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What makes models zoom

Is it to do with wing section, or not

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John Cole19/01/2009 15:56:28
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In February 2009 RCM&E there are I think two references to models zooming: Peter Miller on page 46 in “To the Drawing Board” and Lindsay Todd on page 64 in “Zulu E” – though he actually refers to ballooning, which I think is the same thing.  In both cases, these flight characteristics are blamed on the use of flat-bottomed wing sections. I’ve heard this statement many times in the past, but I’m not convinced it’s true. In my opinion, flat-bottomed sections have only one thing in common: they have flat bottoms, and so can be built easily on a building-board. They have nothing in common aerodynamically. Flat bottom sections often have cambered centre-lines, but not always: flat-plate sections are flat-bottomed but symmetrical and the same applies to Eppler 472. As they’re symmetrical, they have flat tops too!

From an aerodynamic (as opposed to building-board) point of view, I see three main classes of aerofoils: symmetric, cambered and reflex. These characteristics all really refer to the shape of the mean-line, around which is drawn a “streamline” shape or body, giving the outer shape. The issue raised here is I think whether a cambered mean-line causes zooming (whist symmetric does not). I can think of no aerodynamic reason why it should, as the only two substantial effects of the camber are angle of attack for zero-lift (and minimum-drag) together with the nose-down couple (or, as older readers may know it, Centre of Pressure movement).

I suspect the zooming is more to do with C of G location (and the consequent longitudinal dihedral), but what do others think? Do any full-sizers know from their book-learning? And please don’t say semi-symmetric in your replies: it’s a contradiction in term. Things are either symmetric or not, there’s no half-way state.

Gemma Jane19/01/2009 17:56:26
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John I cannot give an informed view as I have only recently come across the issue though I can give some food for thought and general observations
 
Firstly, if we are talking aerofoils I think we  have to lose the term 'zoom' it must be ballooning as a 'zoom' generally refers to  aircraft gaining height from excess momentum.
 
Secondly, the flat bottom aerofoil is not often encountered on full-size.
 
Generally during my current research into aerofoils used for models I have read comments referring to flat bottom aerofoils such as 'difficult to trim', 'best avoided' etc.
 
Lets look at the dynamics, the usual logic is that as the aoa increases so does lift, the CP moves backwards. Giving a nose down pitching moment (assuming the centre of gravity is where it ought to be) and a nice stable situation.
 
 What is often observed though during analysis of aerofoils is that the CP moves forwards before it moves back.
 
I have a feeling that flat bottom aerofoils may have undesirable traits in this critical area of the CP movement, if it does move say a long way forward or rapidly forward it may be introducing a nose up pitching moment which will further increase the aoa and could give an undesirable 'balloon'. One could then assume as the lift increases that eventually the CP moves backwards bringing the nose down moment into action but not without an untidy balloon in the process. Certainly the 'ballooning' must be associated with an increase in aoa one way or another.
 
As said food for thought not an informed view, nothing but some informed guessing! I look forward to others shedding light.
 
Certainly C of G location will have a profound effect if there is anything in what I have proposed in the above assumptions so I await with interest the replies!
 
 
Bob Cotsford19/01/2009 22:29:55
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in my opinion (empirical experience only, no theoretical background) it's because flat bottomed sections are normally associated with sedate trainer type models, with appreciable longtitudinal dihedral.  These models are balanced relatively nose heavy to reduce their sensitivity to control inputs, which makes them speed sensitive.
A less well controlled turn without enough up elevator gives a gain in speed, straighten out of the turn and the excess speed produces a zoom climb. Single channel models were the original culprits as there was no elevator control.
 
