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Old 11-17-2010, 01:44 AM   #1
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Default Aerodynamics - does size matter?

I have built two similar planes but one is only 1/3 the size.

They have the same wing section (Clark Y) and wing loading yet the big one will easily do a glide loop whereas the small one will not.
No matter how steep or long it is dived it never gains enough momentum to carry it over.

In aerodynamics it appears size matters!
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Old 11-17-2010, 01:45 AM   #2
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Originally Posted by quorneng View Post
I have built two similar planes but one is only 1/3 the size.

They have the same wing section (Clark Y) and wing loading yet the big one will easily do a glide loop whereas the small one will not.
No matter how steep or long it is dived it never gains enough momentum to carry it over.

In aerodynamics it appears size matters!
Bigger flies Better...

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Old 11-17-2010, 02:23 AM   #3
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Originally Posted by quorneng View Post
I have built two similar planes but one is only 1/3 the size.

They have the same wing section (Clark Y) and wing loading yet the big one will easily do a glide loop whereas the small one will not.
No matter how steep or long it is dived it never gains enough momentum to carry it over.

In aerodynamics it appears size matters!
Don't know if its true or not, but they say that those full size military fighters have the wing loading of a cast iron man hole cover!
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Old 11-17-2010, 03:58 AM   #4
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Yes it does. Fluid dynamics does not scale up linearily so things with a low reynolds number behave differently than things that fly at a high reynolds number. Flying slow, or with a short wing chord, or in a more viscous fluid all tend towards low reynolds numbers, whereas flying faster, with a longer wing chord or in a thinner fluid tends towards high reynolds numbers.

It's one reason why insects can maneuver so well, but also why it's so damned hard to build micro-UAVs.
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Old 11-17-2010, 07:25 AM   #5
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Yes, as pointed out in the above post it's largely due to Reynolds Number changes (usually just referred to as 'Re'). Re number is proportional to flying speed multiplied by wing chord, so a smaller slower model will have much lower Re.

An airfoil operating at very low Re produces more drag and less lift than one at higher Re.. This explains the old adage 'bigger flies better'

Steve
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Old 11-17-2010, 12:41 PM   #6
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Originally Posted by kyleservicetech View Post
Don't know if its true or not, but they say that those full size military fighters have the wing loading of a cast iron man hole cover!
This grabbed my imagination!
After surveying available manhole covers on the net, I think a fairly typical example would be:

Manhole cover weight: 50 kg. Diameter: 60 cm = 0.28 m² for a wing loading of 178.5 kg/m².

F-16 wing loading: 431 kg/m² (88.3 lb/ft²) (from Wikipedia.)

So yeeeah. Those fighter jets are enormously heavy! Twice the wing loading of a manhole cover
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Old 11-17-2010, 12:48 PM   #7
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The next question. What sort of wing section works best at low Reynolds numbers?

From my days building small chuck gliders I found that a thin wing section had a better glide ratio than a thicker one. In fact a flat plate worked pretty well provided a rounded LE and a really thin TE was used.

So my small 20" span plane will have new much thinner wing to see if it makes any difference.
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Old 11-17-2010, 05:34 PM   #8
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Originally Posted by mesh View Post
This grabbed my imagination!
After surveying available manhole covers on the net, I think a fairly typical example would be:

Manhole cover weight: 50 kg. Diameter: 60 cm = 0.28 m² for a wing loading of 178.5 kg/m².

F-16 wing loading: 431 kg/m² (88.3 lb/ft²) (from Wikipedia.)

So yeeeah. Those fighter jets are enormously heavy! Twice the wing loading of a manhole cover
Wow! Thanks for the update

Guess that goes to show anything can fly, given enough power or thrust.
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Old 11-17-2010, 06:29 PM   #9
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And that good old manned missile the Lockheed F104 Starfighter was even higher at 514 kg/m² (105 lb/ft²) - over 2 manhole covers!
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Old 11-17-2010, 08:08 PM   #10
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Originally Posted by quorneng View Post
The next question. What sort of wing section works best at low Reynolds numbers?

