this hand waving ignores a lot of physics. If you treat the problem with Navier-Stokes equations, you find you cannot have a total pressure differential between the top and bottom of the wing without having a downwash. You simply can’t have one without the other. I don’t find this to be surprising.

Unfortunately, many high school (entry level) physics courses have been taught using the hand-waving picture with a very flawed “artist conception� of streamlines around the wing -- with no downwash.

Ben, you should know better. It took force to change the direction of the particle; once it is changed there is no force acting. Small or not, it is a force and with 10^19/cm^3 these little forces add up fast. Anyway in the fluid picture, particle directions are ignored (or more exactly lumped into a bulk fluid property involving thermodynamics), after all it’s hard to keep track of 10^19 particles each colliding with another every nanosecond. A fluid moving over a lifting wing starts with a horizontal mass flow and after the wing ends up with a vertical component of the mass flow. The vertical force applied is Mass*DeltaV(vertical)/DeltaT. I agree for engineering purposes, this is very hard to keep track of, especially the DeltaT part. The Aero-engineering methods where you just deal with pressures at the wing surface are very elegant and much more useful.


Carl

BMatthews

We had a BIG discussion on this about a year ago.

The way I finally glued all the seemingly disparate bits together was that the airfoil induces a faster flow with lowered pressure on the upper surface (assuming lift is being produced) which then comes back down with a higher energy or velocity at the trailing edge and forces the lower airflow to deflect downwards. And at that point the lift from the pressure differences (potential energy?) is reflected in the downwash of the air mass (kinetic energy?). The bracket energy references are new and a perhaps simplistic and lame way to connect the two together.

However it was generally agreed that you can't have downwash without a pressure difference and if you have a pressure difference then you get a downwash. So they are apparently connected but much like the old wave vs particle theory of light it seems that we see it one way when it's convienient and the other when that version fits.

LouW

Ben, the fact that we both made a living for years doing all that aero engineering stuff is not diminished in the least by acknowledging the momentum explanation of lift. It is just another way of looking at the same thing. ************Edited by BMatthews********

Ben Lanterman

How to make it simple?? Let's look at F=ma.

Mass of wing is accelerated as desired by? --- a force. We need a force equal to F.

Where can we get this new force? ----- air molecules impacting on the wing.

What have we always called that? ---- oh yeah, pressure.

Is it easily measured? --- yes, pressure taps, done all the time. It's easy to develope an accurate field from very few taps.

Then we can numerically integrate the measured pressures and they are equal to --- the lift, drag and pitching moment forces and moments on the wing.

Well that was easy wasn't it.



I mentioned my friend, here is his letter.


Tom,

First, I have been a subscriber to your magazine for a year or two and I must compliment you on a fine magazine. However, at Ben Lanterman's urging, I read Jef Raskin's article. I rarely read such articles as I have found that attempting to reduce any complex subject, such as aerodynamics, to a two or three page article is at best a waste of time. At worst, it can lead to protracted arguments between individuals who are otherwise rational and even friendly.

I believe Mr. Raskin's article brings out the worst. It was not only a waste of time, but I found it somewhat insulting to those of us who have spent our careers making airplanes fly. Ben forwarded me his correspondence with you and I have to agree with Ben. The explanations Mr. Raskin offered are, at best, overly simplistic and in some cases just wrong. I won't go into details because it is too difficult to explain these principles in an email that you would have time to read.

My credentials: I've been a modeler for 45 to 50 years, I don't remember when I actually started flying control line. My first R/C aircraft used a single channel with a button on the transmitter and a rubber band powered escapement in the airplane. I graduated from the University of Kansas Aerospace Engineering school in 1968. I worked in the McDonnell Douglas and later Boeing, after we merged, Aerodynamics department for nearly 36 years. I had been the Department Head for Aerodynamics and Flight Controls in St. Louis for six years when I retired last year. Ben Lanterman worked for me on a couple of projects during our careers.

