This is intended to be an exercise by me for ME as an attempt to try to explain what is actually happening when these little dragons are whizzing around. I thought it might be fun and it is.Do not make any assumptions based on what is written here.
I choose the name "Farmers Guide" as a joke, something we used to say in New Hampshire when we were working on something we didn't understand but out of necessity had to try.
So it is with myself and airplanes.
Did you ever see those invisible dog gags? You know the one with the dog collar attached to a leash. The leash is stiff and curved so it looks as though you are walking an invisible dog. What makes the gag so believable is the curved leash. It looks as though it is hanging between your hand and the dog collar.
I am gonna make a point here.
How about an invisible C/L airplane? Imagine a handle with a wire [or wires] attached to a bellcrank [or a torque unit]. You could spin in a circle and tell people you were flying an invisible airplane.
You would have to make the wire curve to make it believable.
We have heard that the curve of a flying wire is determined by the following:
I think an invisible airplane would not be as much fun as an invisible dog.
That being the case, lets build an airplane to go with our wire.
If you will, a C/L speed plane is flying the wire, centrifugal [centrepidal- WHATEVER-ask Ralph] can keep the plane suspended as soon as a certain speed is achieved. The airplane is needed only to keep the engine pointed in the right direction and to land the plane after the engine cuts.
We will start with the weight of the plane- 42 oz. -obviously a D.
Next, the wire. Since we have a D, we will use .033 x 70' wire. MONOLINE, of course.
Since it is an imaginary D, lets make it a fast one. Say 200mph!
The first thing we will install is the torque unit. Since the wire is curved in flight, lets keep the unit in line with the curve so we will have no binding. The most important feature in Jim Walker's [with apologies to Ova] first C/L airplane was the bellcrank. The next most important feature was the wing tip line guide. So let's add the line guide next.
Let's put the line guide on the curved control-line. Going back to geometry, if we shoot a ray from our handle to the CENTER OF GRAVITY, we will NOT touch the tip guide. The tip guide placement then is essentially aft of where the "ray" enters the airframe. Therefore it is aft of the C/G.
How far aft?
We can use Bob Fogg's program to compute how far behind the C/G our line guide should be. In order to plug in we need a wing span. Let's go PINK LADY here, say a 28" span.
If you plug into his program, this gives us 100 lbs of line pull and places the leadout guide at .607" behind the C/G in order to keep the flight path TANGENT to the circle.
Next, Let's add the engine.
the engine must be a good consistent performer, as should the fuel system. Without this as a given, you cannot know how to set up your trim etc.We have read that an airplane is a machine that can be traveling in one direction while being pointed in another. When the thrust line of the engine is not parallel to the line of travel, this is described as YAW. If the thrust line represents power, then yaw results in a waste of power.
Some say that since the flight path on a C/L airplane is curved and the thrust line should follow that curve. Others maintain that the flight path is NOT curved, only the force exerted on the plane by the wire causes it to turn. Therefore the thrust line should be tangent to the circle, or a straight path. I am of a mind to side with the latter.
It takes more power to turn than to fly straight. Why waste engine power turning when the wire will provide this service? Fun huh?
We forgot to mention the propeller. What kind of prop on our D? That's a secret! But let's consider yaw and the thrust line again.
Let's say we are flying clockwise with a counter clockwise engine rotation. Imagine an ANGEL hovering above the path of our D and looking down at the prop. Let's FREEZE everything for a moment. Hopefully the prop is frozen so that the blades are straight up and down. If the plane is flying straight with the thrust line in line with the path of travel, then the top and bottom blades are of the same relative pitch to the path of travel.
If the plane is yawed to the path of travel, then the top blade has less relative pitch to the path of travel and the bottom blade has more relative pitch. Can this be good? A counter-clockwise flight produces the opposite effect.
Jet guys don't have to think about props, but a yaw still wastes power.
So, let's not waste any more power. Let's get that engine in-line with the flight path, tangent to the circle.
That done, it's about time to think about a wing. I cannot define a wing, but I can give examples. The stab, rudder and engine cowling are all wings, as much so as the "main wing". I do know that a wing can exert "lift" and therefore effect the flight path of our airplane. If you will, lift amounts to using air pressure exerted on the wing[s] in the same manner as a boat uses water pressure against it's hull. If you were able to see air pressure, you would observe that a wing passing through the air has various high and low pressure areas along it's surface. The wing's job is to use air pressure [and/or the lack of it] to defy gravity and/or to defy momentum. As good as lift is, it has one bad feature. Lift produces drag. More lift = more drag. The idea is to get lift to work for you and not against you.