Now, where's PDR with the true facts? 
Eric Bray19/01/2009 23:43:59
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Close, Rob. The wing section is less significant than the longitudinal dihedral, which forces the model to fly at one (trimmed) speed. Slower, and the nose dips, faster, and it tries to loop, because it is now out of pitch trim.
If you have a model with excessive longi di, and slow it down, the nose will dip, the model will accelerate, the nose will lift, the model will slow, the nose will dip, etc. The motion is called a phugoid oscillation.
The same if you increase power somewhat. The nose will lift, the model will climb, then slow, allowing the nose to dip to level, etc.
Obviously, with too much longi di, the model will stall, and incorporate a turn or roll into the gyrations, the novice panicks, hauls back on the 'up' stick, and makes matters worse!
Gemma Jane20/01/2009 11:48:27
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I've been digging deeper, many model sites talk of 'speed sensitivity' of flat bottom aerofoils which made me wonder why. I have found confirmation that the CP is not moving conventionally on a flat bottom aerofoil, it does indeed move forward as the aoa is increased as I guessed it might, in fact it may continue to move forward all the way to the stall.  So John there is a mechanism which could point the blame at the aerofoil, this would have been found empirically but I can picture exactly why it would be so on a foil with a drag ridden lower surface.
 
Though I'm looking at the aerofoil in isolation the normal pitching couples of nose up with power and nose down without, which are usually desirable and stable become something quite different when one is considering a section that is promoting its own stall and therefore reversing one of the couples involved.
 
The question is would these type of models be displaying the same traits given the factors Eric and Robert have given if the foil was changed to a foil with a non flat bottom, my guess is no the model would behave better and be more forgiving, being less sensitive to speed changes and longitudinally more stable.
 
Simon Chaddock20/01/2009 15:52:54
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From my own experiments I can only agree with Gemma's comments on the 'speed sensitivity' of flat bottomed sections or to put it another way at reasonable speeds a very small change in the angle of attack produces a larger increase in lift when compared to "reflex" type aerofoil.
This characterisitic makes it more difficult to 'round out' nicely on landing and also makes the model more sensative to the effects of longitudinal dihedral and thus more difficult to trim for constant level flight.
 
Having said all that it does not mean that flat bottomed sections are inefficient, indeed, all other things being equal and if you can live with the characterisitics, their L/D ratio can be better than a less "speed sensitive" section.
 
Well that my two pennies worth!
 
 
Former Member20/01/2009 18:48:23
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Peter Miller20/01/2009 19:36:51
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Just a couple of comments. I don't want to get involved in the long and obviously learned discussions going on.
 
To say that Flat bottomed sections are not often encountered in full size isn't quite true, in the 30s Clark Y was a very common section, Also some of the USA sections were used. The Curtiss P-6E used Clark Y.
 
I use a flat bottomed section quite often.I used to use Clark Y but now use a NACA section, 3412 or 3514 etc.
 
I set them at 0 degrees incidence and they work exactly like a symmetrical section. No zooming and in flight you cannot tell that they are not symmetrical when flying inverted.
 
The zooming or ballooning is caused by the extra speed combined with the angle of incidence creating extra lift usually when coming out of a turn if the nose has been allowed to drop in the turn.
 
My aerodynamic knowledge is limited to Kermode and experience with my own designs.
Gemma Jane20/01/2009 19:50:06
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I did consider after I had posted that someone would say the Clark Y  was a common section, so lets correct that to not often encountered on modern full size designs. I could go into a vast discussion of the history, but lets not.
 
Peter what you have given as an explanation of zooming or ballooning is entirely pilot induced, the pilot allows and then observes the nose has gone down and pulls back on the stick, the lift equation takes care of the rest. It could happen with any aerofoil section.
 