Again, I enjoy your magazine and look forward to its delivery each month. I thoroughly enjoy product reviews and articles on model construction techniques and flying techniques. If your readers really want to know about aerodynamics, I would direct them to Martin Simons' book "Model Aircraft Aerodynamics" published by Special Interest Model Books in the United Kingdom. It is a 340 page, including appendices, simplification of model aerodynamics.

Dave Evans
Retired Department Head of Aerodynamics and Flight Controls
Boeing Integrated Products Division - St. Louis

Siefring

It really is simple. In fact the pressure part is just a definition, no physics involved.

I completely agree. If you want to know how an airplane behaves all you need to know is what the air does to the aircraft, for lift all you need are the pressures. If you want to have a philosophical discussion about the source of lift then you need to include what the aircraft does to the air.

Are you saying that you do not believe downwash has anything to do with lift?

This brings up a question. Can you really determine drag from pressure sensors?

I have no idea if you can do this for the nice neat laminar flow case[sm=confused.gif]. How about for determining drag from the 3D wing shape (tip vortices)? In the highly sheared flow, turbulent boundary layer case, I doubt it is possible. Maybe I am wrong?

Carl

Ben Lanterman

Never said downwash didn't have anything to do with lift. Re-read the thread, as Bruce so eloquently put it - you can't have one without the other.

Several experiments can be done. Take the 1/16 x 3 x36 inch piece of balsa you just bought, hold it level out in front of you like a wing and shove it forward like you were launching a flying wing. I did this experiment and saw that to no surprise the it rotated leading edge toward me. Why? When I shoved it forward it developed the typical pressure field we see around an airfoil. The net result of the pressure field is an aerodynamic center at the 25% chord location lifting up. That rotates the balsa about the CG which is at 50%.

If it were downwash coming off the back of the balsa then it would seem that it should rotate nose down.

Drive down the road in my long nosed 1968 Pontaic. Release all of the hood catches. The hood lifts up and over the car. Why. Certainly not downwash. It's differential pressures on the front of the hood.

Professional aero engineers that get paid for making airplanes that work look at pressure fields, not momentum changes, Why? Because that is the direct cause of lift. Later the military worries about downwash blowing down buildings on low level practice bombing runs.

Amateurs thinking they have discovered the Holy Grail of aerodynamics talk about momentum changes. It can't be work with in any practical way. It is there but totally useless in life.

I have a photo for you from the Feb 2005 Quiet Flyer magazine - These guys did a heck of a lot of work if pressures are meaningless. The airplane is a full sized competition oriented glider. Note that they believe they can measure drag with the probe just aft of the wing. No where are they measuring momentum characteristics of the air.

Bob.R

My Feedback: (2)

I wonder what Mr. Raskin would say about a rotating cylinder in an airstream.

I'm not a trained aerodynamicist, but I've spent time over the years reading some of the literature - Simons, Eppler, Abbot and Von Doenhoff. I don't claim to fully understand the material, but what I do understand has helped me in my model design activities.

It would be nice if there was a scientifically correct, neat, simple, easily understood theory of lift that could be printed on one page and understood by the typical modeler. I haven't seen one yet (although it may exist). Does this mean writers such as Mr. Raskin should not try? I'm not sure, but at least he helps by dispelling the myth of the separated air molecules rejoining at the trailing edge. In the twenty years I heard that one, I never understood how it could be. I was greatly relieved when I eventually learned it was just wrong.

The air deflection explanation may not be completely and scientifically correct, but everyone has held their hand out the car window and generated lift, and the hand is a really bad airfoil. If this approach gives the average modeler a better feel as to what holds his model up, maybe it is not a bad approach. As for advising modelers to go read Martin Simons, only a very small percentage will. As for me, I finally decided to believe that air deflection and pressure distribution are inextricably linked. Maybe that is not technically and scientifically correct, but now I can sleep at night.