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RE: WACO YMF - 1/17/2013 1:45 AM   
hopkimf


 

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Status: offline 		Bob,

If a plane is flying steady state, the four forces Lift, Weight, Thrust, and Drag are balanced. Take the same plane and add 10 lbs while retaining everything else. Lift is less than weight. It's going down. To overcome that, more thrust (higher speed) is needed to generate more lift. Higher speed means more drag. So addition of power is needed with weight change. It's back in balance again. I believe the relationship still holds.

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RE: WACO YMF - 1/17/2013 1:49 AM   
sensei



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ORIGINAL: hopkimf

Bob,

If a plane is flying steady state, the four forces Lift, Weight, Thrust, and Drag are balanced. Take the same plane and add 10 lbs while retaining everything else. Lift is less than weight. It's going down. To overcome that, more thrust (higher speed) is needed to generate more lift. Higher speed means more drag. So addition of power is needed with weight change. It's back in balance again. I believe the relationship still holds.


I agree, after reviewing the post over and over, that is why I change my question.

Bob


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RE: WACO YMF - 1/17/2013 1:56 AM   
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Status: offline 		I think the answer to your question is yes! All of those elements, in steady state (not accelerating or decelerating etc.) are balanced. Change one element and the others will automatically adjust. To get a specific result though, an element will have to be intentially changed.

Jaybird

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RE: WACO YMF - 1/17/2013 2:10 AM   
hopkimf


 

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Status: offline 		Jaybird,

Appreciate your comments. But I still don't know where the center of lift for wing and tail are. If you draw a simple moment diagram, you could need tail down force or tail up force depending on location of centers of lift.

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RE: WACO YMF - 1/17/2013 2:25 AM   
Jaybird


 

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Status: offline 		I'm baffled too when it comes to the models. The flight training manuals and operating handbooks for the full size aircraft I fly clearly show the center of gravity ahead of the center of lift which is where the tail downforce is needed to balance. The full size obviously have total weight limits for combinations of fuel, passengers and cargo. Performance changes depending on that total weight. It also has to be be within a certain acceptable range which varies with the total weight. Too much weight too far forward and you may not have enough tailforce to pick the nose up at slower speeds (like during the landing flair). Too much weight too far back and you may not have enough tailforce to push the nose down once it is airborn. I believe the full size trainer planes are designed with the C of G ahead of the C of L so that they will be more stable and "better behaved" and have a tendancy to seek a balanced state without a lot of pilot input. Perhaps higher performance aircraft have the two closer together so that they are less stable and therefore easier to move around it's axis.

For any particular aircraft design, the points where those elements occur are part of the engineering process. Most model plans only show a center of gravity point and not center of lift.

Jaybird

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RE: WACO YMF - 1/17/2013 2:52 AM   
hopkimf


 

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I fly full scale as well. Much better than RC planes. Never wrecked one yet. I brought this up because I thought the subject has been ignored by the model side. Just balancing it isn't enough in my view. I would really like to know the theoretical approach needed to get the answers. Beside me is a text by Barnes McCormick,who was a prof at Penn State and a world class expert on Aerodynamics. I'm not about to grind through all of that, but would like a more simplified answer appropriate to RC planes.

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RE: WACO YMF - 1/17/2013 3:34 AM   
Jaybird


 

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Status: offline 		Sorry, didn't mean to preach to the chior.

I agree 100%...the model side of aerodynamics seems more of a mystery than full scale. I look at a big Telemaster for instance and notice that the stabilizer is a large flat bottom UPRIGHT airfoil just like the wing! How can that NOT produce upward lift all the time? I look at my 1/6 scale Concept Fleet and the stabilizer is also a flat bottom upright airfoil with positive incidence. That kind of puts the tail downforce out the window too, or at least would seem so.

Calculating the information you are looking for is far beyond anything I could get even get close to.

I do know the GP WACO ARF has -1 incidence on the top wing and +2-1/2 on the symetrically shaped stabilzer and it flies great!

Good luck on your search. If you do find a answer please share it.

Jaybird

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RE: WACO YMF - 1/17/2013 12:05 PM   
sensei



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ORIGINAL: Jaybird

Sorry, didn't mean to preach to the chior.

