I'd like to collect links or material for a good Aerodynamics FAQ sticky thread. Particular areas I'd like to get stuff for are....

[ul][*] Tail Volume Coefficients and stability calculations for CG placement for various planforms[*] Airfoil drawing and analysis and low Reynolds number information that pertains to our world.[*] Stuff on wing analysis like MAC calculators and lift coefficient tools.[*] Links to sites that host similar material or tools.[*] Anything else that I've missed in this short list....
[/ul]

To start the ball rolling ...

1) [link=http://www.lerc.nasa.gov/WWW/K-12/FoilSim/index.html]Foilsim airfoil applet download[/link]...

This is a very handy tool that can be used to determine to a close approximation the lift coefficient of a model and roughly what the stall speed will be and the effect of airfoil camber on the angle of attack and stall speed. It's a wonderfullly simple tool that has many uses when you see how it fits into the various aspects of flight. Highly recomended and great fun to play with. It reallly gives a good feel for how a wing behaves in flight and is a great aid to understanding how to read the lift drag curves that come with airfoils.

2) A page of links to [link=http://www.b2streamlines.com/winglinks.html]Flying Wing Information[/link]

In particular the Panknin wing twist [link=http://www.b2streamlines.com/Panknin.html]Excel spreadsheet calculator[/link]

And if you do not have Excel then here's a link to the free Open Office package that can open much of the Office 2000 material and should work with the Panknin package.... www.openoffice.org .

More when I get time..... Hopefully you folks will offer up enough that I don't need to do more than fill in a few more nooks and cranny's...

BRING'EM ON ! ! !

Well, what you are proposing has a noble effect, however down the line the column gets littered with chit-chat, people asking "where can I find......" , I agree with.... and someone trying to promote their friend's interest. This has happened in numerous columns at RCU.

I would moreover like to read INFORMATION or sources, than the above noted exceptions. The old brain does not connect idle chit-chat between respondants with original comments. They so often tend to veer the subject matter away. It would be better if the subject matter was heavily moderated, and only applicable things be included or listed.

Also, on occasion, ofr instance once per month, it would be helpfull for the moderator to see that any posted link still works. There are a few dead ones in other forums.


Wm.

Not to worry Coos, I've learned my lesson with the last one. I'll collect info in this thread and then do up a new one and lock it down to it's only the meat and NO brussel sprouts (I hate brussel sprouts). Credit to the contributors will be given and new additions can be directed to the moderators and included. No idle chit chat to mess it up. Folks will be encouraged to ask questions as they wish in new threads.

So 'fess up with the links man!

Another thought.

How about some of you volunteer to research and assemble or type up some articles for this FAQ? It may be as easy as going back into past threads and bringing the material all together in one place. Or maybe a bit of that plus some Google time. It's just that there's enough topics that I can't do it all myself. Many hands makes for easy tasks.

A partial list of topics...
[ul][*] Tail Volume Coefficients for both horizontal and vertical sizeing and relating this to CG placement. We need the equations and links to some sites that may have calculators or other explanations available to go with out own homegrown model related version.[*] A simple newbie level discussion on basic model layout sizing and how different aspects relate to each other[*] A nice newbie article on airfoils. How they work, how the aspects are measured re camber and thickness, basics of lift coefficients and Reynolds numbers.... stuff like that.[*] A more advanced article on airfoils getting into the curves and how to relate them to actual model flying. High lift and anti stall devices would fall under this as well.
[/ul]

I'll add to this once I get some time to go back and see what the more common topics have been over the past year.

Bruce,

Here's a website about tail volume coeffecients called 'How Big The Tail': http://www.eaa62.org/technotes/tail.htm

Baron

I've mentioned this one on these forums before... it's meant for full scale pilots, but it has one of the best online explanations of lift, and stability that I've ever seen.

[link=http://www.av8n.com/how/]How it Flies[/link]


Tom

Great links so far but we need a lot more than these to fill out a FAQ. I know some of you regulars have a stash of good sites. Come on, pony up!

