Most of those who contribute here are probably well versed in aerodynamics. Some perhaps not quite so. The following link will take you to a good site if you desire to read a little about the basics, how controls surfaces work etc

When you get there, go to the bottom left and click on Beginners Guide to Aeronautics. When that page comes up, go to the bottom left and click on the green box titled Aerodynamics Index.

That brings up more information than you will ever want about the theory of flight, and also has some interesting links, including one to where it all started, Kittyhawk, now over a hundred years ago.

The blocks on Aircraft Parts, Motion, and Forces will give you a good grounding.

Sorry if this information has appeared elsewhere.

http://www.grc.nasa.gov/WWW/K-12/airplane/bga.html

It might be nice to have this as a link at the beginning of the forum. It's pretty nice.

This should definately be linked to somewhere in this forum, that is a great resource.

I've seen this before and overlooked it. Thanks for bringing it to my attention once again. I just put a link to it on my club's web page (ssrcc.org)

Thanks again,

Bob

More sites for the collection:

Aerodynamics animated using flash animations.
http://142.26.194.131/aerodynamics1
(I hope that's a static IP address - I can't locate a domain name for it.)
This comes from Selkirk College in British Columbia, Canada.

Ground School, by the Australian Ultralight Federation
As an online resource, I thought it worthwhile.
http://www.auf.asn.au/groundschool/contents.html

"See How It Flies - A new spin on the perceptions, procedures, and principles of flight. "
One persons self published, on line, book about flying.
http://www.av8n.com/how

Wow - lots of great info out there .
Here are some rules I larnt:
1- If the plane is extremely light - the CG does not matter
2-If the plane is too heavy - it still don't matter.
3- If you got enuf power - nuthin else matters .
4-If you ain't got enough power - same thing.

Dick,

I don't think you should make the claim here that CG doesn't matter, no matter how light or heavy the plane is. This is simply not true, and it's not good to give anyone the impression that he/she can ignore CG location if their plane is light enough.

banktoturn

But it is true .
If the plane weighs ZERO- why bother with CG
If it is too heavy to fly - then the CG is of no importance.
These were both comments made to show that there are designs which -if taken to the extreme-- no longer fit into the accepted envelope of design criteria.
Once a craft has a finite criteria - then the relative design needs can be determined.
All of the carefully worked out airfoils for a B24 bomber - are of no use for a EXTRA 330 acrobat.
In fact the airfoils of the EXTRA are look unflyable to some.
The flate plate wing on a 8 ounce electric is the best airfoil - but is counter to any design needs for a General Aviation light plane.
On my IMAC models - the CG is very important -but on my extremely light electrics - it is of very little importance- these things have wing loadings of only a couple of ounces per sq ft.

Dick,

I don't think your analysis of the two extreme cases ( zero weight and too heavy to fly ) are of much use for someone looking for some "basic aerodynamics". Your final sentence is simply false: CG is of no less importance for a plane with low wing loading than for a plane with normal wing loading. This would be a bad nugget of false wisdom to pass on to someone looking for some advice to be used in building/flying his/her own plane.

banktoturn

OK-- but have you ever worked with a model having 2 oz ft loading?
basic aerodynamics -is a moving target in my book.
that is - what is basic for a 4 ounce flying plate - is not related to what is Basic for a - - Cessna 172 -
I use the Extreme examples - because I have found it to be very helpful in seeing how things are affected.
If you stay within set boundries - you miss a lot.
You can't make omlettes without breaking eggs.

If you start with a flawed understanding of what is going on, you miss more.

absolutely -
and I was fed the old theories of how wings lift and that's how planes flew - Once I finally got over that - things made sense.
for example --On our models - the airfoils are probably the least important part of why they fly -
wing loading and powerloading can replace almost any other "important " stuff.

The choice of the best airfoil, which is not what I was commenting on, does depend on the speed at which the plane flies, and is often not the most important factor affecting a plane's performance. The effect of CG placement on stability, which is what I was commenting on, does not depend on the speed at which the plane flies. It is a disservice to anyone expecting to get good advice here to make the false statement that CG doesn't matter for slow flying planes.

I have had some exposure to the 'old theories', as opposed to the popular misinterpretations of them, and I have also read several of your explanations in various threads. I have not yet had any reason to abandon the 'old theories'. You just need to know when they apply and when they don't.

I guess my explanations make no sense to those who have not built/ experimented with models at the extremly low weight / slow speed end of the spectrum .
I do understand that .
My reason for the seemingly absurd comments about weight and balance -are simply to give some other lines of thought -
I am oft times put off by answers given to novice modelers -when they ask about why their model snaps out on application of elev - etc.
I see formulas and answers which may be of importance on some full scale stuff - but on these small models - the real reason is simple and direct - the durn thing is too heavy -.
One can speculate on airfoils/ cg's etc.. which are less critical etc--but on the models - the real culprit is --99.99% of the time - simply weight .
if the weight is low enough - the cg importance is greatly diminished.

Your disdain for formulas and the "old theories" has been well documented in several other threads. It is not the topic here. In what sense is the importance of CG diminished when the weight is low enough? Are you claiming that the well established requirements for stability do not apply below a certain wing loading? Can you explain this in any way?

Your absurd comments are a poor way to "give some other lines of thought". As an alternative, you might consider simply articulating some "other lines of thought" directly, as the absurd comments are simply inaccurate, not thought provoking. No one was throwing unnecessary formulas around, or citing any obscure theories in this thread. You volunteered your homespun anti-old-theory right out of the blue, and it is unmitigated nonsense. If you can justify it as anything else, I'd love to hear it.

