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Speed,Combat,Scale,Racing => Speed Talk => Topic started by: Andrew Tinsley on October 22, 2014, 03:39:35 PM
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This is probably not a very smart question to ask. However I might as well carry on. If you use a symmetrical airfoil, then by definition the plane must fly at a small angle of attack. Hence increasing the drag.
Surely there must be some free flight type of airfoil that would produce sufficient lift and keep the model level and hence make it go faster.
Now this assumes that the free flight style airfoil doesn't have as much drag as the symmetric one at a slight angle. It also assumes that the lift is necessary at high rotational speed, maybe the required lift is very small?
I am sure that I haven't discovered a new way to go faster, so what assumptions do I have wrong? You usually learn faster when you ask stupid questions!
Thanks,
Andrew.
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Will try and answer this as simply as possible.. Get a brick tie a string around it and swing around your head.. Question does the brick fly?? yes amazing.. You only need a bit of lift at the start with most speed models then most of the time centrifical force will hold it up.. The smaller planes require slight lift but the more lift in a aerofoil the slower the plane will fly..
Robert
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That's not a wing! That's a streamlined fairing for the lines!
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1st 21SP I made had a wing with slight negative lift, not on purpose btw. It had to take off on prop power. If you didn't get it off the ground before it built up speed you were in trouble. Once it ran around on the dolly for a couple laps until it hit an old milk carton, then slowed down enough to take off. My next plane had a slight lifting section.
MM
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This is probably not a very smart question to ask. However I might as well carry on. If you use a symmetrical airfoil, then by definition the plane must fly at a small angle of attack. Hence increasing the drag.
Surely there must be some free flight type of airfoil that would produce sufficient lift and keep the model level and hence make it go faster.
Now this assumes that the free flight style airfoil doesn't have as much drag as the symmetric one at a slight angle. It also assumes that the lift is necessary at high rotational speed, maybe the required lift is very small?
I am sure that I haven't discovered a new way to go faster, so what assumptions do I have wrong? You usually learn faster when you ask stupid questions!
Thanks,
Andrew.
Hi Andrew,
well if you consider that if Team Racing models used the humble Clark Y section with its flat bottom, they would be at about -0.5 degrees angle of attack in level flight.
Now imagine where that would put a true Speed model that used a highly cambered free flight section?
(I would guess it may well fly level with -2 or 3 degrees!)
It seems far better to simply use a symmetrical section at those speeds to present less frontal area for drag and stay right away from critical high lift sections.
There was something about compressibility at those speeds too - can't remember off the top of my head though.
Cheers.
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That's not a wing! That's a streamlined fairing for the lines!
Closer to the truth than humorous!
Some mind exercises:
1) A speed model flying level at an altitude exactly equal to the pilot's hand. The centrifugal force is exactly countered by the tension on the wires. The weight of the model is supported by lift generated by the model.
2) Same model flown at a level below the height of the pilot's hand. Same centrifugal force but now a part of the weight of the model is being supported via the lines by the pilot with correspondingly less by the lift of the model. Tension on the lines will be higher.
3) Same model flown above the pilot's. Lift and induced drag must be higher since the pilot is actually pulling the model down.
Most speed flyers will tell you that a model is faster when it is flown very low. There are reasons. It's all a matter of vectors. And real physics!
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Most speed flyers will tell you that a model is faster when it is flown very low. There are reasons. It's all a matter of vectors. And real physics!
Ground effects Bill?
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Ground effects Bill?
Yes, that considered as well.
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Thanks guys,
It is good to do a little real physics now and again, certainly better than theorising!
Andrew.
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Every airfoil section is evaluated Lift / Drag or as the Wright Brothers defined it, Lift versus Drift. Just because a symmetrical section needs a certain angle of attack to generate positive lift does not necessarily make it a drag burden. There will be drag with any section. The thing that needs to be understood is the out of trim drag generated when the plane is trimmed to fly level. Lifting sections like the venerable Clark Y will likely need some elevator to fly level. This section needs to be at a negative angle of attack to generate zero lift.
My D Speed model with a wing section along the lines of a NACA 63-010 flew with no discernable trim input. It worked.
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It is good to do a little real physics now and again, certainly better than theorising!
Speaking of real physics, I ran the numbers and came up with a 50g of centripetal acceleration for a 200mph speed ship on 60 foot lines, which would mean that it balances with a line sag of about 1.3 degrees or so. 12.5g and 5 degrees for 100mph.
But I'm not sure I'm doing it right -- 50g seems like a lot, and all the speed guys I know have right arms the same length as their left.
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Hi Andrew,
Actually there are differing opinions on speed plane wing (& tail) airfoils. MOST of them all work. Your most excellent question is: WHY?