If the airplane has sufficient speed, the combination of centrifugal/centrepidal forces alone can help to keep the airplane suspended in the air. Many old speed designs had very small wings in an attempt to minimize drag . While the idea seems sound, I have noticed that landing speeds of these are very high due to high wing loading. This can result in spectacular landings. To keep landing speeds to a minimum, a low wing loading is in order. This means that the wing area must be increased per given weight of an airplane. To many people, a larger wing equals more lift and more drag. It's true that increased surface area will increase friction, or parasitic drag. However, careful consideration of both angle of attack of the wing as it travels through the air and incidence of the various wings to the engine thrust line will reduce both parasitic drag and drag from lift. It appears as though a zero incidence plus a minimal angle of attack will minimize drag yet still allow for larger wings. Giving up trim after power cuts will change the attack angle, increasing lift and softening landings. Incidence to the thrust line is no longer an issue since you are now flying a GLIDER!I think that the size of the wing is not as critical to the amount of drag produced as is the way the wing works.
Since the wire can amount to 80% of the total drag, a long inboard wing serves another purpose. It covers up more line than a short wing. The line it covers is traveling further than any other part of the wire so it is sweeping more air. I've read that a streamlined cylinder can be 10x's thicker than an unstreamlined cylinder and produce the same drag. The wire is just a very long cylinder and the more you can cover it with a nice streamlined airfoil, the better. I am not convinced that the airfoil shape is that critical at the low speed's that we are talking about.
Now we must consider some additional things:
Since we believe that any change in direction of travel requires power and sacrifices speed, we must strive for a stable airplane, in speedster jargon, one that grooves.
Any mass [object] ROTATES around it's center of gravity and TRANSLATES[moves from point to point without rotation] through it's center of gravity, an airplane being no exception. Since rotation of an airplane will change attack angles of some or all of it's wings before it changes the direction of travel, we have to minimize unwanted rotation.
Common sense tells us that to keep the nose up on a nose heavy airplane we must give up elevator as a flying trim. This diminishes the ability to manuever as well as causing drag from the negative lift produced by the control surface.
The stabilizer is also a wing, capable of providing lift. The position of the stab in relation to the c/g, is that that it is well behind the c/g, making us conclude that it can only impart rotational force to the c/g.
This is true unless you apply the concept of TAIL VOLUME. If you figure the average chord of both wing and stab, find 25% of both, draw a line from each of those points and find 25% of that line length, THAT is the effective C/L for both "wings". If the C/G is placed at that point, the airplane will remain stable. That line is known as the tail moment. The longer that moment is, the smaller the stab can be with the same level of pitch stability.
By considering the concept of tail volume, we can get away with a smaller wing area if we figure the area of the stab as part of the total lifting area . Consider a KEG of beer.
A man comes along with a wheelbarrow, sees the keg, loads it on the wheelbarrow and away he goes. The weight is being carried by a single wheel. Two guys see the man, offer to buy his keg and he agrees [since he stole it anyway] He sells them the keg but refuses them use of his wheelbarrow.
One man is large , the other is small. They decide to carry the keg suspended from a pole, the ends of which each will shoulder. The larger man, being stronger has the keg closer to him, giving him less pole and making him support more of the weight of the keg. The smaller, weaker man has more of the pole but less of the weight.
What does this have to do with C/L speed? It's an analogy to the concept of tail volume.
THE WHEELBARROW WHEEL represents a single wing lifting the keg . The axle represents the C/L of the wing. The axle supports the weight of the keg, the c/g is over the axle. The pole bearers represent a WING and A STAB. The larger man is the wing, the smaller man is the stab. The weight of the keg is shared UNEQUALLY because of the strength of the men relative to each other , yet the weight is supported.
The wing on a C/L speed model serves another purpose. It is the entry point of the control wire into the airplane. It's already been stated that the wire is the greatest drag producer on a typical speed model. If the wing is attached to the fuselage above the thrustline, so is the wire. This results in a high center of drag. This airplane will have a tendency to fly pitched nose up. A low drag center will have an opposite effect.
If you could place the wing/wire on the thrustline, the resultant drag center would have no effect on pitch.
I have just described the three most common configurations for reciprocal powered control-line aircraft, that is: UPRIGHT ENGINE, INVERTED ENGINE and LAYDOWN ASYMMETRICAL.Any other configuration, such as upright asymmetrical, will not be considered.