I agree 100%...the model side of aerodynamics seems more of a mystery than full scale. I look at a big Telemaster for instance and notice that the stabilizer is a large flat bottom UPRIGHT airfoil just like the wing! How can that NOT produce upward lift all the time? I look at my 1/6 scale Concept Fleet and the stabilizer is also a flat bottom upright airfoil with positive incidence. That kind of puts the tail downforce out the window too, or at least would seem so.

Calculating the information you are looking for is far beyond anything I could get even get close to.

I do know the GP WACO ARF has -1 incidence on the top wing and +2-1/2 on the symetrically shaped stabilzer and it flies great!

Good luck on your search. If you do find a answer please share it.

Jaybird

All airfoils with the exception of an asymmetric airfoil require a AOA component to produce lift, when your flying your wing will assume it's own AOA needed to carry the load, but that doesn't necessarily mean that your horizontal stab is carrying any + or - AOA, it is really more of how the designer set up the aircraft, the chosen airfoil, the AOI in regards of wing, fuselage center line and horizontal stabilizer relationships and so on. In this picture you can clearly see that I set up the horizontal stabilizer with 3 degrees positive AOI, and had the wing set at 0 AOI in relation to the FCL. Note that the paint trim line is running parallel to the FCL.

Bob

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RE: WACO YMF - 1/17/2013 12:07 PM   
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Status: online 		The issue that we get into have to do with Reynold Numbers. Full Scale airplanes differ from models due to the effects of Scale Volume, not Linear scale. The bigger the model, the more like the full scale it becomes. For example, a cube that is 4"X4"X4" will have a volume of 64 cubic inches. (4X4=16X4=64) A quarter scale model of that cube is 1X1X1 which yields a volume of 1. Even though the linear scale is one fourth the size of the full scale, the volume is much different. Hope this helps you see the difference.

Bill, Waco Brother #1

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RE: WACO YMF - 1/17/2013 12:55 PM   
sensei



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ORIGINAL: Stickbuilder

The issue that we get into have to do with Reynold Numbers. Full Scale airplanes differ from models due to the effects of Scale Volume, not Linear scale. The bigger the model, the more like the full scale it becomes. For example, a cube that is 4''X4''X4'' will have a volume of 64 cubic inches. (4X4=16X4=64) A quarter scale model of that cube is 1X1X1 which yields a volume of 1. Even though the linear scale is one fourth the size of the full scale, the volume is much different. Hope this helps you see the difference.

Bill, Waco Brother #1


This is actually a good point to bring up, for the very reason that the effects of scale volume are not linear. The larger an airframe, the more efficiently it will carry weight, that is why lets say a full scale Extra 300 weighing in at around 1400 lbs. empty performs very well at that weight. Now take a half scale Extra 300 and weigh it down 700 lbs. and it is not going anyplace, in fact at half scale weighing it down to 100 lbs. and it is a real pig. Now lets get down to small aircraft anything 1/4 scale or less. The smaller the airplane the more important it is to build as light as possible, because the Reynolds numbers are working against you from a load carrying efficiency standpoint.

Bob


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RE: WACO YMF - 1/17/2013 1:58 PM   
Dash7ATP



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Status: offline 		This is a very interesting discussion.  One thing does not change. The wing has to fly at an angle of attack (AOA) at any given speed that will allow it (the wing) to produce enough lift to support the weight of the aircraft..If the plane appears to be "dragging it's tail", but still maintinaing altitude, then lift vs weight (gravity) is balanced.  The plane is either too slow, too heavy, or the wing is mounted at the wrong incidence angle on the fuse. What ever is going on with the stab/elevator is just what is neccesary to maintain that AOA.

IF the aircraft is tail heavy, You just can't trim it out to fly in a stable mannor; ie, hands off. I was a commercial pilot, and on a charter flight I did early on in my carrier, I flew a load of water bottles to a lab to be analyzed for a new treatment plant.  I was in a Cessna 206. All of the seats were taken out and the bottles, about ten gallons each, were spread out on the floor and strapped down with a cargo net.  As soon as I broke ground, the nose pitched up.  I went forward on the elevator, and it pitched down.  This continued all the way for the 300 mile flight. I pulled my seat as far forward as possible, but it was not enough.  I was afraid I would not be able to land as the oscillations continued all the way to touchdown.  We had all assumed we would be OK with the CG.  Wrong,

As for the lifting tails on some models, Guess what?  Some models are designed with a very aft CG . Competation  free flight models ,YES, often have a CG back around 50 to 60 percent of MAC, but they all have stabs that are a large percentage of the total area., and they produce lift. The idea is that it's more effecient to have all of the flight surfaces contributing to overall lift, rather than having the tail exererting a down force

I know, this has not helped this discussion, but only muddied the water.  It sure seems to me that a positive incidence on the stab (LE up), would seem to indicate a slightly aft CG by design. The large tail plane would seem to support that. 

