adam_one

Dick: the main issue here is about whether or not the CG location affects stability.
The answer for me is still - yes, it does.
A computer controlled full-scale fighter aircraft, as you mentioned above, is not stable enough to be controlled by the pilot alone.

Your friend saved the model thanks to his skills and/or luck, as the model was not easy to fly in such circumstances due to the increased instability.

I really do like new things and new ideas, but one should not disregard basic physics principles.

LouW

I have read this thread with some interest and will add a few thoughts of my own (based on the “old theories�).

Does cg location affect stability and handling characteristics? Most definitely. There are limits of cg location beyond which an aircraft is no longer stable. This is true no matter how light or heavy or how much power is available.

Does it matter? It all depends. Unstable aircraft can be quite flyable. A good example is a conventional helicopter, which is an inherently unstable machine but is quite flyable.

Stability is merely the tendency of an airplane to return to the original flight condition after an upset. An airplane in free flight must have some positive degree of stability or it will quickly return to earth in some divergent manner. Once the element of control is added, stability becomes more of an optional factor.

An aircraft that flies on the wing is much easier to control with some degree of positive stability. For this reason, a trainer type aircraft (and most sport aircraft) need reasonably positive stability. An aircraft that depends more on thrust than lift for its flight can get by with being somewhat unstable as long as the response time is slow enough (or the thumbs are quick enough) to allow timely reaction to any upset. A typical 3D model can be balanced near the neutral point or maybe even a little aft. As long as the flight is in the hovering, or harriering regime, this lack of aerodynamic stability is really no factor. However in the normal flight regime, they must be attended to constantly. An aircraft with a very light wing loading can be flown slowly enough so that even clumsy thumbs can react, so that it may be flyable though unstable.

Dick Hanson’s observations are not too far “off the wall� but are adequately covered by the “old theories�.

rmh

Lou- I very much appreciate your input -as well as Adam's.
If Adam is from Sweden -he may even be a shirt tail relative
Papa was Hakan Anderson. An my uncles were Olaf , Axel etc..So I also am just another descendant of poor folks from Sweden.
Anyway - I am not a compete stranger to the physics of flying.
I never disregard the laws of physics -
I simply try to handle those involved.
BTW-the guy who landed the plane which had the engine "dismount"- won the TOC once and had two second places - so luck wasn't really a factor.
He also likes the funny little planes like mine.
They are a blast -
the great part is that if you bash them - you may be out -- five bucks?
So why not push the envelope ?
cruisin around in/with a Cessna is my idea of terminal boredom.

davidfee

My point is just that if he had let go of the sticks, that model would have crashed. It was only "flyable" because he significantly altered the trim to adjust for the new CG. Even then, it required constant attention. I trust he got it down immediately... I don't think he flew it around that way for fun.

We are coming from different angles... I want a model to be neutrally stable, if not slightly positive, while you are happy with divergent behavior. CG position, along with the associated trim, is largely what determines which condition you have so, in my mind, CG is important.

-David

rmh

I absolutely understand that - I designed,built ,sold and setup hundreds of pattern planes - really.
Typically , we could set them up for hands off level - then very small corrections for other attitudes . (Tipo's, Runarounds , Buckers, Dalotels EMC2 etc..)
A very neutral but solid flyer can be done with just about any design.
The CG is very important in these.
This stuff I am playing with is completely different.
BUT- they both fly.
My original point was that , given the right combos, you can take huge liberties with what has oft been consided a hard, fast rule.
Both "truth and fact" are moving targets.

antslake

My Feedback: (1)

WOW! that is just so wow.
Ready for me to prove you wrong?

1, if the plane is too light the CG still does matter. If you had a plane that wieghed 2 oz. and all the weight was in the tail, or the nose, it wouldn't fly coreectly, and pssibly not at all. Getting the CG right not only will make the plane fly better, it will make it fly more effiecently. Just because its flying doesn't mean its doing well. I guess if I throw a paper ball out the window, its flying for a little bit?
So CG matters if you want the plane to fly correctly and to the best of its ability.

2 if the plane is too heavy? No such thing. Just add enough power and the right wing and it will fly. After all thats how you are claiming that the cg doesn't matter, just add enough power and it will fly, you are contradicting yourself.

3 <see #2

4 Not enough power? well then the CG does matter, and so does the wing design. Again you contradict yourself.


If a plane weighed zero lololololol. Please please show me a plane that weighs zero MR.wizard.
Wing design is based on wings, that wiegh. Flying through air, in gravity. Its all part of the formula. Any other scnerio is not flying in the sense that these formulas are created.

Just because you sing, doesn't mean that you are in tune,
Just because you are flying, doesn't mean your doing it as best you can.