I have seen lifting to symmetrical, straight symmetrical, & semi-symmetrical w/ undercamber airfoils all used on the same type of model and they all work!
My experience is that a 2/3rds -> 1/3 semi-symmetrical airfoil to be slightly better, & reasonable to build by "Joe -Average" i.e myself! Such a wing for starters can be installed at zero degrees incidence. PERFECTLY STRAIGHT models always fly better & faster than (slightly) crooked models. If you laminate sheets of wood with a fiberglass (or carbon) center, then you can sand to shape and your center line is maintained (& determined) by the cured hard cloth. Keeps everything nice & STRAIGHT. OR, with a power disc, mill a slot around the entire wing periphery & glue in carbon pieces.
That works well too.
From perhaps 15-20 years or so ago, the very best builders molded composite wings, with & without solid cores. Often CNC'd aluminum milled molds are involved. They are the most accurate, lightest and aerodynamically the best as everything ( i.e. undercamber) can be very carefully controlled. I envy the one (or two) people I know who can do this kind of work...
One of the problems about theorizing, it that a number of things are happening that are not evidently apparent. So we go off making assumptions without ALL THE INFO that we really need..
R/C glider people & the like have made careers out of studying to find the "perfect airfoil". The struggle continues ....still!
Flying height is also not so well understood. Jari Valo would have won this years world F2A speed champs with the highest speed but he was DQ'd for flying too high! Paul Isner & myself have also suffered the same DQ at W/C's competitions losing what was going to be our fastest flights... It turns out out perceived aerodynamic loss is overcome by flying a slightly shorter flying circle. So the model might be slower (flying high), but it's overcome by the shorter distance traveled, resulting in a lower elapsed time....possibly? All fascinating stuff!
Cheers, Paul
This is probably not a very smart question to ask. However I might as well carry on. If you use a symmetrical airfoil, then by definition the plane must fly at a small angle of attack. Hence increasing the drag.
Surely there must be some free flight type of airfoil that would produce sufficient lift and keep the model level and hence make it go faster.
Now this assumes that the free flight style airfoil doesn't have as much drag as the symmetric one at a slight angle. It also assumes that the lift is necessary at high rotational speed, maybe the required lift is very small?
I am sure that I haven't discovered a new way to go faster, so what assumptions do I have wrong? You usually learn faster when you ask stupid questions!
Thanks,
Andrew.
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The critical point in understanding all of this requires an understanding of the importance of the Chord Reynolds Number. For our small wing speed models the Reynolds Number is relatively low in magnitude. Flow over the wing tends to be laminar and easily separates from the wing surface. So, the air doesn't much care what the airfoil section really is.
The Reynolds Number, a dimensionless quantity, brings together the inertial or mass effects of the fluid (in this case air) with the viscous effects. Its magnitude predicts the transition between laminar flow and turbulent flow. It is a bit complicated. Brew a big pot of coffee or whatever and start Googling.
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Just to add to this, aren't most speed models quite nose heavy?
Does this determine wing section shape at all?
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Hi Chris,
Even I know the answer to that one. They are really nose heavy because gyroscopic effect will try to lift the nose of a speed plane.
When I was talking about free flight sections. I was NOT talking about highly cambered sections, even I am not that dense. I was thinking more like a flatish bottom and the normal speed section on top. I have done the brick on the string calculation and realised you don't need much lift at speed! I was thinking that it might just give you the odd mph advantage with the right section. Looking at some of the FAI results, that could easily a mean a place or so difference.
I appreciate that the major tricks have been learnt by now, so to go faster will need more HP and just any minor tweak that can be done. My query was really based at the latter aspect, I.E. the small tweaks.
Has anyone actually done any experiments with wing sections? The ones used now, appear to be little different from those of the 1950s, seems strange that the old practitioners got it right first time.
Andrew.
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Carl China covered this subject in his construction article for the Viper II B Proto published in the September 1966 issue of Model Airplane News. He also did a presentation on the subject at one of our club meetings. It was very informative. I have the pages scanned but they are too large to post. I don't know how to resize them right now.
In a nutshell, he demonstrated that a thin symmetrical airfoil was the optimum solution. His Viper II required a coefficient of lift of 0.04 at design speed. The wing generated necessary lift at +1 deg angle of attack.
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Hi Chris,
Even I know the answer to that one. They are really nose heavy because gyroscopic effect will try to lift the nose of a speed plane.
When I was talking about free flight sections. I was NOT talking about highly cambered sections, even I am not that dense. I was thinking more like a flatish bottom and the normal speed section on top. I have done the brick on the string calculation and realised you don't need much lift at speed! I was thinking that it might just give you the odd mph advantage with the right section. Looking at some of the FAI results, that could easily a mean a place or so difference.