From my days building small chuck gliders I found that a thin wing section had a better glide ratio than a thicker one. In fact a flat plate worked pretty well provided a rounded LE and a really thin TE was used.

So my small 20" span plane will have new much thinner wing to see if it makes any difference.
Yes, thin works best at low Re numbers. Prof Mark Drela has designed some terrific airfoils for use at low Re, primarily aimed at gliders but they would work just as well on power models. The AG-03 airfoil used on Mark's Epogee glider should work well: http://www.charlesriverrc.org/articl...gee36_wood.pdf
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Old 11-17-2010, 09:11 PM   #11
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Originally Posted by quorneng View Post
And that good old manned missile the Lockheed F104 Starfighter was even higher at 514 kg/m² (105 lb/ft²) - over 2 manhole covers!
Yep, anything can fly, even if only for a few seconds!

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Old 11-17-2010, 09:19 PM   #12
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Talking

Hi
Try telling that to the Humble Bumble Bee
Flight

Bombus pratorum over an Echinacea purpurea inflorescence; a widespread myth holds that bumblebees should be incapable of flight.


According to 20th century folklore, the laws of aerodynamics prove that the bumblebee should be incapable of flight, as it does not have the capacity (in terms of wing size or beats per second) to achieve flight with the degree of wing loading necessary. The origin of this myth has been difficult to pin down with any certainty. John McMasters recounted an anecdote about an unnamed Swiss aerodynamicist at a dinner party who performed some rough calculations and concluded, presumably in jest, that according to the equations, bumblebees cannot fly.[27] In later years McMasters has backed away from this origin, suggesting that there could be multiple sources, and that the earliest he has found was a reference in the 1934 French book Le vol des insectes; they had applied the equations of air resistance to insects and found that their flight was impossible, but that "One shouldn't be surprised that the results of the calculations don't square with reality".[28]
Some credit physicist Ludwig Prandtl (1875–1953) of the University of Göttingen in Germany with popularising the myth. Others say it was Swiss gas dynamicist Jacob Ackeret (1898–1981) who did the calculations.
In 1934, French entomologist Antoine Magnan included the following passage in the introduction to his book Le Vol des Insectes:
Tout d'abord poussé par ce qui fait en aviation, j'ai appliqué aux insectes les lois de la résistance de l'air, et je suis arrivé avec M. Sainte-Lague a cette conclusion que leur vol est impossible.
This translates to:
First prompted by the fact of aviation, I have applied the laws of the resistance of air to insects, and I arrived, with Mr. Sainte-Lague, at the conclusion that their flight is impossible.
Magnan refers to his assistant André Sainte-Laguë who was, apparently, an engineer.
It is believed that the calculations which purported to show that bumblebees cannot fly are based upon a simplified linear treatment of oscillating aerofoils. The method assumes small amplitude oscillations without flow separation. This ignores the effect of dynamic stall, an airflow separation inducing a large vortex above the wing, which briefly produces several times the lift of the aerofoil in regular flight. More sophisticated aerodynamic analysis shows that the bumblebee can fly because its wings encounter dynamic stall in every oscillation cycle.[29]
Additionally, John Maynard Smith a noted biologist with a strong background in aeronautics, has pointed out that bumblebees would not be expected to sustain flight, as they would need to generate too much power given their tiny wing area. However, in aerodynamics experiments with other insects he found that viscosity at the scale of small insects meant that even their small wings can move a very large volume of air relative to the size, and this reduces the power required to sustain flight by an order of magnitude.[30]
Another description of a bee's wing function is that the wings work similarly to helicopter blades, "reverse-pitch semirotary helicopter blades".
Bees beat their wings approximately 200 times a second. Their thorax muscles do not expand and contract on each nerve firing, but rather vibrate like a plucked rubber band.
Take care
Hank

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Old 11-18-2010, 01:47 AM   #13
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Thanks JetPlaneFlyer
At 36" it is quite a bit bigger than my 20" but the new thin wing has a similar section. A bit thicker at the root (8.5%) but thinner at the tip. (5%)
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We shall see.


