basic aerodynamics
 

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

Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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.

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

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-

RE: Suggestion for moderators
 

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

RE: Suggestion for moderators
 

hahaha !! :D

RE: Suggestion for moderators
 

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 !

RE: Suggestion for moderators
 

gee -I started with Spike Jones compositions - so I just can't resist the chance to tweak noses .
But on another parallel - I also had a classic music education from age seven -till sixteen - when girls and cars suddenly upset the the cart.
My home spun technical background really served me well -I had a few very successful technical patents - two dealing with fail safe pneumatic circuitry.
So when I state the patently absurd - I look to see who understands the subject - If they really do understand it - they see the gag .
You obviously saw it.
Some did not ---
I really do not understand all the reasons why the flat plates work so well but the fact that they are tnin- means that they can be stalled at low speeds -and these can be commanded to do very precise snap rolls - yet they are very docile.
I have a friend who, along with his son - live in Merseyside are active club members and he also acted as a Team Cap. at the prior Polish World Champs
both he and his son fly my designs in competition
An EMC2 and an Ariel FAI aerobatics and a Staudacher I whipped up for scale aerobatics.

RE: Suggestion for moderators
 

Yes, the main problem very low Re airfoils have is laminar boundary layers and the fact that this laminar boundary layer can't support large pressure gradients without separation. In layman's terms, fat airfoils don't work at low Re because the airflow can't stick to a really curved surface. An extreme example would be a ball.

Curved plates work well, but real airfoils work better. Often times, a turbulator will be used to energize the boundary layer to prevent separation at high Cl. This turbulator can be as simple as a length of thread glued to the upper surface of the wing, at about 15% chord. Attached is a pic of a couple airfoils suitable for the Re = 40k-80k range. Less camber is normally used at lower Re.

-David

RE: Suggestion for moderators
 

Hey Dick! You must be an F4 pilot. That plane proves that if you got enough thrust and have a large, government-sized reserve of JP4, a rock will fly.

RE: Suggestion for moderators
 

Yeah, and the F-101 too. Standing joke at Vandenberg AFB was that with those J-57's a barn door would fly!

RE: Suggestion for moderators
 

Yes!!!
Vectored thrust is a real part of today's aircraft design.
I pulled a few tails to see how many guys actually flew stuff (models or biggies ) using this method of flying.
I think I also stepped on a few tails --
I said early on " If the thing is light enough -cg doesn't
matter -and if it is too heavy -it still doesn't matter".
Watch a close up of an F16 tailplane as the craft takes off - it is wiggling to keep the craft from going ass over teakettle .
OR fly a tiny flate plate wing aerobat with wing loading measured in a few ounces and 2-3 x thrust from the motor.
With a proper computer (or some quick thinking) and these parameters , you can rethink the old rules on flying.
The flate plate wing has a characteristic of going from low drag (at a low AOA) to instant high drag at a fairly low AOA. (predictable also)
on a very low wing loading - this provides a wing that will fly fast -then instantly snap on command.--OR if slowed and power is increased - it allows you to steer the model around like a surfboard.
Low wingloading and high power loading means everything on these designs.
The cg - -oh somewhere convenient -further aft the more fun.

RE: basic aerodynamics
 

Hi ya'll,
Boy what a go around this outing has and is seeing. Creates a desire for everyone to get
his 2 cents into the mix, however I don't have the 2 cents, but>
When you have been there, did it al and learned you find certain things are for sure and as
a result most necessary
In that respect I would suggest that one never forget that BASIC AERODYNAMIC LAWS apply to any and all aircraft. Details can vary but performance is always asociated with the basics.
As Dick has said at extremes it makes little difference what is what but we surely do not
work even close to extremes so must not go beyond he basics, OK?
Good luck,

Hal [email protected]

RE: basic aerodynamics
 

Ah, banktoturn you contradict yourself. CG is dependent on speed. The faster a wing travels through the relative air the more lift it attempts to create. Therefore, a fast pylon racer can utilize a cg farther forward of the center of lift. While a slower flyer will benefit more with a CG farther aft than that of a pylon racer. By the way, I understand the points that dick hanson was trying to make. I also respect his opinions (whether correct or incorrect). Calling them absurd is harsh. Reading his threads, his opinions and theories come from personal experimentation. He understands what he is doing and it works for him. Just because his logic is not traditonal doesn't mean it doesn't work. dick you should thank banktoturn for the criticism, for it will motivate you to be more thought provoking. dick you provoked my thinking with the threads in this forum.


Quote from banktoturn

“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.”

“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.”

“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.”

RE: basic aerodynamics
 

95tequesta,

Maybe, but I don't think so. What did I say that contradicted something else I said? I didn't find any such contradictions in my postings in this thread.

You claim that "a fast pylon racer can utilize a cg farther forward of the center of lift. While a slower flyer will benefit more with a CG farther aft than that of a pylon racer". This does not make sense to me, contradicts what I have learned about stability, and, I think, is a false statement. Can you explain it to me in more detail so that I can understand it? I'd appreciate that.

