WIng Loading question.
09-16-2002, 08:21 PM
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WIng Loading question.
I have recently been designing and building my own planes. I usually start with a target wing span and area, however the overall weight comes out less than expected which gives a low wing loading. Although the planes fly well they tend to float more on landing approach, more than I would like anyway.
So I thought I'd work from a known wing loading and let that determine the area and therefore the span.
My question then is what is a good number (oz/sqft) to shot for, for a medium size sport type plane, is there a maximum? I'm more interested in all out speed than airbatics.
So I thought I'd work from a known wing loading and let that determine the area and therefore the span.
My question then is what is a good number (oz/sqft) to shot for, for a medium size sport type plane, is there a maximum? I'm more interested in all out speed than airbatics.
09-17-2002, 03:05 PM
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WIng Loading question.
Wing loading alone is not a good criteria for planes. Wing volume loading is much more pertinant. For instance, a half A model with an 18 oz./sq. Ft. will be a lead bomb while a 1/4 scale job at that loading will float forever.
09-17-2002, 03:49 PM
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WIng Loading question.
It depends on what you mean by "float."
If the model has a shallow power glide on final approach and you would like it to be slow but steeper then there are two possible solutions. If the shallow glide is caused by too much thrust when the engine is idling, then the answer is a lower pitch prop or an engine that idles slower. The excess thrust can also be overcome by adding a lot of drag in the landing approach. Lots of rudder and opposite aileron will side slip the model and the huge drag of the side slipping fuselage will bring the model down more steeply. You just have to straighten it out before touch down. Yet another solution is to add inboard split flaps. The flaps will slow the glide and also steepen it. You can even use flaps with lots of holes in them like the dive breaks of the Douglas Dauntless.
Increasing the wing loading will reduce maneuverability and raise the stall speed. I think you will find it is a poor way to compensate for too much thrust from an idling engine.
If the model has a shallow power glide on final approach and you would like it to be slow but steeper then there are two possible solutions. If the shallow glide is caused by too much thrust when the engine is idling, then the answer is a lower pitch prop or an engine that idles slower. The excess thrust can also be overcome by adding a lot of drag in the landing approach. Lots of rudder and opposite aileron will side slip the model and the huge drag of the side slipping fuselage will bring the model down more steeply. You just have to straighten it out before touch down. Yet another solution is to add inboard split flaps. The flaps will slow the glide and also steepen it. You can even use flaps with lots of holes in them like the dive breaks of the Douglas Dauntless.
Increasing the wing loading will reduce maneuverability and raise the stall speed. I think you will find it is a poor way to compensate for too much thrust from an idling engine.
09-17-2002, 04:32 PM
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WIng Loading question.
The plane in question is a Half-A with 185 sq inches and weighs about 14 oz. It has a 34 in wing span with a 5.5 in chord. The wing loading is about 10.5 oz/sqin. I land it dead stick since it has no landing gear. It glides as well as my 2 meter sailplane when coming in for a landing. This isn't really a bad thing execpt I fly it from a soccer field, a very short runway.
My Q500 planes are about 16 oz/sq in and my 40 size pattern plane is about 23 oz/sq in. This seems to be a big spread, I thought there might be a magic number to shoot for, but it may depend more on what the plane is intended for.
I designed my little half-a as a pylon racer. I have since cut the wing down to 30" but have not flown it yet. My thinking was this would increase wing loading and also increase top speed due to less frontal area. I think in this case it was just to much wing area for the weight. Are there any guidelines for this design issue some where?
My Q500 planes are about 16 oz/sq in and my 40 size pattern plane is about 23 oz/sq in. This seems to be a big spread, I thought there might be a magic number to shoot for, but it may depend more on what the plane is intended for.
I designed my little half-a as a pylon racer. I have since cut the wing down to 30" but have not flown it yet. My thinking was this would increase wing loading and also increase top speed due to less frontal area. I think in this case it was just to much wing area for the weight. Are there any guidelines for this design issue some where?