Now we must consider the cowling around the engine and how it effects both drag and lift. The engine, a typical one lunger, has it's cylinder assembly jutting out into the breeze[drag]. If mounted in an upright position, drag would be produced to create a pitch up attitude. Inverted mounting would reverse the situation. If the engine cylinder is mounted on the same plane as the wing, drag from the cylinder would not induce pitch .
FAI sidewinder asymmetrical designs have all of their wings on the same plane, for lack of a better term I'll call it the pitch plane. These airplanes are designed to truly fly in a circle. The single long wing has only one fuselage junction, one wing tip vortex, has it's wire enter on the thrustline and the cowling and stab do not effect stability in terms of drag.
Since we like old style airplanes and posess limited flying and building skills, our airplane is an upright engine design with symmetrical wings.
The thing we must consider first is how large a wing is needed to make for easy landings. A Pink Lady D has about 80 sq. in. of wing area. These are said to be sweet fliers, so let's go with 80. If we can build somewhat skillfully so as not to be too heavy in the stabilizer area, we see the Pink Lady design has a very large stab and a fairly long fuselage. This gives us a fair sized tail moment and a large tail volume. As described before, if you wish, you could balance this airplane well BEHIND the quarter chord of the wing and still have a stable airplane. The question is, at speed is your wing and or stab producing lift and how much? This is where it becomes tricky.
Ralph Lindsay makes a point of the words SYNERGISM and HARMONY. What I think he means is that your systems must be chosen so as to work together for the given design.
As sleek as a PINK LADY is, it has more in common with a PIPER CUB than it has with a P-51.
Consider again the concept of C/G [center of gravity]. C/G is the point where the 3 axis of pitch, yaw and roll meet. Remember that if you apply force to the C/G, your airplane will translate not rotate.
A Piper Cub, with it's high wing probably has it's C/G located below the wing. A P-51 probably has it's C/G above the wing. The PINK LADY, as a standard upright speed design, probably has it's C/G below the wing.
Wings , in order to function must have air flow. At the start of a run, the plane is moving too slowly for the wings to function as intended. On a jet this means one thing. My experience has been with a skid launched jet that I whip like a F-40 till it becomes airbourne. On a reciprocal engine propped airplane it means that the spiral wash from the prop is effecting the wing[s]. Spiral wash and C/G along with engine torque and thrust are the determining factors of how your airplane behaves on takeoff roll.
A dolly takeoff design has to be flown off the ground much differently than a wheeled design.
Thec wide wheelbase and launch platform of a dolly helps to control pitch, yaw and roll on takeoff. A single wheeled design has only the tip guide to control yaw and roll and the effect of the spiral wash on the elevator to control pitch. A wheeled takeoff requires the pilot to whip or pull the airplane so as to use the line tip guide and the wing as a lever on the C/G so as to offset the effects of the prop/engine.
As speed increases, lift and drag on the wing surfaces, thrust and line drag begin to exert force around the C/G. The position of the C/G stays constant [as long as the fuel tank has been placed properly], but the new forces can serve to effect the pitch and yaw of the plane. The path of the plane is controlled by the balanced centipedal/centrifugal forces. Roll stability is of course controlled by the line tip guide.
The object now is simply to keep the airplane thrust line pointed in the direction it's going to maximize efficiency of the powerplant.
A stable airplane is described as one that returns to normal level flight after a disruption [wind]. That is an airplane that is FLYING. The question is then at speed should we be flying an airplane or hanging on to a rock on a string, which is faster?
I've read that a symmetrical airfoil must have a slight angle of attack to produce lift.
The following is from Dave Rolley in response to some questions I had,
- QUESTION Is it true that a symmetrical airfoil needs incidence to produce lift?
Incidence is measured with respect to some datum on the aircraft. A symmetrical airfoil needs some angle of attack (AOA) to the realitive wind to generate lift. How much of an angle depends on the aerodynamics of the aircraft, the mass of the aircraft, and how fast aircraft is flying.
- QUESTION If so, is it the designer's objective to get the airplane to fly at 0' angle of attack, so as to minimize lift and drag, relying solely on centrifugal [centrepidal?] force rather than lift?
According to Doc Davis, we are not so much flying an airplane as we have a rock flying itself around us on the end of a string. He suggests not bothering with anything but a thin symmetrical airfoil that provides enough area for take off and landing and then wants to fly about waist high (or lower) pretty much hands off.
- QUESTION Will a Clark Y airfoil produce lift at 0' angle of attack?