A link to the UIUC airfoil database seems appropriate:

http://www.aae.uiuc.edu/m-selig/ads.html

I have some online calculators at http://pages.sbcglobal.net/limeybob/
Check them out.
Bob

You may also find some useful info here:
https://rcplanes.online/

Here's an academic link on "how it flies"...

http://www.av8n.com/how/

Andy Lennon (Model Airplane news GURU), developed an Excell spread sheet for RC aircraft design a few years ago. He's about a sharp as you can get concerning RC design.
You might be able to get a copy through MAN along with permission to use it.

Link to a Neutral Point and CG calculator thanks to Ben Lanterman...

http://www.geistware.com/rcmodeling/cg_super_calc.htm

this is a valuable source for all types of online calculators pertaininag to RC airplanes...
[link]http://www.coloradogliders.com/tools.htm[/link]

Our own Cafeenman has a site with some good material on it.

Link to another CG and neutral point calculator using slightly different equations. This one is supposed to be closer to the true neutrial point if you input for a 0% static margin. Thanks to our own Adam_one for this site and calculator.

http://adamone.rchomepage.com/cg_calc.htm

Bruce. I think E-Flight use to have a refrence that had short titles of ONLY downloaded or ONLY veiwable information.
---------I would love that here. You have a huge data base to have people help you on a dedicated, Direct P M to you, to check out and post after you feel it is of use.
-----------------------Like you need another hat on your head to fill in all the free time. HA.
I would appreciate the efforts. Thanks, if you and some of us can help you. [sm=thumbup.gif]

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:

*********************************
"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?
*********************************

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:
*********************************
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.

Mesae, I hate to break it to you but it is quite possible that the site's author is correct and that the stabilizer is lifting positively when at the aft end of the allowable spectrum.

If you do some checks around here you'll find that unlike full sized aircraft that are mandated to limit their CG's to fairly forward values our models can and do operate with further aft limits and the stabilizers are often, no, actually most commonly operate with a positive lift value.

Based on the fact that this is how the full sized stuff is generally set up it's no wonder that the instructional books are written the way they are. They are just discussing the parameters as they are normally encountered in the full sized world and with the generally accepted full sized aircraft range.

It all comes down to how stable we need our aircraft to be. In the full sized world the needs for stability are great and this shows up in relatively far foward CG's in order to provide a very linear and highly self restorative reaction to pitch displacements. However our models often toss all this aside in the quest for higher performance and more pattern like model flight response to pitch variations.

Even in the full sized world I suspect that if you do a check into it you'll find that the aerobatic pilots in their Sukois and Extras are operating with CG's that have more in common with our models than they do with commercial or the usual private aviation types.

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Yes, any aircraft be it of any sort will go unstable if the CG goes aft of the neutral point. But in many cases long before that happens you will find the stabilizer working with a positive lift coefficient and the aircraft will still be admirably stable. This is not an "us and them" situation between full sized and models. It just applies. And I'm also not talking about the aircraft with computer augemented stability either, just the regular stuff.

Without seeing a running of the numbers for the Cessna I'm not in a position to say yes or no to the question of if it has a positive lifting. And truthfully I'm not that comfy with some of the advanced math required to show what needs to be shown.

However I'm basing this on a few key bits of info.

The first is that a few aeronautical engineers hang around here and they have talked about this subject in the past. They often don't agree on much between themselves but they all agreed that the tail lift coefficient moves from negative with an extreme forward CG to neutal and into the positive values as the CG moves back towards a standard value for our models.

Mark Drela also had a lovely chart that showed the relationship of wing to tail area and the location of the nuetral point as the tail gets larger right from a very small stabl through to tandem wings and ending with a canard configuration. Oddly enough the same stability calculations can be used across the entire spectrum with the "front wing" of the canard being inputed as the "main wing" and the larger rear wing being inputed as the "stabilizer". The neutral point, and thus the related slightly forward CG position, comes out of the exercise just fine as it should. But the key point here was that the cross over of the tail lift went from negative to positive while the configuration was still very much like a normal aircraft should look.