09-17-2002, 05:56 PM
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WIng Loading question.
I wish we had an objective wing loading chart for model airplanes. I believe it would have volumetric wing loading (oz/in^3) on one axis and cruise speed (mph) on the other. Maybe three charts for zero, medium, and high % camber lines. The simple subjective "rules-of-thumb" don't help too much. On the other hand, trial and error can be more fun.
09-17-2002, 06:45 PM
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WIng Loading question.
The wing loading is a compromise. The allowable wing loading range depends mainly on the size of the model, the airfoil and, the purpose of the model.
The smaller the model, the lower the maximum lift coefficient of a given airfoil and the lower the the wing loading must be for a practical landing speed.
The tighter the model has to turn, the higher the maximum lift coefficient of the airfoil and the lower the wing loading must be. Aerobats require a symmetrical airfoil for equal performance upright and inverted. Other things being equal, symmetrical airfoils will have a lower lift coefficent than mean line cambered airfoils and will require a lower wing loading for a given stall speed.
The maximum speed depends on drag minimization and thrust maximization. One of the most effective ways to minimize drag for a given thrust is to make the plane as small as possible for the powerplant. This makes the wing loading very high.
A plane that has to take off normally, land normally and go fast must compromize the wing loading to establish a balance between conflicting objectives. If you catapult the plane on launch and land it into a net, then the balance will fall to a higher wing loading. If you use flaps, the speed range can be extended on the low end for a high wing loading. Any parasitic drag coefficient reduction extends the top speed without having to resort to a higher wing loading.
The smaller the model, the lower the maximum lift coefficient of a given airfoil and the lower the the wing loading must be for a practical landing speed.
The tighter the model has to turn, the higher the maximum lift coefficient of the airfoil and the lower the wing loading must be. Aerobats require a symmetrical airfoil for equal performance upright and inverted. Other things being equal, symmetrical airfoils will have a lower lift coefficent than mean line cambered airfoils and will require a lower wing loading for a given stall speed.
The maximum speed depends on drag minimization and thrust maximization. One of the most effective ways to minimize drag for a given thrust is to make the plane as small as possible for the powerplant. This makes the wing loading very high.
A plane that has to take off normally, land normally and go fast must compromize the wing loading to establish a balance between conflicting objectives. If you catapult the plane on launch and land it into a net, then the balance will fall to a higher wing loading. If you use flaps, the speed range can be extended on the low end for a high wing loading. Any parasitic drag coefficient reduction extends the top speed without having to resort to a higher wing loading.
09-17-2002, 07:14 PM
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WIng Loading question.
Here is a photo of the above mentioned plane. It has an MH30 airfoil. I'm really pleased with its flight performance. It does about 80 mph and loses very little speed in the corners.
Ollie,
Would you agree that shortening the span decreases frontal area and should increase top speed? Similar to what they do to full scale unlimited pylon racers, like Strega and Rare Bear. They cut several feet off each wing tip, the planes are lighter without guns, ammo and armor plating which means they need less wing area. The side benefit from that is increased speed. Or do they do it just for a speed increace reguardless of wing loading.
Ollie,
Would you agree that shortening the span decreases frontal area and should increase top speed? Similar to what they do to full scale unlimited pylon racers, like Strega and Rare Bear. They cut several feet off each wing tip, the planes are lighter without guns, ammo and armor plating which means they need less wing area. The side benefit from that is increased speed. Or do they do it just for a speed increace reguardless of wing loading.
09-17-2002, 07:51 PM
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WIng Loading question.
Cutting the wing span and area could increase the speed in the straight away. If you are going to decrease the wing area it will be far better to decrease the chord rather than the span because that improves both the straight away and the turns.
Chopping the wing tips off will have a very detrimental effect in the turns because the plane can't turn as tightly and because it will have more drag in a wider radius turn than it did before the change. Not only will the induced drag in the turn be seriously increased but the induced AOA will be seriously increased which in turn will seriously increase the fuselage drag. Not only that but it will be in the high drag state longer. What is the frontal area of the fuselage when it is at an 8 to 10 degree pitch up compared to the frontal area of the fuselage aligned to the direction of flight?