I am not an aeronautical engineer, but with common sense can point out that your theorys are flawed.
I think you need to re-word what your trying to say.

antslake

My Feedback: (1)

BTW, I love your foamie.

adam_one

Here's a cool video clip for those who have a broadband connection (size about 4.7Mbyte).
http://www.80electric.rchomepage.com/Su-31mik.wmv

rmh

Ponder this -you have to see the extremes in anything -even if you never use them.
1. A plane can be done at ZERO weight -
Rubber inflated aircraft -man carrying were once considered as a possible emergency vehicle.
Theoritically one could do a powered airplane which was simply ZERO weight - not lighter than air - just at ZERO weight -
2. too heavy is too heavy - you can add bigger wings etc - but then you have changed it--from being too heavy to simply heavy.
3.enough power nothin else matters - no matter how badly you bungle the size/shape/ cg/whatever , enough acceleration will keep it moving.
4. not enough power ? Then if it will not move -of what value are of the other parameters?
Honest- I do know what makes a plane fly - and fly well or badly.
I also like to look at what may seem to be absurdities- Some "absurdities" are the real answer to a question about what is wrong with a particular airplane.

Greg Morris

With a fully symmetrical wing should i be using aileron differential and why

rmh

Well - the two things are really not related -
Here are two examples :
a long span narrow chord wing - which -if flying at a fairly high angle of attack -in level flight - will need differential -or the DOWN aileron will /may drag heavily and just make that wing drag enough to cause a yaw - in the opposite direction of the turn..but....
A very short span large chord - extremely low wing loading setup -flying at a very low angle of attack- may not need any .
your plane is -- somewhere in between these extremes.
so - maybe your setup needs a little -or a lot -
You can calculate your a-- off read formulas till the cows come home - some of which is good readin but this is a cut 'n try on a model
.None of my aerobatic planes have any differential -low wing - raised wing etc - makes NO difference.

Greg Morris

Thanks Dick for your reply
I'm flying a Goldberg xtra 300.Some of my club members think i should use aileron diff. and some think i shouldn't. I have dialled in a small amount and will try it.
Thanks Greg

adam_one

Differential ailerons are used to prevent stall due to aileron drag.
Stall due to aileron drag is more likely to occur on landing approach with flat bottom wings.
When the pilot applies aileron to roll upright during low speed, the downward movement of the aileron on the lower wing might take an angle on that part of the wing past the critical stall angle.
Since differential ailerons will have the opposite effect when flying inverted (but you're not supposed to land inverted), most aircraft with symmetrical airfoils designed for aerobatics don't use this system.

some_cool

hey guys
well this is my first post...im an undergrad aerospace engg student...in my 4th semester...studyin in Pakistan!
n i want to know how can i make my own mini aircraft...wat are the requirements,wat sort of material shud i b requiring??
or just tell me some relative site..
thanxx
byee

raabid

some cool, go to www.foamyfactory.com or www.3dfoamy.com .

I'd just like to thank Dick for stepping out of the box. I've designed and scratch built 10 - 15 planes now, starting from when I knew nothing about aerodynamics to the present. Some of you guys need to loosen your grip on the slide rule, all Dick was trying to point out is that the CG isn't at the top of the list of reasons why your plane is flying well. A no time did he say that the CG on a 33% Extra, flying a .91 Saito radial 4 stroke, doesn't matter. Everything's is relative to the plane your flying, and what you want it to do.

Flying electrics I typically push my motors far beyond their recommended limits. Why? Because somone has to. I have baffled, confused, upset, several people telling them how I can fly a 4 lb airplane with the same motor they're flying a 2lb plane with. I'd rather be inventive and experimental, than sit back and be spoonfed universal truths. Otherwise the sun might still be orbiting a fixed, flat planet Earth.

raabid

Oh, I did have a question. My aileron rates while in a dive are below sluggish, how can I increase this?

rmh

OK I will giverago.
IF--- you have enough servo power AND your ailerons do not bend -there is still another potential culprit.
High speed aileron flutter.
Been there -
On an old 4star model - a guy had at the field - he complained that the aileron servo quit in dives.
I tried it and it was simply flutter - the noise was drowned out by the engine.
the strip ailerons never came adrift - they just buzzed until speed was reduced - then the aileron control resumed.
On foamies - I use a highly tapered aileron so that it will not flutter.
My Spitballs do super fast rolls even at full speed . over 720 degrees a second.

