RE: Suggestion for moderators
 

Don't let the theories get in the way of making your Katana fly .
First -remember the task at hand.
That is: how big is the plane and what do you want it to do.
On small models -such as your Tigre 60's, wing loading is kept fairly light for best results -you could change the airfoil from 10% to 15% and see little performance difference
Also you could taper the wing some or keep it a rectangle -with no noticable differences in performance.
If you put in an extreme taper say 4-1 the results will change but -if you reduce the wing loading quite a bit as you make this change - you may find the model flies very well .
Trying to randomly pick a characteristic from a large 500,000 cargo plane, then apply it to a 5 lb model is absurd.
You did not say how large your Katana was to be -so absent that - it is hard to correctly assess how good/ bad either wing planform will perform. Having done /flown various Katana's - they are aerobatic setups and less wingloading is always better for powered aerobatic models full size or tiny models .
That is one hard,fast rule.
Bottom line -it is physically impossible to get your Katana too light and the lighter it is, the better it will fly AND the less critical it will be to CG positions .
CG does matter of course - but if you can't tell that a lighter model has a broader effective usable cg range - you simply have not tried enough size/weight combos.
If one flies the so called 3D models , it becomes very clear that flying with vectored power permits CG ranges that are completely off the textbook ranges for controllable flight -yet -these models fly extremely well.
I started modelling flying free flight and control line stuff, a loooong time ago so I do understand CG - both from a practical and a theoretical standpoint.

RE: Suggestion for moderators
 

Hi Dick

I have found that aerodynamics has very little to do with getting model and full size aircraft to fly. Its Money that makes 'em fly, pure and simple. :)

Carlos G
AKA
The BaronVonEvil

RE: Suggestion for moderators
 

Fairly new on RC Universe, but I have a formal education in aerodynamics, a practical education in aerodynamics and full size aircraft building, maintenance and piloting, as well as a long running education in flying models.

I've been cringing reading some people's claims on this thread. Some people are right, others are right but irrelevant, others have a half baked understanding and hence insult the theories proclaiming them to be useless whilst others still talk total nonsense.

I'd like to put some perspective on things.

Almost any aircraft that's been built was designed by:

Listing the NEEDS
Listing the WOULD LIKES
Doing some calcs
Reviewing the numbers with an experienced mind
Revising the calcs
Turn the calculated values into drawings (areas, moments etc etc etc)
Review and revise. Refer to NEEDS and WOULD LIKES
Fiddle the drawings to improve the visual appeal.
Prove the drawings by testing a model
Review results
Revise Calcs
Modify Drawings
Build a prototype
Assess the flying characteristics
Review findings and compare with calcs
Revise as neccessary
Produce

Anyone who thinks building an aircraft is only about TLAR is a fool. TLAR is only safe to use by the experienced. What looks about right on any particular airframe to one person may look distinctly wrong to another person with more experience.

No-one in the right mind builds an aircraft on looks then sets about finding out it's capabilities. Aircraft are built for function first, form second. There was little room for TLAR and trial and error on a project such as the B-2 or A380. These were designed with maths, confirmed with models then expected not only to fly, but to perform as per the numbers.

Anyone who doesn't understand aerodynamics, I appeal to you NOT to proclaim them as worthless.

Terminology - some people advertise their ignorance by using the wrong words to express their thoughts on engineering matters. If you don't know the "fancy words", then you don't know what you're talking about - SIMPLE!

GRIPE OVER

Of course, there comes a point when calculating becomes trivial with model aircraft because we still fly in full size air molecules. Models of full size aircraft can be tested in higher pressure air to compensate for differences partially, but there remains a need to confirm findings and expectations with a test flight programme. In addition, models are often extremely crude aerodynamically compared to full size aircraft (exposed servos under wings and all that really screws up the slip stream)

I back up the previously stated comments that there is a distinction between flying and flying well. Whilst some modellers may feel proud that they can handle an unstable model, others may simply view them as crap builders. The modelling holy grail lies somewhere between building a perfectly stable aircraft that happily flies without pilot intervention, whilst retaining the ability to be manouvered as the pilot wants.

May I state that "extreme flying" as some people call it (3D????) isn't actually flying at all. Neither are the full-size aerobatic counterparts. It's simply phenominal power and large control surfaces influenced by propellor slipstream. Nothing more. I'm not saying it doesn't require great skill, it certainly does, but it's not aerodynamic flying in the classical sense.

RE: Suggestion for moderators
 

((comments retracted))

Roger

RE: basic aerodynamics
 

I'm sorry, I know this post has been out there a while but I just have to say something here.