BTW, the power off glide angle is not much affected by wing loading. Increased wing loading increases the sinking speed and the horizontal speed in the same proportion. Cutting off the wing tips degrades the lift to drag ratio and steepens the glide angle because of increases in drag, not because of increases in wing loading.
Chopping the wing tips off will have a very detrimental effect in the turns because the plane can't turn as tightly and because it will have more drag in a wider radius turn than it did before the change. Not only will the induced drag in the turn be seriously increased but the induced AOA will be seriously increased which in turn will seriously increase the fuselage drag. Not only that but it will be in the high drag state longer. What is the frontal area of the fuselage when it is at an 8 to 10 degree pitch up compared to the frontal area of the fuselage aligned to the direction of flight?
BTW, the power off glide angle is not much affected by wing loading. Increased wing loading increases the sinking speed and the horizontal speed in the same proportion. Cutting off the wing tips degrades the lift to drag ratio and steepens the glide angle because of increases in drag, not because of increases in wing loading.
09-17-2002, 08:22 PM
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WIng Loading question.
I can see how it would be hard to design a plane that was both fast in a straight line and fast around a race course.
I should get to fly my little plane this weekend, will be interesting to see how its flight characteristics changed by shortening the wing. Even though I built it as a pylon racer, there is none of that in my area, so I'm trying to optimize it for straight line speed as much as possible.
I'm going to build a plane for straight line speed next. I have chosen an airfoil but was struggling with wing area and wing loading. I was hoping someone would tell me, "don't exceed XX oz/sq in" or "don't use less than XXX sq in for a given weight". I'm realizing its not as cut and dried as that, and there are many things to consider.
I should get to fly my little plane this weekend, will be interesting to see how its flight characteristics changed by shortening the wing. Even though I built it as a pylon racer, there is none of that in my area, so I'm trying to optimize it for straight line speed as much as possible.
I'm going to build a plane for straight line speed next. I have chosen an airfoil but was struggling with wing area and wing loading. I was hoping someone would tell me, "don't exceed XX oz/sq in" or "don't use less than XXX sq in for a given weight". I'm realizing its not as cut and dried as that, and there are many things to consider.
09-18-2002, 12:09 AM
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WIng Loading question.
Yes we can talk about all the interactions and how airplane design is a balance of many compromises, blah, blah, blah, but someone must have created an attribute optimization program or pencil and paper technique for model airplanes by now. It seems so fundamental with the technology we have available.
Where's the Machinery's Handbook for model airplanes?
Where's the Machinery's Handbook for model airplanes?
09-18-2002, 01:26 AM
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WIng Loading question.
I would agree that model airplane designers must have a list of "do's and don'ts" that they follow. Whether it was derived by experience or by formulas and calculations, I don't know. I wonder if full scale design is a one to one corrolation to model design, or are there some scaling effects to take into consideration. I'm sure the info is out there somewhere.
Setting a target wing loading would be hard because its hard to predict what the finished model will weigh, at least for me, but maybe close is good enough. Its fairly easy and cheap to experiment with models but I'd like to be "in the ball park" when I start a new design intended for a single purpose, like going as fast as possible in a straight line.
Ollie,
One could argue that a 34" x 5" wing at 2 degrees AoA would have the same amount of drag as a 30" x 5" wing at say 4 degrees AoA? No?
I agree that a high aspect ratio (HAR) loses less speed in a turn than a low aspect ratio (LAR) and a LAR wing is faster in a straight line than a HAR wing. Just depends on what your after.
Setting a target wing loading would be hard because its hard to predict what the finished model will weigh, at least for me, but maybe close is good enough. Its fairly easy and cheap to experiment with models but I'd like to be "in the ball park" when I start a new design intended for a single purpose, like going as fast as possible in a straight line.
