weight vs. drag
06-18-2008, 10:14 PM
#2
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RE: weight vs. drag
ORIGINAL: 1fasthitman
If we could compare apples to apples, what is more difficult to overcome, weight or aerodynamic drag?
If we could compare apples to apples, what is more difficult to overcome, weight or aerodynamic drag?
As it is stated, I don't think there is a clear answer to your question. What type of plane are we talking about here?
Cheers,
Rich
06-19-2008, 06:12 AM
#4
Senior Member
RE: weight vs. drag
If you're designing a new airplane, it's easy to do things like work out the least frontal area, or draw a good pressure cowl design.
When you're choosing what design structure, you're locked into what you think is at least strong enough structures. No wiggle room there.
If you're building a kit, there are often lots of places where you can choose to build parts of the plane with a different, lighter design. Or use lighter, stronger wood. Or add lightening holes. But little opportunity to change shapes or sizes. If you're assembling an ARF, ain't no wiggle room to do much of anything if you're like most ARF builders and pressed for time. On the other hand, awhile back, I chose not to use the horizontal tail from an ARF. The stab was made out of crap wood and the design was sticks. And the center section wasn't any larger than the fuselage so the LE was the only stiffener the stab had from it's TE forward. I laid out the same shape with a wider center piece, a LE doubler that went out about .3S, and added a couple of diagonals. Also used balsa that fit the task of each piece. After all, I wanted strength and no excess weight. Apples and oranges, but both good fruit. And the elevator was two solid pieces of some kind of wood that'd been good flooring. So that got flipped into the trash after tracking the outline. The new pieces were quarter grain, medium balsa. And since movable surfaces flutter relative to their mass, I used some hole bits and made a pretty row of holes along them. Looked sweet and holes are light, very light. Nah, it wasn't really an ARF. For sure it wasn't an ARFWVL. But it wound up as one. It didn't come out of the box Almost Ready to Fly Well and for Very Long. But it took to the air that way.
About the only place I can think of where weight and drag shows up together is wing fillets. You're doing a fillet to reduce drag. It adds weight. So you can choose to make it as small as possible to keep the weight down. And build it with as little weight as you know how. And you can control both. But it's easier to not do fillets at all.
So, are you looking for hints on where to cut holes in heavy ARFs? Or whether or not to throw out heavy ARF parts? If you're building a kit, you've got some room to do some things. Building a scale plane, only room to design in lightness. Almost no room with everything to reduce drag unless you're the designer. The word "reduce" suggests a starting point you don't control.
For the most part, you change shapes to reduce drag. The lesser part, you keep things smooth. Two different fruit.
Using lighter components won't change drag, except for induced drag that is. And components that don't change size or shape usually don't change the drag.
I apologize for not understand what you want to do. I would guess all this blather hasn't helped you at all.
When you're choosing what design structure, you're locked into what you think is at least strong enough structures. No wiggle room there.
If you're building a kit, there are often lots of places where you can choose to build parts of the plane with a different, lighter design. Or use lighter, stronger wood. Or add lightening holes. But little opportunity to change shapes or sizes. If you're assembling an ARF, ain't no wiggle room to do much of anything if you're like most ARF builders and pressed for time. On the other hand, awhile back, I chose not to use the horizontal tail from an ARF. The stab was made out of crap wood and the design was sticks. And the center section wasn't any larger than the fuselage so the LE was the only stiffener the stab had from it's TE forward. I laid out the same shape with a wider center piece, a LE doubler that went out about .3S, and added a couple of diagonals. Also used balsa that fit the task of each piece. After all, I wanted strength and no excess weight. Apples and oranges, but both good fruit. And the elevator was two solid pieces of some kind of wood that'd been good flooring. So that got flipped into the trash after tracking the outline. The new pieces were quarter grain, medium balsa. And since movable surfaces flutter relative to their mass, I used some hole bits and made a pretty row of holes along them. Looked sweet and holes are light, very light. Nah, it wasn't really an ARF. For sure it wasn't an ARFWVL. But it wound up as one. It didn't come out of the box Almost Ready to Fly Well and for Very Long. But it took to the air that way.