Joe

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RE: WACO YMF - 1/17/2013 2:25 PM   
peterduplessis


 

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quote:

ORIGINAL: sensei

quote:

ORIGINAL: Stickbuilder

The issue that we get into have to do with Reynold Numbers. Full Scale airplanes differ from models due to the effects of Scale Volume, not Linear scale. The bigger the model, the more like the full scale it becomes. For example, a cube that is 4''X4''X4'' will have a volume of 64 cubic inches. (4X4=16X4=64) A quarter scale model of that cube is 1X1X1 which yields a volume of 1. Even though the linear scale is one fourth the size of the full scale, the volume is much different. Hope this helps you see the difference.

Bill, Waco Brother #1


This is actually a good point to bring up, for the very reason that the effects of scale volume are not linear. The larger an airframe, the more efficiently it will carry weight, that is why lets say a full scale Extra 300 weighing in at around 1400 lbs. empty performs very well at that weight. Now take a half scale Extra 300 and weigh it down 700 lbs. and it is not going anyplace, in fact at half scale weighing it down to 100 lbs. and it is a real pig. Now lets get down to small aircraft anything 1/4 scale or less. The smaller the airplane the more important it is to build as light as possible, because the Reynolds numbers are working against you from a load carrying efficiency standpoint.

Bob


Using your example of the Extra 300 weighing 1400 lbs (power to weight ratio of 0.2143)... a third scale would theoretically be 1/27th (3x3x3) =51 lbs .. (mm a little heavy on the wing loading!) with 11hp (300hp/27) same power to weight ratio... will fly probably quite realistically compared to the fullsize in terms of take-off and stall speed performance. As modellers, Have we not just become more accustomed to having more power to weight (that 1/3rd scale Extra would typically weigh around 30-35 lbs) to get the adrenalin pumping? Peter, #224


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RE: WACO YMF - 1/17/2013 3:50 PM   
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Status: offline 		" The wing has to fly at an angle of attack (AOA) at any given speed that will allow it (the wing) to produce enough lift to support the weight of the aircraft"

Only for symmetrical airfoils.

Les

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RE: WACO YMF - 1/17/2013 4:10 PM   
sensei



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quote:

ORIGINAL: peterduplessis


quote:

ORIGINAL: sensei

quote:

ORIGINAL: Stickbuilder

The issue that we get into have to do with Reynold Numbers. Full Scale airplanes differ from models due to the effects of Scale Volume, not Linear scale. The bigger the model, the more like the full scale it becomes. For example, a cube that is 4''X4''X4'' will have a volume of 64 cubic inches. (4X4=16X4=64) A quarter scale model of that cube is 1X1X1 which yields a volume of 1. Even though the linear scale is one fourth the size of the full scale, the volume is much different. Hope this helps you see the difference.

Bill, Waco Brother #1


This is actually a good point to bring up, for the very reason that the effects of scale volume are not linear. The larger an airframe, the more efficiently it will carry weight, that is why lets say a full scale Extra 300 weighing in at around 1400 lbs. empty performs very well at that weight. Now take a half scale Extra 300 and weigh it down 700 lbs. and it is not going anyplace, in fact at half scale weighing it down to 100 lbs. and it is a real pig. Now lets get down to small aircraft anything 1/4 scale or less. The smaller the airplane the more important it is to build as light as possible, because the Reynolds numbers are working against you from a load carrying efficiency standpoint.