The other kick in the pants was a version of the old David Fraser DOS program Sailplane that I have. In it the wing and tail lift coefficients are calculated by what I would assume is the standardly accepted formulas for such work. In designing my own sailplanes I was shocked to see that as I made the tail larger and pinched the CG back towards the 35 to 37% mark the tail was operating at a positive lift coefficient and that only when sped up to higher speeds did it shift to a negative value. I was so shocked that I inputed the values for my own and some buddies' sailplanes that were flying well and got the same sort of results.

So it is possible to operate at a postive lift coefficient and still have a stable flying platform. I see no reason to suspect that this would not apply to large models and up on to people carrying airplanes. But as I said before the earlier indications in previous discussions said that by agreement in the industry the designers out there choose to set the permissable CG range based on a number of CG related safety issues and the limits they set are very conservative. These conservative settings will USUALLY result in a negative lift coefficient at the tail of the majority of planes. However it does not specifically deny the idea that some of the aircraft out there may shift to a neutral or slightly positive lift at the stab if the CG is shifted to the rear of the permissable limits. As long as the aircraft still operates with the desired stability in pitch it isn't an issue.

I went through all this too and had to re-educate myself as all the books I ever read talked about the weight ahead of the wing being countered by the downwardl lift behind the wing. But in reality the answer is that "it depends". It depends on the tail size and tail length and the CG location. As I said it would not be possible to confirm or deny the Cessna example without running the numbers and doing so for the various speeds across the typical performance range.

I know I don't have any charts or other math formulas to show you but there's been a lot of discussion in the past about this and various proofs offered by the folks that have made the study of aerodynamics more than a hobby. This includes that old DOS program I have.

deleted

The whole thing in my opinion is more of an art then a science. There are too many variables to conquer that aren't even acknowledged by traditional teachings much less what they are still trying to put together formally now in this area. I found real world test to be the most straight forward way to learn this stuff, it's about starting with the power/weight and scaling it all correctly from the drawing board. There are multi thresholds in flight characteristics within the scale/design/conditions planes fly through, most are not even known exactly and these are what throw off any theory or "system", you are trying to get an even curve in your data, but should be looking at a stock market symbol when it all comes down to it.

deleted

hey, I'm just talking RC, small fliers from 3" to 40", it indeed is a different animal when you are talking full scale, hehe.

Actually, you can do plenty of real world observations in this area as long as you low tech it, ie. tape the weights on, carve some simple foam for it's entire construction, use the glue gun to attach things and see how the glide tests go with your original prototype, I can build many planes in one day with simple hand tools and spending pennies on materials. When you get into this on a larger scale, we have a much larger set of molecules to deal with for the air that it's cutting through as well as the structure needs to be addressed in a larger way just to hold it's own weight much less the stress, so it becomes much more predictable yet clearly requires more thought into the initial construction, along the full scale end compared to the small RC's, they figured out there was as threshold when they broke the sound barrier, the same mini thresholds are in play with the smaller models, can you imagine the performance any 3d RC plane would have if it were scaled up to true scale, it would be horrible.

To accurately use "physics" on this, you have to take one plane, one wing, one weight ratio, one, scale one temperature, baromter, etc., it goes on forever guys. You then have a working model to base your "math" upon, but it will only apply to that one plane, one model, one wing size, etc. The experts do NOT have it all down, the chapter upon the proper anything is not written in stone, I have plenty of "real world" expamples, you want vid, PM me........ My designing criteria are into lift bodies, flying wings and of course saucers, traditional planes are just cake, they are just beyond predictable and by nature are really just a counter weight in the air.

Edited it here, just wanted to give this new guy some kind of thumbs up for his effort, where is your web site you assembled about this stuff?