The zero lift AOA for a Clark Y is -4 degrees. (slightly leading edge "down"). So, yes, at zero AOA the Clark Y is producing lift.
- QUESTION If you want to produce zero lift at top speed, does where you put the C/G become a big factor?
(the sound you hear is that of opening a can of worms...) Think of a lever with the fulcrum at the areodynamic center pressure (CP), the rock to be moved at the CG and the tail at the handle. (now let's see if ASCII graphics work) ------------------------------ ^ CG CP Tail The position of the CG does not affect the angle that the wing obtains zero lift. What moving the CG around does is change the amount of tail force (and possibily the elevator displacement) required to hold a specific AOA. Reducing the tail force either reduces the lift required (reducing the down force required) or increases the lift required (reducing the up force on a lifting tail). The elevator displacement required for a given AOA creates drag. A tail system that can provide the required AOA (at a given speed) without displacing the elevator should provide a faster plane than the same model that required some elevator displacement. Up to the point that the drag from the tail incidence (and the rsultant tail AOA) becomes equal to the drag of the displaced elevator. This is when you start moving parts (or weight) around to change the tail force required to hold a particular AOA. Your goal is stable flight at some speed (hopefully your fastest) with the wing at its zero lift AOA (remember the rock on the string) with minimum tail displacement.
- QUESTION If your engine is powerful enough, are all of these details trivial?
Only if you aren't trying to be the top dog at the NATS. (or that level of competitior) Details make the difference. Anyone can do the gross level stuff (even me). It is the attention to and understanding the details that make the difference at the top. Basically, how much do you want to work on your hobby.
- QUESTION It seems as though the object is to build so as no trim input is needed, since this adds to drag.
Actually what you want is for the controls in a speed model to be the trim and the model is setup to fly almost hands off. Of course a model set up this way will eventually crash because you will not have the control effectiveness to handle all possible circumstances that can arrise in the circle. It also requires a pilot that can think far enogh ahead of the aircraft that they can anticipant what needs to be done before the problem gets here.
- QUESTION Is balancing a better way to trim?
Only if the model is properly configured to start with. (airfoil, wing area, tail volume, etc)
- QUESTION What are the implications of putting the C/G at, behind or ahead of the 1/4 chord?
First, remember when you are talking about 1/4 chord, you are talking about the Mean Aerodynamic Chord (MAC). Most general Avaiation airfoils (what our stuff is based on) have their center of pressure (CP) some where between 30% and 50% of the MAC. (the exact location is one of the characteristics of a specific airfoil) One of the goals is a stable aircraft which usually requires the CG to be ahead of the CP through the normal range of AOA the aircraft flys. (fly by wire systems with computer intervention are not included in this discussion. :) :) :) ) The closer the CG comes to the CP the less stable the aircraft becomes. On full size aircraft, the general guidelines for the CG range is the forward CG limit will permit the elevator enough authority to lift the nose of the aircraft to a sufficient angle for slow flight and landing. The aft limit will permit sufficient elevator authority to recover from a fully developed stall. A CG toward to rear of the acceptable range is faster (see discussion above) but less stable. I fly my full size glider in the rear 20% of the allowable range, but the ship is pitch sensitive. Dave Jr. flew his 1/2A Proto at the Nats balanced at 36% of MAC. He flew it out of the pylon for the first time last week and said he never wants to fly it out of the pylon again. It is too sensitive! I normally shoot for 25% - 30% of MAC. BTW, for simple wing shapes (ones with straight lines) figuring MAC can be as simple as taking the average of the root chord and the tip chord and finding that chord length on the wing. (that is what the lines are on the bottom of my wings.) If the wing shape is more complex, curves or breaks in the LE or TE you have to factor in the percentage that the component shapes contribute to the overall wing area to find the true MAC and its location. That is more work than I want to do right now. (translation, I don't remember how and don't have the bookhere that tells me how to do it)
- QUESTION On C/L speed planes, is it worthwhile to try to figure in tail volume?
Based on the above discussion, once you can quantify the impact, it is useful in the anaylsis of your current model's handling (and determining the corrective action required to get the model to behave the way you want it to) and design of your next model.