Dick's claim that I disagreed with was that "1- If the plane is extremely light - the CG does not matter". This is not untraditional logic that works for Dick, it is simply untrue, and is an absurd statement. I'm sorry if that seems harsh to you. I may not have made the comment, except that this thread is in the context of offering aerodynamics advice to beginners. In that context, claiming that CG location does not matter for a certain type of aircraft is not an appropriate thing to do.

banktoturn

RE: basic aerodynamics
 

If you would have read the thread I posted you would have seen the quote that I copied and pasted from your own words. Yes, your correct in that these discussions got away from the intended subject. Your attitude resembles that of a crusty old military person, that takes information presented to them as the gospel, but rarely thinks for themselves or questions "theories" or opinions. I believe you would argue with a brick wall!!!!!!!!!!!!!!!!!!!

You pick and choose information out of the posts. Before I mentioned the pylon racer I said, "The faster a wing travels through the relative air the more lift it ATTEMPTS to create." "Therefore, a fast pylon racer can UTILIZE a cg farther forward of the center of lift. While a slower flyer will BENEFIT MORE with a CG farther aft than that of a pylon racer." Read the small words!

RE: basic aerodynamics
 

95tequesta,

I am sometimes a bit argumentative, a fault that I struggle with. For some reason, I seem to be having that problem today, in particular.

I think I see the apparent contradiction you were referring to, but it is not really a contradiction. There is a difference between saying that CG location does not matter for a light plane, and saying that a slow plane may be flyable even if it is unstable. CG location DOES matter, at any wing loading, in the sense that it can determine whether the plane is stable or not. The fact that one could decide to fly a very slow plane with an unstable location of the CG does not in any way contradict this. In particular, one would not want to send the message to a beginner that stability, and hence CG location, does not matter. If Dick had made the observation that a sufficiently light plane could fly slowly enough to be controllable despite static instability, I would not have taken issue.

Your comment about the faster wing generating, or attempting to generate, more lift is not relevant to the topic of CG placement's effect on stability. I read all the small words.

It's interesting that you accuse me of not questioning theories or opinions, while the cause of this little exchange is the fact that I questioned Dick's. Is it only well established, proven theories that I should question?

banktoturn

RE: basic aerodynamics
 

OK banktoturn,

lets get on a friendly note. I realized what I wrote after I sent it and you are correct and I am wrong in that you did enter this debate questioning hanson's theory. However, I am still correct in saying that speed does play a part in cg location. If you are flying a fast plane you would rather have the cg more forward than aft. However, we are getting into a totally different realm. A more forward cg for a fast plane in relation to a slow plane would be more desireable than an aft cg. Especially if we are trying to make a sport airfoil (which tends to have better lift characteristics than an aircraft designed and built for speed) fly faster. Most of the RC pilots out there are sport pilots and find ways of increasing performance on the sport planes they are already flying. If I wanted a aircraft that would fly balistic I would build one for that intended purpose and nothing else. In this case the cg would be factored into the overall design of the plane and would resemble the % of most other planes in the different categories.

RE: basic aerodynamics
 

95tequesta,

I'm all for a more friendly note. Generally, a faster plane (a pylon plane for example) will be set up with a nearly neutral CG location (just a little ahead of the main wing's aerodynamic center), because this minimizes the load on the horizontal stabilizer, which reduces drag. A sport plane, whose pilot is less concerned with drag, would usually opt for a more stable CG location, or further forward.

banktoturn

RE: RE: basic aerodynamics
 

I fly a lot of models - and some are extreme in their power to weight and wingloading
So to explain to those who don't fly models and /or lots of different stuff- or who are new to all of this:
Here is my silly postulate

"if the plane is light enough -CG does not matter
If it is too heavy -it still does not matter."

Why would I say that?
Imagine a fixed wing aircraft that is so light that it is almost bouyant.
Add an engine -
It is still a fixed wing powered craft.
Power applied and control surface applications will do all of the controlling.
The CG is of little or no importance.
Stall is not going to happen.
Ludicrous craft?
perhaps
Now make the craft too heavy - If it is too heavy to fly , the cg is again, of no value.
Silly?
maybe to you -You fly it.
What I see - over and over is some poor newcomer who has a plane that is too heavy to perform and when asking why, he gets all kinds of theories and rules and worse yet - formulas presented to show why the model does not perform as expected.
The real problem typically -It is just too durn heavy.and or underpowered
the CG can be moved to place the crash site further down the road - but tell it like it is .
All of the model flying I have done shows that "the critical nature of the the CG increases with wing loading."
Be honest - the same thing is true with a 1940 DC3
On racers - we always kept it forward- to prevent easily increasing the AOA past safe limits
On aerobatic stuff -such as designs we did for pattern, or for the TOC ( and they were good enough to place and win ) to little foamies which are grossly powered feathers --I find that weight and power are far more relevant than CG
CG is of course important but first the weight and power must be addressed.
Then place CG parameters for that situation.

RE: RE: basic aerodynamics
 

Guys, Guys, Guys, (Any Gals here?),

You're ALL missing the basics: Airplanes (R/C, glider, full scale, rotorhead, etc) don't fly because of CG, Thrust, airfoil, etc.

They fly because of $$$$$$$

No bucks, no Buck Rogers...:D

RE: RE: basic aerodynamics
 

I thought it was a joke... or sarcasm...

Not only doesn't CG matter but engine, structure, controls, tail, aerodynamics, etc don't matter either. Any minor wind will make it crash and/or blow away. Start thinking about your next plane and stop chasing it.

Same thing but put all the pieces in the trash can before you leave the flied.

If neither of these cases are true, it (CG) matters.