I have... and much lower than that also. How about a 0.2oz/ft^2 rubber powered indoor model? CG and stability definitely matter. The same formulas apply as for larger/faster/heavier models, as well as full scale. (and thank goodness for that)

-David

Looks like I hit a nerve .
Not that I disrespect the work done in the past on figuring stability margins , etc..
My point is ----- in actual practice - hands on testing -we find that the C of G can be changed -a alot ---IF the wing loading is very low --and especially if the power loading is also increased.
The currently popular "3D " type models have many examples of this.
We have tried this many times .
The CG can be placed at say 25% and the model exhibits solid forward stability and for accurate aerobatics - this is desireable.
However -when we get into aerobatics which include tumbles and controlled flat spins etc., - the CG can move aft - and in very lightly loaded models - way aft.
In actual practice - back past 50% of the avg chord.
The nice stability is gone - but the model is still very flyable.
I said if the wing loading was low enough - the CG does not matter .
Absurd?
If the loading is ZERO- Theoretically- where would you put the cg?
Why?
Have you seen or worked with these super light aerobats?
If not - I would suggest you take a peek.

David -I don't know which rules apply to all sizes -
Sure some do apply
But - some of what has been considered hard and fast aerodynamics - when it gets to the small aerobats - just isn't that important.
The rules for a good supersonic craft are not the same as for subsonic- so
Why is it so strange to find that very light controlled aeroplane can work well under a different set of parameters?
One thing I have noted in each of the cases where I get negative feedback on my absurd "theories"
The rebuff is always from those who have never tried these setups .
Practical -hands on experience does count for something .
You fly "indoor rubber?"
That is a real skill and the setup and CG are very important .
I learned freeflight trimming back in 1950 - using a Wasp .049 and a OK Cub .049.
I really did not excell at it - but I got the hang of it .
The ability to RC control the little thingiies I am playing with now, allow incredible latitude in setup and design.

Hi Dick,
I dabble in indoor rubber and have flown outdoor rubber extensively (mostly F1B), but for the last few years I have been flying mostly electric pylon racers (R/C). Odd mix, I know. I've been building and flying models for about 25 years... and I'm only 29. Guess that makes me an addict.

I have an electric 3D plane and, while it's not super-light, I do fly it with a rather aft CG of about 35%. This CG would make a lot of R/C planes "unflyable," but the point is that with R/C we have active control. CG and stability still matter, but we get to act as the avionics computer which is keeping the thing going the right direction. So in this regard we are in complete agreement. If the thing is slow and light (and you have R/C), then there is a lot more lattitude in CG placement. Speed things up a bit, like in my 170MPH+ F5D racers, and CG placement is a little more critical.

-David

On racers - you bet it is critical - I always started at the leading edge -then went forward .
The guys who don't actually fly these things simply don't believe it.
Understandable ,as it does not cross readily into the typical full scale underpowered Cessna stuff.
I once read a bunch of stuff (?) about cg on racers - which was obviously from someone who had never been there -and they did not appreciate how important correct trim drag was on these things.
I once built midgit racers - with small KB 15 engines - (about 1972 as I recall )
We always set them up very "nose heavy " so that they were more stable on hard turns.
My favorite was a take off on the Spirit of St Louis - with a stubby wing.
it looked clumsy being square etc., but it was very fast --and very predictible.
Also- it was easy to see

David,

You've pointed out the critical distinction between wing loading and speed. While the stability of a plane with low wing loading is dependent on CG placement, a plane which flies slowly enough can be flown even if it is moderately unstable. The Wright brothers demonstrated this with their unstable flyers. A plane with light wing loading, if flown fast, would be just as hard to fly with an unstable CG location as a plane with high wing loading at the same speed.

banktoturn

I also noted that my models were very slow and light - still you said I was absurd, innacurate, homespun (I accept that one )
And for the record -I understand your viewpoint-

davidfee, indoor rubber? outdoor rubber? Kinky diversion from the thread!! Joking, before anyone gets upset.

hahaha !!

regarding low speed flight:-

At the extreme of v.low power to weight, yes CG is nonsensical: who cares where the CG of a brick is if it can't fly. (this is a rather pointless observation though isn't it!- it doesn't really get us anywhere)

but in v. high power to weight - e.g. rockets/missiles - I bet they have to have their CG in the right place. - probably head of the stabilising fins if in use - Can someone fill me in the aerodynamics of rockets out there? I suppose an-auto stabilising rocket with fly by wire and the CG in the wrong place could still fly. But I wonder if the drag would decrease or increase. I expect "un-stable" aircraft like the eurofighter are actaully higher drag than if they were stable because of the drag increase from continually moving control services. However drag is not the driver here - manouverability is.

Certainly in the v. low Reynolds number regime of indoor fliers, flat plates do remarkably well. There are a lot of Gottingen studies of flat plates and curved flat plates at low Re, and they seem to be pretty good at v.low Re - e.g. sub 60 000.

Am I right in saying these sections work because the more boundary layer is a more significant component at v.low Reynolds number rather than high reynolds number. Is it because the boundary layer is laminar at these Re's?

The point about where you sit on this discusssion, is not whether CG matters - because clearly you can mess with it and still have a plane you can fly, or not whether high Power to weight allows you to thro your theory book out the window; It's that what set up produces a plane with the best performance and/or the best flighting characteristics for what you want to do.

There is a competition aerobatics pilot at my club who puts his CG as far back as 45% on his Capiche (a 3D pseudo-Cap) because he likes it back there for prop hanging type manouvres. Hand's off it will dive or climb, but its still stable-ish.

My position is that although I wouldn't try this (because I am nowhere near a good enough flier), HE IS NOT WRONG. And neither is Dick. Though I think if you are starting out with aerodynamics and design, its good to follow the rule-book to the letter before you start messing with it.

If you want to be a great classical composer - start with early classical - Bach and Talis, not Stockhausen !