Bob


Using your example of the Extra 300 weighing 1400 lbs (power to weight ratio of 0.2143)... a third scale would theoretically be 1/27th (3x3x3) =51 lbs .. (mm a little heavy on the wing loading!) with 11hp (300hp/27) same power to weight ratio... will fly probably quite realistically compared to the full size in terms of take-off and stall speed performance. As modellers, Have we not just become more accustomed to having more power to weight (that 1/3rd scale Extra would typically weigh around 30-35 lbs) to get the adrenalin pumping? Peter, #224

No, the smaller you go, the less efficient the lifting capacity is, your forgetting about the Reynolds numbers working against you the smaller you go.

Bob

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RE: WACO YMF - 1/17/2013 4:13 PM   
sensei



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'' The wing has to fly at an angle of attack (AOA) at any given speed that will allow it (the wing) to produce enough lift to support the weight of the aircraft''

Only for symmetrical airfoils.

Les

All airfoils need AOA to produce lift with the exception of an asymmetric airfoil.

Bob

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RE: WACO YMF - 1/17/2013 4:16 PM   
Jaybird


 

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Status: offline 		Don't all airfoils need some AOA to the relative wind to produce lift?

AOA (angle of Attack) happens during flight in relationship to the relative wind and is different than AOI (Angle of Incidence) which part of the structural design.

What was the question again?


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RE: WACO YMF - 1/17/2013 4:26 PM   
sensei



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Don't all airfoils need some AOA to the relative wind to produce lift?

AOA (angle of Attack) happens during flight in relationship to the relative wind and is different than AOI (Angle of Incidence) which part of the structural design.

What was the question again?


Jaybird

Yes, except an asymmetric/cambered airfoil. Don't take my word for it, Google it.

Bob

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RE: WACO YMF - 1/17/2013 8:12 PM   
nine o nine


 

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Status: offline 		Who is this guy Reynolds??? Why did he screw everything up??? Lift generation??
According to the Bernoulli venturi principle it's the "separation of air molecules" allowing a low pressure area on the top surface of the wing????? That's what I taught when I was a certificated flight instructor....how many years ago was that???
Just kidding around. The whole basic aerodynamics discussion is fun. Mitch


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RE: WACO YMF - 1/17/2013 8:15 PM   
Jaybird


 

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Status: offline 		Almost as much fun as the "why don't people build kits anymore" converstion on the other thread!

Jaybird

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RE: WACO YMF - 1/17/2013 8:23 PM   
FMBB


 

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Status: offline 		Hi Joe,

symmetrical airfoil was chosen to have a better inverted flight performance. If you fly upside down with Clark Y your AoA has to be awfully high which in my opinion does not look nice. With the symmetrical airfoil you AoA is much lower and your nearly can fly straight and level. That was the reason for this airfoil. A customer did built it with the Clark Y flies good as well but inverted see above.

Peter

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RE: WACO YMF - 1/17/2013 8:32 PM   
FMBB


 

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sorry fro not mentioning the CG in the plans but the CG is max. 2 cm in front of the aft cabane struts on the upper wing center section if you weight it in more to the front it is nose heavy which is not such as critical as tail heavy but....


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RE: WACO YMF - 1/17/2013 8:41 PM   
FMBB


 

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just to make it clear:

upper wing 0 to minus 2 degrees
lower wing 0 degrees
horizontal stab 2 degrees leading edge up
CG just in front of the aft cabane strut about one to two fingers width.

That's how the Barth Wacos should fly and I do fly them.

Peter

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RE: WACO YMF - 1/17/2013 8:55 PM   
Dash7ATP



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Hi Joe,

symmetrical airfoil was chosen to have a better inverted flight performance. If you fly upside down with Clark Y your AoA has to be awfully high which in my opinion does not look nice. With the symmetrical airfoil you AoA is much lower and your nearly can fly straight and level. That was the reason for this airfoil. A customer did built it with the Clark Y flies good as well but inverted see above.

Peter

Waco Brother #170


Thanks, Peter,

I assumed it had to do with aerobatic preformance, but I wanted to make sure I wasn't missing something.

Joe



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RE: WACO YMF - 1/17/2013 9:24 PM   
sensei



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Status: offline 		The designer bearers the burden of making all these structural and aerodynamic decisions, and he must consider many tradeoffs along the way.

Bob

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RE: WACO YMF - 1/18/2013 8:37 AM   
RCplanman


 

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Status: offline 		jaybird,What is the incidence of your bottom wing.?

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