Ollie,
One could argue that a 34" x 5" wing at 2 degrees AoA would have the same amount of drag as a 30" x 5" wing at say 4 degrees AoA? No?
I agree that a high aspect ratio (HAR) loses less speed in a turn than a low aspect ratio (LAR) and a LAR wing is faster in a straight line than a HAR wing. Just depends on what your after.
09-18-2002, 01:26 AM
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WIng Loading question.
PAINLESS & Ollie,
I don't agree entirely with all of Ollie's comments. Clipping the wings will definitely reduce frontal area and increase top speed. It may also cause a significant increase in induced drag during high-lift tasks, like turning, landing, and slow flight. If you look at Martin Simon's Model Aircraft Aerodynamics, you will see a chart that shows the profile drag of the wing to be much more significant than induced drag, during straights and turns. The profile drag of the wing is what we reduce when we clip a wing.
I think that the wing loading corresponds pretty closely to the glide angle, although landing is certainly a time when the increase in induced drag due to clipping the wings would be most significant.
As I contended in an unrelated thread, if you want the highest top speed, forget induced drag, and make the wing as small ( highly loaded ) as you can tolerate for turning, landing and launching. Also, low aspect ratio ( short span ) is definitely a drag win for top speed. Obviously, as Ollie points out above, if you really pursue top speed single-mindedly, you start running into odd compromises like launching rather than taking off, and landing on skids.
banktoturn
I don't agree entirely with all of Ollie's comments. Clipping the wings will definitely reduce frontal area and increase top speed. It may also cause a significant increase in induced drag during high-lift tasks, like turning, landing, and slow flight. If you look at Martin Simon's Model Aircraft Aerodynamics, you will see a chart that shows the profile drag of the wing to be much more significant than induced drag, during straights and turns. The profile drag of the wing is what we reduce when we clip a wing.
I think that the wing loading corresponds pretty closely to the glide angle, although landing is certainly a time when the increase in induced drag due to clipping the wings would be most significant.
As I contended in an unrelated thread, if you want the highest top speed, forget induced drag, and make the wing as small ( highly loaded ) as you can tolerate for turning, landing and launching. Also, low aspect ratio ( short span ) is definitely a drag win for top speed. Obviously, as Ollie points out above, if you really pursue top speed single-mindedly, you start running into odd compromises like launching rather than taking off, and landing on skids.
banktoturn
09-18-2002, 01:40 AM
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WIng Loading question.
Banktoturn,
I work with a guy who has pit crewed for Rare Bear for the last several years. I'm going to pick his brain about clipping wings and the theory behing it. I don't think full scale pylon racers turn very tight so its not a drag issue but more just to increase top speed. We'll see what he says when he returns from travel.
As far as models go, a model built for pure speed would be so overpowered you could launch it straight up and accelerate vertically. Landings would be hot but if you have the room.........
I work with a guy who has pit crewed for Rare Bear for the last several years. I'm going to pick his brain about clipping wings and the theory behing it. I don't think full scale pylon racers turn very tight so its not a drag issue but more just to increase top speed. We'll see what he says when he returns from travel.
As far as models go, a model built for pure speed would be so overpowered you could launch it straight up and accelerate vertically. Landings would be hot but if you have the room.........
09-18-2002, 01:52 AM
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WIng Loading question.
Engineering schools do a great job of teaching how to analyse but have been struggling with the best way to teach design which is a process of synthesis rather than analysis. The best way to teach the design process is to use design competitions. The competing designs are quite varied and do not seem to converge on a single solution. Incidently, model airplane competition has generated more innovation than any other aspect of the hobby.
In a system as complex and interrelated as a model airplane there is never a set of design requirements that fully states all the things necessary for success. That's why testing is required to find out the things the requirements left out and the things the design does that were not intended by the designer.