 
Eric Bray21/01/2009 18:06:30
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Is the Clark Y a true flat-bottomed section, as forward of the spar it is convex!
To my way of thinking, a flat-bottom wing is flat from the aft end of the leading edge, right back to the tip of the trailing edge.
(Retreats to dugout after stirring pot)
Gemma Jane21/01/2009 18:15:11
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I would say not Eric, it is only flat from about 25% back from the LE. I would put the Aquilla in the class 'flat bottomed' but then again I guess we could have a whole thread on the definitions used for aerofoils in modelling, they are all a bit different to what I'm use to.
Clark Ross26/01/2009 13:28:16
34 forum posts
a raised entry on a flat bottomed airfoil can make a big difference depending on how far it is raised. and a rounded leading edge helps to seperate the airflow while changing angles of flight. having the corect stabilizer size helps to. back in the fifties a close friend of mine larry conover won the world chapionship fai free flight with a model called the lucky lindy.
it had a flat bottomed airfoil with a sharp leading edge that had a glide ratio that had to be sean to be believed. but it was specialised for free flight and would not have worked well any where else
Evan Pimm30/01/2009 10:48:26
146 forum posts
Really chaps, we must get away from this 'flat bottom/semi symmetric etc' nonsense and start to use 'cambered sections' and 'non cambered sections'. Then it becomes much easier to understand this 'zooming' thing. The camber line of a section (that line that separates the streamline section into equal areas above and below) gives a clue to how much drag and/or lift the section can generate (in broad, laymans terms) and how 'speed sensitive' it may be. Generally the greater the camber, the more 'lift'  (and drag) the section can produce at an airspeed. That being so, the greater the difference in lift and drag with changes in speed. So, if your turn in any direction in relation to the wind direction (the aeroplane does not know the wind is blowing) results in a speed change, then the model will climb or lose altitude, depending whether you allow the nose to drop or rise during the turn. In the full size there is both an ASI and Altitude indication, and it is relatively easy to turn without changing you airspeed, but with models this is not easy, the best indication is the engine note, if it speeds up then your airspeed is increasing, and the model will 'zoom' when you straighten up, and if it reduces, then the model will lose speed and altitude when straightening. The whole thing is pilot induced, and practise is required to get to know just how much 'back stick' is needed to produce a smooth, level turn. Obviously a non cambered section is much less sensitive to this malady, and another good reason to use such a section on almost any sport R/C type.
Evan.
John Cole30/01/2009 17:58:26
615 forum posts
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Gemma: modern terminology is to describe the lift forces on a wing surface as a couple (or moment) plus lift, and not to talk about centre-of-pressure.  Symmetric-section aerofoils do not display this moment (this is equivalent to the CoP not moving at varying AoA).  One reason this terminology is preferred is that it is simpler, as for most aerofoils with a cambered  midline the two descriptors can be an AoA-related lift force running through the 25% point, and a couple which is independent of AoA (but both of these DO of course vary with airspeed: roughly in line with the square of the airspeed).
 
The couple for a cambered section is NOSE-DOWN.  See the appendices to the excellent paperback Model Aircraft Aerodynamic by Martin Simons.  As a plane dives and the speed increases, the nose-down couple increases.  So that's not the explanation.
 
A second reason that CoP fell out of favour derives from the two characteristics I've described.  As the AoA reduces the couple remains the same but (at constant airspeed) the lift decreases.  If you represent this by CoP movement, the CoP moves backwards as AoA decreases.  At very low AoA the "CoP" is aft of the trailing edge!  Unless you believe in magic (or action-at-a-distance) this all starts to sound a bit silly!
Terry Whiting07/02/2009 16:13:36
154 forum posts
Evan,
The term SEMI SYMMETRICL has been used by builders of full size and models for as long as I can remember. I have been modelling almost sixty years, and you are the first person I have heard of that has a bee in his bonnet  about the term 'semi symmetrical'  
 
What is this 'camber' line you mention which is able to devide a rib section into two equal areas? please do not tell me it's STRAIGHT  line from the tip of a rib to the centre of the trailing edge.                                                                             

 

Gemma Jane07/02/2009 23:33:02
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John, it is interesting that you can dismiss the explanation based on the fact that you do not see my terminology as fashionable and then disregard it by discussing an entirely different analysis based on entirely different criteria. A flat bottom section displays a nose up couple with increasing alpha, I had not considered V at all nor did I need to (neither do I need a lesson on basic aerodynamics).
 
Though it is logical to consider lift in terms of camber, it is entirely illogical to completely ignore the physical attributes of a given section. I will refer Langley Research Center to Mr. Simons book, they will be fascinated to see that they have it wrong, whilst obviously he is right, as it was the LRC with whom I checked my assumptions.  
John Cole08/02/2009 08:46:42
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TJW: semi-symmetric is a contradiction in terms.  Something is either symmetric or it isn't.  So I agree with Evan.