If the wing loading is zip, then the weight is zip. Then how do you calculate a 2:1 power to weight ratio? An engine capable of ANY thrust would result in an INFINITE power to weight ratio, which is meaningless. The reference to a 2:1 power to weight ratio on a weighless aircraft is nonsense.

RE: basic aerodynamics
 

Have to agree - the zero wing loading concept is impossible and totally trivial. It cannot be imagined because it doesn't make sense. The assumptions contradict one another.

As to having fun, well great. I for one care about aerodynamics, as do others or else this forum wouldn't exist.

If a question is asked, it deserves to be answered fully and correctly.

When I see things written which I know to be inaccurate or only partially true then I feel a responsibility to other readers to set the record straight.

RE: Suggestion for moderators
 

First sentence: You are still stating an approximate aft CG limit. All airplanes have their limits for acceptable controllability. A 50% limit means that CG very definitely DOES matter.

Second sentence: If the CG is so far back that the pitch oscillations are too large and/or rapid for a given pilot to correct, it will be a very short flight, even with a 100:1 power to weight ratio.

I think you have destroyed the original purpose of this thread.

RE: basic aerodynamics
 

You GO Jamie!

RE: Suggestion for moderators
 

Nice to see that you boys read !
The whole idea of my "uninformed bits of worthless formula" was to see who actually did fly and evaluate the capabilities of extremely low weight -high power models
apparantly you guys are not into that type of thing
The stark reality of the "non flying 3D models tho --is that --- they do fly -
The rules YOU may choose to believe are set in stone - may be OK for you.
However ; when the rule says one thing --and the results say another -- it does not take long to figure out that something is amiss.
I do understand the so called "classical " aerodynamics . And I do use them -all the time
To deny that other types of flight are simple abberations -is simply a flawed response.
They work and if you can not learn from them - it's your loss- not mine.

RE: Suggestion for moderators
 

That's exactly it though, high power, light weight 3D flying is simply thrust vectoring. Nothing more.

There's nothing else to understand. I don't think I'm missing anything as I already understand what physics allow that aircraft to perform those manouvers.

If you're only airborne because of sheer power and control authority then no, I don't suppose CofG really does matter quite that much.

RE: Suggestion for moderators
 

An odd way to find out who flies a certain type of aircraft: Make untrue statements about physics.

"To deny that other types of flight are simple abberations -is simply a flawed response." I'm trying to be careful here but it looks like you are trying to say that people should acknowledge that other types of flight are abberations. Is that what you meant to write?

No flight is an aberration. If it doesn't behave the way we expect it to, it's because we don't fully understand it. There is no escaping physics. If it flies the way you like, then so be it. It's not violating any laws, it's complying with them all. You don't have to understand them to have fun, but please read on.

Just because you can modify or design a foamy and it flies, or flies better, doesn't mean you know more than an engineer about aerodynamics, which you seem to be implying (Please correct me if I have misread). Anyone can do that, without understanding the physics thereof. I take no exception to anyone experimenting with foamies (or whatever) using trial and error or eschewing math and physics. I do take exception with people making untrue statements, and claiming to disdain physical laws that have been established for decades, and passing that off as "wisdom" for newbies.

If you understood "classical" aerodynamics, it seems unlikely you would have made reference to a weightless airplane possessing a 2:1 thrust to weight ratio.

I don't understand what rules you feel are being violated by lightweight planes. Their behavior can be explained by well understood principles, which is supported by reasonably good aerodynamic computer (mathematical) models in RC simulators available right now, as one example.

You also seem to be drawing unwarranted conclusions about what we are into or not, or what we learn or not, simply because some of us point out that you have made untrue statements about aerodynamics. I fly foamies too. I have also professionally flown (and helped modify with engineers; I'm not an engineer but I've studied it recreationally) very underpowered 900 lb UAVs with 27 foot wingspans, and a 210 lb UAV with an 18' wingspan that was powerered by a 120 CC engine, and many full-scale airplanes.

It would be good if more people could just admit when they have posted erroneous information.

RE: Suggestion for moderators
 

Looks like I wounded your ego - sorry about that -
You did however take literally what I said about zero weight - and cg- 2-1 thrust etc..

I was simply trying to explain a concept; "it the weight is practically zero ,the cg really can be abused thru the use of power to correct attitude /direction/speed."
I distain laws of physics?
How do you know that?
My comment about abberations Read it again .
I was referring to the comment that "extreme flying is not flying in the classical sense! (made in early post by Master Duff--)
Aerodynamic flying--- what does that mean?
If it flys -it flies
If it is a craft which is a compromise made to do a specific task ( a 747) or a toy which flits wildly about for entertainment - what is the difference ?
both fly
Oh here is a project that a friend is working on - I did some of the structural work -and have flown it -- probably not worth mentioning in this column -anyway --

RE: Suggestion for moderators
 

My ego??? That was a bit presumptuous

As far as taking literally the zero weight statement, you did write "zip", which to me means zero. You then stated a mathematical relationship between power (actually thrust) and weight that resulted in a "divide by zero" situation. Since this is a thread on aerodynamics, it pays to word one's statements carefully.