Almost any design with sufficient power, low enough weight and and a modicum of stability and control can be made to fly. However, something that is more economical, beautiful, durable, reliable, faster, slower, contollable, maneuverable, longer range, longer duration and with a bigger pay load than anything else that has gone before isn't something that will come from a program or handbook but will come from the creative mind of the artist-designer. An elegant design is one that synergistically combines several functions into one element and produces simplicity out of complexity. A sparless, ribless, stressed skin wing structure is an example of an elegant design approach only made practical by the light weight, strength and stiffness of composite materials.
In a system as complex and interrelated as a model airplane there is never a set of design requirements that fully states all the things necessary for success. That's why testing is required to find out the things the requirements left out and the things the design does that were not intended by the designer.
Almost any design with sufficient power, low enough weight and and a modicum of stability and control can be made to fly. However, something that is more economical, beautiful, durable, reliable, faster, slower, contollable, maneuverable, longer range, longer duration and with a bigger pay load than anything else that has gone before isn't something that will come from a program or handbook but will come from the creative mind of the artist-designer. An elegant design is one that synergistically combines several functions into one element and produces simplicity out of complexity. A sparless, ribless, stressed skin wing structure is an example of an elegant design approach only made practical by the light weight, strength and stiffness of composite materials.
09-18-2002, 02:14 AM
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WIng Loading question.
Bank to turn,
I think you are misinterpreting Simon's drag budget illustration.
Induced drag is proportional to the coefficient of lift squared divided by the effective aspect ratio. In a highly banked tight turn at a constant altitude, the wing is operating near its maximum lift coefficient and the induced drag is more than half the total drag of the aircraft. The component of the lift force that equals weight is the lift force times the sine of the angle of bank and the component of the lift force that resists centrifugal force is the lift force times the cosine of the angle of bank. Simons drag budget illustration applies to wings-level flight, not a highly banked, tight turn.
I think you are misinterpreting Simon's drag budget illustration.
Induced drag is proportional to the coefficient of lift squared divided by the effective aspect ratio. In a highly banked tight turn at a constant altitude, the wing is operating near its maximum lift coefficient and the induced drag is more than half the total drag of the aircraft. The component of the lift force that equals weight is the lift force times the sine of the angle of bank and the component of the lift force that resists centrifugal force is the lift force times the cosine of the angle of bank. Simons drag budget illustration applies to wings-level flight, not a highly banked, tight turn.
09-18-2002, 02:15 AM
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WIng Loading question.
I could not agree more. I am a mechanical designer, using CAD, by trade.
Simply knowing CAD does not make one a designer. Spending time in the shop, learning the capabilities of lathes, mills, 3 and 5 axis machines, wire EDM, different types of welding from EBM to TIG to MIG will teach you more than any class room course. Some hands on experience doesn't hurt either.
At least with models you don't have to risk your life trying to prove a point. Still there must be a few guidlines for designing models, such as choising the right airfoil for a particular application.
Simply knowing CAD does not make one a designer. Spending time in the shop, learning the capabilities of lathes, mills, 3 and 5 axis machines, wire EDM, different types of welding from EBM to TIG to MIG will teach you more than any class room course. Some hands on experience doesn't hurt either.
At least with models you don't have to risk your life trying to prove a point. Still there must be a few guidlines for designing models, such as choising the right airfoil for a particular application.
09-18-2002, 10:45 AM
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WIng Loading question.
Of course there are guidelines. For sailplanes, speedsters, duration and range, minimize drag. For aerobatic models, speedsters, duration, range, load carriers and sailplanes, minimize weight. For aerobatic models add drag to limit acceleration on the down line. For durability maximize the strength to weight ratio. For beauty don't employ a design team (a camel being a horse that was engineered by a committee). For aerobats, speedsters, and load carriers, maximise thrust. For an all around sports model kit provide low landing speed, provide lots of maneuverability, provide adequate stability, provide good vertical performance and do it at a minimum cost in materials and labor.