I appreciate that the major tricks have been learnt by now, so to go faster will need more HP and just any minor tweak that can be done. My query was really based at the latter aspect, I.E. the small tweaks.
Has anyone actually done any experiments with wing sections? The ones used now, appear to be little different from those of the 1950s, seems strange that the old practitioners got it right first time.
Andrew.
Agreed, as shown here mate.
http://www.supercoolprops.com/articles/gyroscope.php
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someone forgot to tell Bill Wisnewski about that as most of his planes ballanced back about 40% of the cord at the fuse and then used a big stab
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So now it's time to bring in Static Margin and Tail Volume Ratio. Brew more coffee.
I fly C/L Speed Clockwise. Can't say I ever felt that the plane was trying to drive itself into the ground.
Also, C/L Speed models are not bricks on a string. They are real airplanes and they really do fly.
Brick on a string only works because all of the energy necessary to run the system is supplied by the human at the center.
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Brick on a string only works because all of the energy necessary to run the system is supplied by the human at the center.
Funny that, because that is exactly how a speed model 'flies' before coming on pipe.
Both are whipped more than a lazy racehorse! ;D
Cheers.
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Funny that, because that is exactly how a speed model 'flies' before coming on pipe.
Both are whipped more than a lazy racehorse! ;D
Cheers.
With all due respect, your comparison is a fallacy.
An F2A Speed model, to which I assume you are referring, takes off on its wing. Yes, it may require a bit of "arm" for some added energy since the engine is running below resonant revolutions. But, the plane is flying. Whipping is required to assist the over propped engine to come into resonance. Since we all know that whipping during the speed run is illegal the basic aero forces of Lift, Thrust, Weight, and Drag are in charge. That is all. No more, no less.
The brick never flies. It is supported solely by the vector resultant of the force due to the earth's gravity and the forces of circular motion as constrained by its tether with energy provided by the strong arm.
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Hi Bob,
I didn't claim that speed models are bricks on strings! They have to fly to get up to their maximum speed. At speed all I was saying was that the aerodynamic lift vector reduces in value as the speed increases, I suppose that means the angle of attack reduces.
I am a little lost for words at your experience when flying clockwise. First reaction is that the opposite effect would prevail re the gyroscopic effect. I must go away and think that one through! As Paul rightly says "There are lots of things going on during a speed run and not all of them are factored in" when we try to do a simple analysis of what is going on!
The fact that Bill Wisnewski had his CG as far back as 40%, certainly made me start! Still I am only trying to get back into the game after a 50 year layoff, so I am not surprised that there are things that I have got wrong! It is intellectually stimulating as well as very pleasurable in the build phase. Glad I decided to give it a go, before my illness prevents me doing so!
Thanks everyone for a very interesting series of replies. It looks as if I need to correct some longstanding assumptions!
Andrew.
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Andrew,
You're right, there are a lot of things happening during a speed run. The good thing is that it's not magic.
For starters, approach the problem from a Systems Perspective. The system in this case is the pilot, line(s), airframe, and engine. Look at the basic factors of flight - Lift, Thrust, Weight, and Drag.
The F2A and F2C models offer a lot of good design practices. Frankly, most of the traditional Speed models were probably not very efficient.
"Science is the great antidote to the poison of enthusiasm and superstition."
~Adam Smith - Wealth of Nations
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."
~Lord Kelvin
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Andrew,
You're right, there are a lot of things happening during a speed run. The good thing is that it's not magic.
For starters, approach the problem from a Systems Perspective. The system in this case is the pilot, line(s), airframe, and engine. Look at the basic factors of flight - Lift, Thrust, Weight, and Drag.
The F2A and F2C models offer a lot of good design practices. Frankly, most of the traditional Speed models were probably not very efficient.
"Science is the great antidote to the poison of enthusiasm and superstition."
~Adam Smith - Wealth of Nations
"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."
~Lord Kelvin
Or equivalently,
"If I can't take a bite out of it, it doesn't exist."
~Ted Nugent
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"I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely, in your thoughts, advanced to the stage of science, whatever the matter may be."
~Lord Kelvin
So, Lord Kelvin didn't have many girlfriends then?
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Lord Kelvin did marry.
But, he did err on a few things, especially considering the subject of this particular thread.
"I am afraid I am not in the flight for “aerial navigation”. I was greatly interested in your work with kites; but I have not the smallest molecule of faith in aerial navigation other than ballooning or of expectation of good results from any of the trials we hear of. So you will understand that I would not care to be a member of the aëronautical Society."
~ Lord Kelvin