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Old 11-18-2010, 04:28 AM   #14
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Originally Posted by JetPlaneFlyer View Post
Yes, as pointed out in the above post it's largely due to Reynolds Number changes (usually just referred to as 'Re'). Re number is proportional to flying speed multiplied by wing chord, so a smaller slower model will have much lower Re.

An airfoil operating at very low Re produces more drag and less lift than one at higher Re.. This explains the old adage 'bigger flies better'

Steve
Not exactly, Reynold's Number is rho*vel*length/mu. Decreasing both velocity and length changes the result proportionally. An airfoil operating at a very low Re will produce more drag and less lift if it was designed for high Re. Low Re flow is laminar with low drag whilst high Re flow is turbulent with higher drag. With an airfoil at high enough Re there usually is a laminar layer from the leading edge until the Re becomes high enough to become turbulent. A transition layer occurs (which can be a short separation layer) and the flow becomes turbulent. A turbulent layer is less likely to separate than a laminar layer due to the higher energy in the boundary layer. With a short chord at low speeds, the boundary layer does not have a chance to become turbulent before the curvature of the airfoil surface causes separation due to the adverse pressure gradient. If the airfoil is designed for low Re the boundary layer will remain attached for most or all the surface. You can make a high Re airfoil work better at low Re by inducing a turbulent flow into the boundary layer before it separates. This is commonly done on wind tunnel models by adding a grit strip or short cylinders perpendicular to the surface at the point where the full scale aircraft will have its natural transition to turbulent flow. Look at the wing of a Whistling Pig (1st generation Boeing 737). There is a line of short blades along the wing and up the vertical tail to induce a turbulent layer and delay separation.
Airfoils designed for high lift and low drag at low Re have a peculiarity, instead of the separation point slowly moving forward as the airfoil angle of attack increases (increasing drag and drop off of lift linearity), the flow remains attached until an angle where the whole surface separates at once. The result is instant high drag and no lift. This would cause an immediate snap roll and yaw to the side that separated first.
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Old 11-18-2010, 08:35 AM   #15
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Originally Posted by magno_grail View Post
Not exactly, Reynold's Number is rho*vel*length/mu. Decreasing both velocity and length changes the result proportionally.
That's what i said.. Proportional to chord x airspeed is the same as saying proportional to both chord and airspeed.

Also.. IMHO ALL airfoils produce better L/D at higher Re numbers than at low (providing we stay firmly subsonic). Low Re airfoilds like the Drela ones I linked have a low critical Re which means their performance is 'relatively' good at low Re but it's still worse than the same airfoil at higher Re.

Attached is a plot for the Drela AG-03 low Re airfoil with plots at Re= 50k 150k and 500k

You can see that performance is better (higher Cl and lower Cd) at higher Re than at lower, even for this specialised low Re airfoil.


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Old 11-29-2010, 04:41 AM   #16
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Originally Posted by kyleservicetech View Post
Yep, anything can fly, even if only for a few seconds!


HA HA HA

RE-POST

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Old 12-06-2010, 02:32 AM   #17
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Originally Posted by mesh View Post
This grabbed my imagination!
After surveying available manhole covers on the net, I think a fairly typical example would be:

Manhole cover weight: 50 kg. Diameter: 60 cm = 0.28 m² for a wing loading of 178.5 kg/m².

F-16 wing loading: 431 kg/m² (88.3 lb/ft²) (from Wikipedia.)