The above sentence about CG is a lot closer to reasonable than saying it doesn't matter for a light airplane. What might be even more accurate might be to say something
like, "As a given airplane lightens, the usable CG range increases", or something like that.

I read and re-read your sentence about aberrations. Still doesn't make sense. It doesn't read anything like your more reasonable and obvious statement above.

Despite the differences in the way some of us word things sometimes, it's likely that we agree on more points than we disagree on.

The tube is interesting. What is it's intended purpose?

RE: Suggestion for moderators
 

It increases lateral area -just another way of doing this
effectively the model -in hesitation rolls or very slow rolling agenda--will have more usable lift--in this area.
Please - don't read all this aerodynamic BS so literally
the concept is what counts
One can formulate till the cows come home
means zip
Unless you actually DO something and test it-- in real world -all the rest of it is simply up ending mouse turds.
frittering away time.
I don't have a formal education--and I have the patents to prove it .

RE: Suggestion for moderators
 

I don't doubt your accomplishments, trophies or that you might be a good stick. I'm sure you are creative and resourceful.

Some of your statements might be interpreted as disdain for those who choose to use mathematics to model the behavior of their as-yet unbuilt or unmodified craft, before even taking knife to wood. Whether this is a waste of time or not depends on many factors. It can be argued strongly that it actually saves a lot of time and expense by eliminating untenable ideas based on well-known principles. No sense trying ideas that are very unlikely to work. Of course, what's the point in designing if you are not going to eventually build and test? So it's all good. The geeks can calculate AND test to their heart's content, and the eschewers of math can perform trial and error all they want, as long as we are talking about our toys.

With full-scale aircraft design, math is imperative. Doing careful design work before building anything is much less expensive than building a blob and then trying to get it to fly the way you want through trial-and-error modification. The Wright Brothers used known aerodynamic principles to design, then built and tested before they attempted powered flight. They knew approximately what specific mathematical performance to expect from thier craft before they flew.

Does the tube fly as expected? Does it have the handling qualities you were shooting for? It looks interesting, I am curious about it's intentional snap roll characteristics.

RE: Suggestion for moderators
 

I just watched what may be an old bit of footage - of a Sukhoi- which does things I thought were usually only done by models -
The CG is obviously way back - the plane goes into/ out of departure stuff with relative impunity .
never saw any footage of a US craft doing these things -
maybe th whole thing is a foamie with good editing --
Sukhoi_SU_30MK.wmv is the file -if you can find it -

RE: Suggestion for moderators
 

I read an article about the SU26 years ago. The guy pulled ten g's in it without meaning to. I think the wing was built for 23 g's ultimate or something crazy like that. It has no pitch stability; the CG is near or at the neutral point. In fact, the author wrote that if you pull the stick back and let go, the plane will do loops until you push the stick forward again. I guess that's what happens when you have a government build an aerobatic plane.

<edited because I was mistakenly thinking SU31 which is similar to SU26>

RE: Suggestion for moderators
 

Just out of topic, where can I find that video Dick Hanson posted, I can´t find it.
Thanks

RE: Suggestion for moderators
 

I could have posted it on my web page - but figured it was something available on the internet and I am dumping th web page it is too much hassle.
I can't put a .wav on the RCUniverse. and don't want to convert it
I thot it was amusing -as I have had a lot of flack on how I really don't fully understand how real planes fly and how real planes don't do this and that and how real --- you get the idea - this guy does all of the things I have been admonished that real planes do not do --
funny --
Looks real to me and it is -
a very neutral craft -the flying stabs are wagging like crazy as he lands it - just like an old F16
Incredible show flight
I am at [email protected] - and can send it to anyone who wants a copy-

RE: Suggestion for moderators
 

Not sure this is the same video. Still impressive. You can definitely see the canards and stabs fighting to keep the thing from flipping out.