The general sorts of guidelines then have to be quantified and the quantities fed into equations, mathematical models or computer programs to generate a range of possible solutions. For example, the structural equations and the aerodynamic equations don't solve the problem of creating something that satisfies both perfectly. How those solutions are integrated and allocated is not a deterministic activity. In other words, the component solutions do not converge on a "best" solution but have to be synthesized or integrated by the designer using his talents of judgement, creativity and artistry including those needs that were not originally specified but are necessary for practicality. Practicality is a moving target that evolves as new materials and processes are developed.
The general sorts of guidelines then have to be quantified and the quantities fed into equations, mathematical models or computer programs to generate a range of possible solutions. For example, the structural equations and the aerodynamic equations don't solve the problem of creating something that satisfies both perfectly. How those solutions are integrated and allocated is not a deterministic activity. In other words, the component solutions do not converge on a "best" solution but have to be synthesized or integrated by the designer using his talents of judgement, creativity and artistry including those needs that were not originally specified but are necessary for practicality. Practicality is a moving target that evolves as new materials and processes are developed.
09-18-2002, 10:58 AM
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Formulas for design
Back in the late 80s and early 90s, I was one of the people that created the kind of design program many of you are talking about. Some people developed these into commercial computer software... "Auto-Design" programs, if you will. I've seen a few advertized since that time, in various magazines, but haven't used one.
Developing my own software finally got me to the point that I could input 2 simple parameters, such as "type" (Pattern, Fun Fly, etc.) and "span", and the computer would then calculate the desired proportions, CG placement, engine required, and a very long list of other details, from which a design could be built. (This was done on an ancient Sinclair QL computer, and I no longer have either the computer or software.)
But...If you can spare a weekend, you can create your own. Here's how:
The process can be done by using a (large) spreadsheet. Simply start entering data into the spreadsheet for each type of design you like, using proven measurements. Do this for different types, different scales, and so on. I would enter the data for at least 15 or 20 aircraft of each type and scale that you're interested in. Specs for most models are published in catalogs, magazines, and on various web sites. Finally, take the averaged results of all this math, and create input-related "forumlas" on another spreadsheet.
Eventually, you can create your own customized "Auto Design" spreadsheet, which will, based on the inputs you provide, spit out what you need to start drawing. It's tedious, but can be created in a couple of days of dedicated work. If you consider the end result, that's really not much time to spend. As you design and build your own planes, keep collecting performance data, to refine your results.
This cookie cutter approach has only one real drawback, which is that we keep designing the same kinds of aircraft over and over, with only small cosmetic differences. The parameters would have to be editable, to allow for new construction materials, engine technology, and radio technology, which can affect performance greatly. While it's true that we shouldn't waste our time re-inventing the wheel, I would like to see some NEW concepts explored, and am always tinkering with ideas to see what happens.
Developing my own software finally got me to the point that I could input 2 simple parameters, such as "type" (Pattern, Fun Fly, etc.) and "span", and the computer would then calculate the desired proportions, CG placement, engine required, and a very long list of other details, from which a design could be built. (This was done on an ancient Sinclair QL computer, and I no longer have either the computer or software.)
But...If you can spare a weekend, you can create your own. Here's how:
The process can be done by using a (large) spreadsheet. Simply start entering data into the spreadsheet for each type of design you like, using proven measurements. Do this for different types, different scales, and so on. I would enter the data for at least 15 or 20 aircraft of each type and scale that you're interested in. Specs for most models are published in catalogs, magazines, and on various web sites. Finally, take the averaged results of all this math, and create input-related "forumlas" on another spreadsheet.
Eventually, you can create your own customized "Auto Design" spreadsheet, which will, based on the inputs you provide, spit out what you need to start drawing. It's tedious, but can be created in a couple of days of dedicated work. If you consider the end result, that's really not much time to spend. As you design and build your own planes, keep collecting performance data, to refine your results.
This cookie cutter approach has only one real drawback, which is that we keep designing the same kinds of aircraft over and over, with only small cosmetic differences. The parameters would have to be editable, to allow for new construction materials, engine technology, and radio technology, which can affect performance greatly. While it's true that we shouldn't waste our time re-inventing the wheel, I would like to see some NEW concepts explored, and am always tinkering with ideas to see what happens.