About the only place I can think of where weight and drag shows up together is wing fillets. You're doing a fillet to reduce drag. It adds weight. So you can choose to make it as small as possible to keep the weight down. And build it with as little weight as you know how. And you can control both. But it's easier to not do fillets at all.
So, are you looking for hints on where to cut holes in heavy ARFs? Or whether or not to throw out heavy ARF parts? If you're building a kit, you've got some room to do some things. Building a scale plane, only room to design in lightness. Almost no room with everything to reduce drag unless you're the designer. The word "reduce" suggests a starting point you don't control.
For the most part, you change shapes to reduce drag. The lesser part, you keep things smooth. Two different fruit.
Using lighter components won't change drag, except for induced drag that is. And components that don't change size or shape usually don't change the drag.
I apologize for not understand what you want to do. I would guess all this blather hasn't helped you at all.
06-19-2008, 07:36 AM
#5
Senior Member
RE: weight vs. drag
I'd like to mention that excess weight can actually add drag. As mentioned by rgunder, lifting capability overcomes weight. More weight requires more lifting capability - more lift always comes with more drag. This in turn can require more thrust. These two things are not completely exclusive of eachother.
DaRock hinted at this when he brought up induced drag.
DaRock hinted at this when he brought up induced drag.
06-19-2008, 01:11 PM
#6
RE: weight vs. drag
The classic design trade-off is probably retractable landing gear. Over the years I've noticed pattern planes add retracts over fixed gear only to shed them a few years later. It would seem the lower weight (and thus inertia) makes the plane more tractable and reactive to inputs. (Changing the direction of a rolling basketball is easier than a rolling bowling ball even though its larger.) I've also been told that a dirty shape helps prevent the plane from accelerating in the down-leg part of a maneuver where constant speed is required.
While the retracted gear gives the plane a higher maximum speed for a given engine power it's probably only important if you have a high priority to go FAST. Parasitic drag starts low and climbs quickly with airspeed because of it's geometric relationship to velocity. Inertia, on the other hand, is directly proportional to acceleration so can effect performance over the entire range of flight speeds including a slow landing on a gusty day.
With that said, I think everyone that designs their own planes wants to build a rocketship at some point. I'm currently working on a twin that has retracts, lengthened nose for better penetration, streamlined nacelles with minimum frontal area, large transition fillets between wing and fuselage. Will it go fast? Man, I hope so....It's been a LOT of work and $$$.
While the retracted gear gives the plane a higher maximum speed for a given engine power it's probably only important if you have a high priority to go FAST. Parasitic drag starts low and climbs quickly with airspeed because of it's geometric relationship to velocity. Inertia, on the other hand, is directly proportional to acceleration so can effect performance over the entire range of flight speeds including a slow landing on a gusty day.
With that said, I think everyone that designs their own planes wants to build a rocketship at some point. I'm currently working on a twin that has retracts, lengthened nose for better penetration, streamlined nacelles with minimum frontal area, large transition fillets between wing and fuselage. Will it go fast? Man, I hope so....It's been a LOT of work and $$$.
06-19-2008, 10:48 PM
#7
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RE: weight vs. drag
I would say that unless you're dragging around a parachute or similar frontal area then weight plays a far bigger role in how a model flies than drag. Now if you're talking about a racing plane then that's a different story. Then you need to balance weight and drag. But even there weight in the turns needs to be supported and a lighter model in the turns will make less drag than a heavy one turning at the same radius of turn.
On our models the small things like exposed wheels, lack of fillets and other fairly minor items don't mean much. We've got pleanty of power to pull the drag around and it's not like we really care about how many miles to the gallon we get with our models.
On our models the small things like exposed wheels, lack of fillets and other fairly minor items don't mean much. We've got pleanty of power to pull the drag around and it's not like we really care about how many miles to the gallon we get with our models.