http://www.irkut.com/common/img/uplo...K_flight-1.wmv

RE: Suggestion for moderators
 

yes-- that is a portion of it maybe a 1/3

RE: Suggestion for moderators
 

Interesting that you would make a connection between this and RC planes. The Su-30 is so unstable in pitch in certain parts of the flight envelope that it would be very difficult if not impossible to fly without additional help. It has a fly by wire system in the pitch axis that affords it artificial stability so it will act like a conventional airplane. During the maneuvers that you see in the video, the pilot actually overrides the angle of attack limits normally imposed by the flight control system. In addition, although it looks like he enters and leaves departed flight with impunity, the maneuvers are actually carefully worked out. This type of aircraft typically is unstable in the linear part of the AoA range (at subsonic speeds) and becomes stable in pitch in the very high angle of attack range, but the lateral and directional stability is seriously degraded in the intermediate angle of attack range. The pilot applies a very high pitch rate so he can quickly shoot through this directional instability to the very high angle of attack range (70 degrees+) where the Su-series apparently becomes stable again in both the longitudinal and directional axis. Here the aerodynamic moments will actually automatically recover the aircraft. Certain versions in the Su range have thrust vectoring that help them attain the high pitch rates required. I believe the Su-30MK is one of these with the vectored thrust.

For fascinating footage of this so-called "supermaneuvrability", have a look at the joint US/German X-31 research vehicle. It does similar things to the Su series, but also in the directional/lateral axis. I believe you will find a link to images and video footage from the NASA website.

To get back to whether any of this applies to models: On models we can tolerate a slight negative static margin since the low inertia of models (especially your light foamies) keep the time-to-double of even a slightly unstable model within flyable limits. This is very different to the artificial stability required on the heavy fighter aircraft to make them flyable. If anything, the Su-30 is an example of how, when something works on a model, it doesn't always translate very well to full-scale or vice-versa. You can do similar maneuvers with your foamy - but the physics are different since the ratios between aerodynamic and inertia forces are different.

Another example of this difference is the modern highly maneuvrable aerobatic aircraft. Have a look at some of the tumbling maneuvers performed by the full-scale aerobatic champions. You will notice these tumbling maneuvers often give the impression of being more aggressive than we see on our models. The reason is that the propeller on the full-scale aircraft has, in relation to a model, a lot more rotational inertia. Many of those tumbling maneuvers depend on the gyroscopic effect produced by the spinning propeller when you apply very high pitch or yaw rates. The result is maneuvers that look noticeably different from what you get with a typical 3D model.

Full-scale and models can mimic each other's capabilities under certain circumstances, but there will always be differences due to the differences in inertia, aerodynamics, etc at the different scales. What stays the same, though, is that they all operate under the same laws of physics. That is why engineers and scientists don't spend much time during their training on the detail of how a certain airplane/bird/insect flies (except when examples are needed to understand certain concepts), but rather on the fundamental physics. Once you know the basic physics and how and why things scale in certain ways, applying it all to anything from a bumblebee to a B-52 becomes fairly straightforward.

RE: Suggestion for moderators
 

of course - the size,weight and airspeed are far greater -and the inertia is in a completely different world.
My point was - the attitudes the craft can operate in are the same -
If you do fly foamies -just for fun - you will find that the CG can be stuck just about anywhere on a really good aerobatic setup and --YOU --acting as the computer (what better computer is there?) , can do all kinds of nutty things with it - take a look on the German modelling sites for the guys who have developed push/pull propeller driven models to a fine art . The models actually back up -
The unusual antics shown in the SU30 were not simply done to amuse someone - they were part of a program to work out the possibilities of going outside the usual limits .
On my precision aerobaic -contest models - I get cg to within 1/2" (on a 25 lb model ) for best use of the design
On my 6 ounce foamies -I fly em with bout the same allowable varience .
Different ballgame .

RE: Suggestion for moderators
 

The first part of what I quoted from Oryx is an excellent explanation of why Dick's "CG can go anywhere" position works for light models. The CG governs static stability, but the dynamics (inertia and damping) are so benign that a human pilot can respond to and control the airplane. Light weight and a large tail are where it's at. The difference being that on the SU-30 the dynamics require a flight computer to respond quick enough -high wieght and slick airframe. But it is a good comparison by Dick, as it is essentially the same thing, with the object of both planes being maneverability due to low static stability. One twitches slow enough that a human can control, one uses 'power steering'.

Most people do not have a grasp of dynamic stability/dynamic response and cannot see further than the CG position in relation to stability. This is why the second half of the quote is so important. I have never seen physics/aerodynamics fail to explain any flying object. When I see someone claim this, it is doubtless someone who knows "just enough to be dangerous" without knowing how to apply all the rules.

RE: Suggestion for moderators
 

This thread is going for years and still not getting anywhere.

What about flat aerofoils on propellers of foamies? If profile of the wing doesnt matter, so should profile of propeller blade.

And if weight is zero, why one need wings? Helicopters have no wings and I bet they have greater than 1 thrust to weight ratio. They fly. Althogh CG placement probably matters for helis.

And hot air baloon fly too! No thrust and very lite indeed.