09-18-2002, 11:53 AM
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WIng Loading question.
I stumbled across an excel spread sheet that does exactly as you describe, but not by type of plane, just some real general design guidelines. The one thing it does have is a CG calculator for many different wing planforms, even biplanes. Very handy.
09-18-2002, 01:55 PM
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pylon drag budget
Ollie,
I don't have Simon's book with me at the moment, but if you look at the diagram in question, you will see a short line segment which represents pylon racers. At the left end of the segment is the 'turning task', and at the right end of the segment is the 'straight speed task'. The left end of this segment certainly does not correspond to level flight, and shows that induced drag is a small component of total drag. While a pylon racer is generating a lot of lift during a turn, I think that the wing is not that close to its maximum CL, because the speed remains high, and because it would be very risky to approach maximum CL during a turn. I completely agree that there is a limit to the benefit of clipping wings for a plane that must turn, because drag goes up in turns. It is not clear to me whether the induced drag penalty is worse than the profile drag penalty however. I suspect that as long as speed remains high, the profile drag increase resulting from increasing the angle of attack is larger than the induced drag increase. In particular, I think it is a mistake to assume that it is better to reduce chord than span when reducing wing area in search of top speed.
banktoturn
I don't have Simon's book with me at the moment, but if you look at the diagram in question, you will see a short line segment which represents pylon racers. At the left end of the segment is the 'turning task', and at the right end of the segment is the 'straight speed task'. The left end of this segment certainly does not correspond to level flight, and shows that induced drag is a small component of total drag. While a pylon racer is generating a lot of lift during a turn, I think that the wing is not that close to its maximum CL, because the speed remains high, and because it would be very risky to approach maximum CL during a turn. I completely agree that there is a limit to the benefit of clipping wings for a plane that must turn, because drag goes up in turns. It is not clear to me whether the induced drag penalty is worse than the profile drag penalty however. I suspect that as long as speed remains high, the profile drag increase resulting from increasing the angle of attack is larger than the induced drag increase. In particular, I think it is a mistake to assume that it is better to reduce chord than span when reducing wing area in search of top speed.
banktoturn
09-18-2002, 02:54 PM
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Banktoturn,
I got out Simon's book and looked at his drag budget illustration again. You are right about Simon's drag budget illustration. It does cover turns.
At the turn end of the pylon racer example, the induced drag is only about 1/3 of the total drag. If you extend the arrow for the pylon racer just a very small distance to the left, the induced drag as percentage of the total pylon racer drag goes up dramatically. It just means that you better not turn too tightly and the lower the aspect ratio, the wider you better make your turns. If the low drag bucket of the airfoil was chosen for a coefficient of lift range associated with a high aspect ratio, tight turning flying style and you chop the wing tips, the best flying style will be wider turns which lengthen the course and eat up at least part and maybe all of the speed advantage. Also, the ideal coefficient of lift range of the airfoil will be shifted to lower values and the old airfoil will no longer be quite as effective.
I got out Simon's book and looked at his drag budget illustration again. You are right about Simon's drag budget illustration. It does cover turns.
At the turn end of the pylon racer example, the induced drag is only about 1/3 of the total drag. If you extend the arrow for the pylon racer just a very small distance to the left, the induced drag as percentage of the total pylon racer drag goes up dramatically. It just means that you better not turn too tightly and the lower the aspect ratio, the wider you better make your turns. If the low drag bucket of the airfoil was chosen for a coefficient of lift range associated with a high aspect ratio, tight turning flying style and you chop the wing tips, the best flying style will be wider turns which lengthen the course and eat up at least part and maybe all of the speed advantage. Also, the ideal coefficient of lift range of the airfoil will be shifted to lower values and the old airfoil will no longer be quite as effective.
09-18-2002, 05:16 PM
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Where is this Spreadsheet?
PAINLESS ...