Here are some comments from Ralph Lindsay based on what has been written so far:
Hi John, Being an old potato farmer I sure enjoyed your analogies. I always thought The Farmers Guide was a common sense guide to doing things right. I seem to agree with almost all you said in the article.I'm going to skip on down past the invisible puppies and planes, that's too abstract for my Potato Farmers brain. To the "We have heard that the curve of a flying wire is determined by the following:" 1. The longer it is the more it will curve. Why? 2. Wire thickness- the thicker it is the more it will drag, therefore the curve will increase. 3. Weight of the plane- the heavier the plane the less it will curve. The pull of the plane will tend to straighten out the wire. All three of these examples are true. #1 doesn't provide an answer, being that centripetal force increases with a reduction in radius (line length) which is just another way of stating #3. #2 the reason is obvious, we're having to move more air out of the way. It seems to me that we have created a problem with 3 variable parameters. Line Length, Line Thickness, and Plane Weight. I think that this will confuse the issue when we go to an example that has 5 fixed parameters. As you do in the paragraph following the centripidal/centrifugal reference ( which is a mute point anyhow) It comes in when we are not aerodynamically "flying" but are "hanging" at an altitude far enough below the handle where no aerodynamic lift is required to maintain altitude. The difference between the forces, "pidal & fugal" is only the weight of the wire and in the opposite direction. The 5 fixed parameters are 1-Wire Length, 2- Wire Dia., 3-Weight of Plane, 4-Speed of Plane, and 5- 1/2 the wingspan. (70 ft.- .033 in.- 42 oz.- 200 mph.- and 14 in.)
The next paragraph appears right on, and the next "Bob Foggs" to my 40-50 yr old vantage point, when the only program we had was "Jack Bennys" and 150 was really hauling on 60 ft and less than 2 pounds on .018 X 2 lines we used 1/4 in behind the centrifugal balance (CG) I remember a lot of guys going the other way to get the plane to turn into the circle and lighten up on the line pull. And using rudders and "inthrust" and relatively high wings (shoulder) Why you could see the top of the outboard wing of most "Hell Razors" from the pylon.
Bob Foggs .607 " fits my way of thinking. He's sure doing something right. John, to me, "tangent flight path and heading is a must.John, I'm going to send what I've written so far now because this AM I tapped the entire message out and when I interrupted my system to send you those 2 websters dict. bits on futal and petal my Webtv got confused and I lost 1-1/2 hours of tapping.
I'll finish it later...,
later...
Hi John,
Now lets get to the propeller and your imaginary Angel. This is the interesting part. Looking straight "down" with "no L or R yaw" the relative prop. pitch will be equal top and bottom-OK but with yaw, either L or R Yaw, the relative pitch on the top blade will differ from that of the bottom blade.-OK Now if the upper and lower blade(s) differ in relative pitch, Is this not going to cause a force we normally describe as "up or down thrust" resulting from L or R yaw? If the bottom blade has more relative pitch than the top blade, wouldn't the net thrust be "up" even with a zero degree propeller shaft line? And create an artificial up thrust input that would require some form of "down aerodynamic trim input" to counteract the tendency of the plane to climb? And just the opposite in the other direction? And the resulting aerodynamic drag would consume or waste power that would otherwise be contributing to "going fast" ya still with me?
Well I'm about to throw a "monkey wrench" into the works. It's a large but little thought about force called "Gyroscopic Force." This very high rpm mass, ( the rotor valve, crank shaft, bearings, half of the con rod, spacers, propeller and spinner and nut) all combine while being forced by the wire to constantly turn L or R or R to L to generate a Gyroscopic Force, always perpendicular to the rotating mass, of almost unbelievable (up or down) proportions that , I believe make all these other considerations we have been discussing of very low priority.
The up or down Gyroscopic Force, depending on which way we fly the circle [CW or CCW] in order to overcome the (equivalent) UP or DOWN FORCE so generated requires, by far, large values of aerodynamic inputs to counteract it [elevator, trim etc.], at a large cost in aerodynamic drag.
The first and most logical way to deal with this Gyroscopic Force, in my potato farmers brain, would be to put it to use and take advantage of it by first determining which CW or CCW circle rotation vs eng. rotation would take advantage of this force by helping the wing carry the load. OK? (I know that this force is significant as I have 11,000 hours of commercial aircraft flying time logged) in craft with high mass-low rpm engines. I am told (having never flown jet turbine craft, that the Gyro effect is much greater in these turbine powered craft with much higher rpm much lower rotating mass than in the reciprocating powered craft I am familiar with.
I "know" fast (flat-uncoordinated) turns cause gyroscopic induced climbs or dives that exceed anything you could imagine, at least with my potato farmers thinker.
Not so in a soaring glider. John, more later, my fingers are tired and btw what do the farmers in Vermont raise?
Ralph
That's New Hampshire, Ralph.
...to be continued...