So yeeeah. Those fighter jets are enormously heavy! Twice the wing loading of a manhole cover
An F-16's combat wing loading is closer to double that. Most people don't realize the Viper carries about two B-17's worth of ordinance at about three times the speed, and puts it through a window about 85% of the time.
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Old 12-27-2010, 05:37 PM   #18
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Another couple of points. When you make a plane half the wingspan, you only have 1/4 of the wing area, so the weight has to be proportionally less.
Anything under about 30 in. WS, a flat plate wing is about as efficient as an airfoiled wing without the bad atribute of the airfoil's tipstall. You can gradually pull the stick back with the flat plate, with added power of course, and not even realize where the stall point is. Just a few small facts to be aware of.

Gord.
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Old 12-27-2010, 11:33 PM   #19
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Originally Posted by flypaper 2 View Post
Anything under about 30 in. WS, a flat plate wing is about as efficient as an airfoiled wing without the bad atribute of the airfoil's tipstall.
I'm not sure about that. You can certainly get a lot more lift and much better L/D from of a cambered airfoil than from a flat plate, and that includes low Re numbers like you would find on small models. Even a flat plate with some camber formed into it (such as a slow stick airfoil) performs much better than a plain 'flat plate'.
Perhaps if you are only comparing symmetrical airfoils then the difference is less but still a 'proper' streamline airfoil gives less drag, if no more lift, than a flat plate.

I plotted a few polars in Xfoil at typical small model Re to illustrate:


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Old 12-28-2010, 12:45 AM   #20
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Good stuff. Shows how close the flat plate is to the airfoiled wing. No guessing with good graphs.

Gord.
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Old 05-27-2014, 02:54 PM   #21
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Originally Posted by quorneng View Post
I have built two similar planes but one is only 1/3 the size.

They have the same wing section (Clark Y) and wing loading yet the big one will easily do a glide loop whereas the small one will not.
No matter how steep or long it is dived it never gains enough momentum to carry it over.

In aerodynamics it appears size matters!
One thing to consider is that the larger airplane has more energy stored up in forward momentum. Think about this you can throw a Basketball farther than a Ping-Pong ball.
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Old 05-27-2014, 03:21 PM   #22
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You can throw a marble further than a baseball... its got a higher density and less wind resistance plus less total weight so you can accelerate it to a higher speed with your arm.

The ping-pong ball has extremely low mass vs air resistance... its the old feather vs a bowling ball issue. With no atmosphere to give them any drag they will both fall at the same speed if dropped at the same time. the biggest factor involved in the difference of how far you can throw the ping pong ball vs the baseball is the rate of deceleration due to wind resistance. F=MA.

You can have a big bird that slows down faster due to wind resistance than an ultra micro sized model...

In fact the way that big 3D style gasoline models slow down when you chop throttle will surprise most glow power fliers because the props get to a lower "pitch speed" and act as a big air-brake disc. They can just suddenly "drop out of the air" on landing approach if you idle the engine.

What matters is mass vs drag for being able to do a loop dead stick. If you have too much wind resistance vs the acceleration due to gravity you can't dive fast enough to do the loop.
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Old 05-27-2014, 05:44 PM   #23
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Quorn.... use the old slope soarers trick ... add a bit of weight to the smaller model and then give it another go.

You would see an increased dive speed ... which results in greater store of potential energy, increased momentum, and the ability to actually complete the loop. Funny enough as you know I'm sure - if you try to pull the loop tighter - it screws out ...

Me - I look at things in a more simple way ...

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Old 05-27-2014, 06:01 PM   #24
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It could also be the freewheeling prop... adding drag and preventing the airplane from retaining enough energy to complete the loop.
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Old 05-28-2014, 01:29 AM   #25
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Originally Posted by Marty79 View Post
One thing to consider is that the larger airplane has more energy stored up in forward momentum. Think about this you can throw a Basketball farther than a Ping-Pong ball.
LOL
Unless you're the TV show Mythbusters, where they launch a ping pong ball at 1100 MPH Wonder how far that ball would go through open air?

https://www.youtube.com/watch?v=msgfm4DHiyc

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