I am interested in taking a look at this spreadsheet as I have a biplane glider project on the cad that I would like to compare the "spreadsheet" data to my own ... and see how smart I really am ... or "STUPID" depending on the results ...
if you can post a link to it, I would apreciate it or maybe just e-mail it to me.
Thanks in advance.
I stumbled across an excel spread sheet that does exactly as you describe, but not by type of plane, just some real general design guidelines. The one thing it does have is a CG calculator for many different wing planforms, even biplanes. Very handy.
I am interested in taking a look at this spreadsheet as I have a biplane glider project on the cad that I would like to compare the "spreadsheet" data to my own ... and see how smart I really am ... or "STUPID" depending on the results ...
if you can post a link to it, I would apreciate it or maybe just e-mail it to me.
Thanks in advance.
09-18-2002, 06:24 PM
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Mike James, I like the way you think. Synthesis through the structured use of practical tools. Of course, even a structured mathematical approach like you have used leaves a lot of room for art and imagination. I bet you have designed and produced some very nice models.
09-18-2002, 06:34 PM
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Thank you!
Thank you!
The sharing of information that goes on here at RC Universe will hopefully make all of us better designers. I especially like it when people successfully design and fly ideas that are out of the ordinary.
As a flyer, I'd try anything, including most popular models. But as a designer, I hope to see new and different planes. There certainly have been some unique ideas proposed on RCU, and I hope everybody concerned keeps at it!
The sharing of information that goes on here at RC Universe will hopefully make all of us better designers. I especially like it when people successfully design and fly ideas that are out of the ordinary.
As a flyer, I'd try anything, including most popular models. But as a designer, I hope to see new and different planes. There certainly have been some unique ideas proposed on RCU, and I hope everybody concerned keeps at it!
09-19-2002, 06:51 PM
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Banktoturn,
Have a close look at Simon's Drag Budget illustration. The horizontal axis is labeled "Slow, high lift coefficient." Strictly speaking this doesn't apply to Pylon Racers unless they slow down a lot in the turns. A plane that is banked 80 degrees is pulling about 5 G's or a bit more in a level turn where the fuselage is not contributing lift. If the lift coefficient is 0.1 or 0.2 in the straights and there is not an appreciable slowing in the turns, then the coefficient of lift must be about 0.5 to 1.0 (5 times as great for 5 G's). At these lift coefficients, the induced drag is very high and high aspect ratio is a big advantage. Also when the chord is narrowed and the airfoil % thickness is maintained the frontal area is reduced in the same proportion as in chopping the wing when the resultant areas are the same for the high and low aspect ratio cases.
If you were to base a drag budget diagram on the same speed in the straights as in the turns, the parasitic drag would be more or less constant and the profile drag would be about the same if the airfoil was still operating in its low drag bucket. However, the induced drag would increase as the square of the lift coefficient increase.
Of course there is slowing in the turns but you want to minimize that with the highest aspect ratio that you can get away with structurally.
Have a close look at Simon's Drag Budget illustration. The horizontal axis is labeled "Slow, high lift coefficient." Strictly speaking this doesn't apply to Pylon Racers unless they slow down a lot in the turns. A plane that is banked 80 degrees is pulling about 5 G's or a bit more in a level turn where the fuselage is not contributing lift. If the lift coefficient is 0.1 or 0.2 in the straights and there is not an appreciable slowing in the turns, then the coefficient of lift must be about 0.5 to 1.0 (5 times as great for 5 G's). At these lift coefficients, the induced drag is very high and high aspect ratio is a big advantage. Also when the chord is narrowed and the airfoil % thickness is maintained the frontal area is reduced in the same proportion as in chopping the wing when the resultant areas are the same for the high and low aspect ratio cases.
If you were to base a drag budget diagram on the same speed in the straights as in the turns, the parasitic drag would be more or less constant and the profile drag would be about the same if the airfoil was still operating in its low drag bucket. However, the induced drag would increase as the square of the lift coefficient increase.
Of course there is slowing in the turns but you want to minimize that with the highest aspect ratio that you can get away with structurally.