Aero Mech

Well, not to break up all ya'lls totally mind numbing discussion of cg placement and wing loading but I have a question. [&:] I have just finished my second scratch built plane and test flew it last weekend. upon take-off it went up and no amount of elevator could bring it back down. A guy at the field helped me get it down by several knife edges and stalls at idle. when I got it down he said I should shim the trailing edge of the wing up, or the nose of the engine down. Here's the question, which is better? wing angle of attack adjustment or engine angle of attack adjustment? BTW it's a OS 91 FX on a 60 size high wing model with a flat bottom wing

adam_one

Flat bottom wings are mainly used in slow and relatively light powered models.
A flat bottom wing often tends to "balloon" up into the sky, keeping climbing
when full throttle is applied or the model is overpowered.
You may start doing the adjustment you think is easiest to do (e.g. shim the trailing edge of the wing up).
Moving the CG slightly forward may also help but you'll get higher landing speed if it becomes too nose-heavy.

antslake

My Feedback: (1)

For Dick Hanson, You seem to be taking things out of their relativity.
You even said yourself that you know what makes a plane well or badly.

Planes that are too heavy, for their given wing design can be made to fly by either lightening them or making the wings bigger. Prbably if you had enough power you wouldn't even have to do either.

If a plane weighs zero, then to me its not a plane anymore, but a blimp. There is a difference, and would be when designing such a craft.

I don't think you have stepped out of the box at all, just out of relativity. No matter what you are claiming, you can still use design technics to make a plane fly more efficiently, and thats what basic aerodynamic design is for. Its not going to decide if the plane will fly or not, only how well it will fly, as you said yourself in other words.

If I throw a garbage pail off a cliff, it will fly for a little bit.

rmh

Unless you peruse the extreme /absurd.-or "out of the box" -how do you determine the real limits?
Example - a zero weight airframe -
I did not say it would float - like a blimp - just zero weight.
So -assume such an airframe -- now apply all of the well documented rules about airflow /airfoils etc.
DO THESE STILL APPLY?
Maybe- but they are far less important.
Have you tried the super light, high powered , flat foam aerobats?
These things will make you wonder why we ever bothered with some "rules"

Throw a pail?
That ain't flying- not even good ballistics.
My point was/is -that the hard and fast rules really are NOT hard and fast rules - just guidelines.
Example - we were once discussing wing construction.
One person commented on their own rules for good wing design and noted he had never had a wing fail.
My view was that -if this is the case - how did he know the real limits of his design?
Personally- I have broken and damaged a number of them - from air loads and vibration.
Each was looked at at part of learning the real limits.

adam_one

Dick,
I'm not sure if I've got your point yet…
Could you please define what an aircraft's stability is?
[I mean an aircraft that's heavier than the air (not balloons or feathers)]

Thanks

rmh

Define stability?
I have no idea . It is not a finite thing.
A stability that allow hands off flying that self corrects?
OR simply flyable -with constant corrections - some full scale stuff works that way also -
What is the stability setup for the twin rotor Osprey?
Is there one?
Or is it a moving target.

adam_one

Dick,
I think that the main problem here is just a matter of definitions.
There are the Static and the Dynamic stability.
Static stability refers to the aircraft's initial response when disturbed from a given angle of attack, slip or bank.
Dynamic stability refers to the aircraft response over time when disturbed from a given angle of attack, slip or bank.
Further there are:
Positive stability (tends to return to original condition).
Negative stability (tends to increase the disturbance - undesirable in most situations)
Neutral Stability (remains at new condition - does not move further away or closer)
If we mix them altogether we may get:
Positive Static stability, Negative Static stability Neutral Static stability or
Positive Dynamic stability, Negative Dynamic stability, Neutral Dynamic stability, etc.

To make a long story short, I would say:

For me, a totally stable aircraft will (after a disturbance) return, more or less immediately, to its trimmed state without pilot intervention.

However, such an aircraft is rare and not much desirable as we usually want an
aircraft just to be reasonably stable so it is easy to fly.
If it is too stable, it tends to be sluggish in manoeuvring, exhibiting too slow
response on the controls.

Too much instability is also an undesirable characteristic, except where an
extremely manoeuvrable aircraft is needed and the instability can be continually
corrected by on-board 'fly-by-wire' computers rather than the pilot, such as a
supersonic air superiority fighter.

So, an aircraft that needs constant corrections during the flight either done by the pilot or by a computer is not considered stable even if it is flyable.
That's why stability is not a moving target it is just stability...

rmh

Yes -I know that -
so that's why I asked.
You just said "stability".
and you ended with your opinon of stability.
By a moving target - I meant "what kind of stable situation."
Example: a plane is locked into a "stabilized turn"
OR
A plane is so "stable" it will return to a level attitude - no matter how much it is upset.
My little foam models are neutrally stable in darn near any attitude .
Have you ever flown any of these?