What's the camber line?  Think of an aerofoil as being built up from two components.  The first is the centre-line - the line which goes from the leading edge to the trailing edge and is always half-way between the upper and lower surfaces.  In a symmetric section this line is flat.  In most other wing sections this line is curved.  The second part is the "streamline" section "wrapped round" this centre-line.
 
The job of this "streamline" section is to stop the airflow becoming turbulent and detaching from the (top surface of) the wing - as happens eventuially in the stall.
 
The job of the centre-line (or camber-line) is to get good lift with little drag, within the design flying speed range (and hence range of angle of attack).  A section with a cambered centre-line will usually give a better lift/drag ratio at normal angles of attack than a symmetric one.
 
Camber-lines can be of many shapes, and so can the wrapped-around "streamline" section.  The "peak" of the camber-line can be well back from the LE or it can be well forward.  Changing  the location of "peak-camber" and the pojnt of maximum thickness of the "streamline" section are the sort of things that alter the conditions where the wing "works best".
 
Camber lines can also have "reverse-camber" towards the trailing edge: a reflex section. 
 
For full-size aircraft and particularly sailplanes the choice of section can make a tremendous difference to performance, but this is less true at model-sizes.  The "best" wing sections lose some of their advantage over "average" ones at model-scale.
Terry Whiting08/02/2009 10:28:22
154 forum posts
JC,
I am really pleased I have been blissfully ignorant of all this theory when designing my own models, and to think the Wright brothers and all our earlier aviators only had birds
for their guidance.
How on earth did we ever manage FLIGHT.
Myron Beaumont08/02/2009 10:37:42
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I'll tell you what makes makes models zoom ! the topic under discussion I believe
.Too much speed for the wrong section for the type of aircraft & its wing section obviously designed for something that is there to provide max lift at a much lower airspeed .End of story  Eh ! & yes Gemma agree with everything you've said except that us old modellers know what we mean by semi-symetrical section ( in other words not quite symetrical  but the air has to travel slightly further over the top surface than the bottom of the wing which in practical terms makes no discernable difference to a model -Except to make it easier to fly inv erted  AMEN
Climbs off soap box - I have one of the few remaining ones left ! Very very old but maybe gets used too little nowadays
John Cole08/02/2009 10:50:20
615 forum posts
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Gemma: I'm sorry if my comments on terminology seemed to be a dismissal of your arguments.  They were intended only to show why CofP is rather a strained concept, and why modern wind-tunnel tests normally show Pitching Moment coefficient (Cm) instead, measured about the Aerodynamic Centre (AC - typically the 25% point).  Note: Wind Tunnel test results often show the Cm graph "upside down" - with negative Cm at the top.

However, let me comment on your arguments themselves.  You say "A flat bottom section displays a nose up couple with increasing alpha".  Well, yes it does if you measure it about the mid-point or centre of area, but not if you measure it about the AC.  In fact almost all sections do as you say, including all symmetric ones - about the mid-point.
 
The AC is defined as the point about which the pitching moment is invariant with alpha (in the usable range), and for symmetric sections the AC is on the 25% point and Cm is zero for all alpha (and hence invariant).  For cambered sections Cm is usually negative.  Negative Cm means a nose-down pitching moment.  Langley data for Clark Y gives a typical Cm of -0.06: nose-down.
 
Here's what Wikipedia says about the theory of thin wing sections: Wikipedia: Airfoil
"  The moment about the 1/4 chord point will thus be,

 \ C_M(1/4c) = - \pi /4 (A_1 - A_2) .

From this it follows that the center of pressure is aft of the 'quarter-chord' point 0.25 c   "
 
As the CofP is BEHIND the AC, the pitching moment about the AC is nose-down.
 
Here's what Wikipedia about Pitching Moment: Wikipedia Pitching Moment
 
The graph shows the Cm for a Cessna 182, consistently negative.  The author quotes from Ira H. Abbott, and Albert E. Von Doenhoff (1959), Theory of Wing Sections, Dover Publications Inc., New York SBN 486-60586-8 :
 
" Pitching moment is, by convention, considered to be positive when it acts to pitch the airfoil in the nose-up direction. Conventional cambered airfoils supported at the aerodynamic center pitch nose-down so the pitching moment coefficient of these airfoils is negative.  "

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