Va and weight?
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ppragman
FLIPY FLAPS!
Think about inertia, like trying to move a wheel barrow filled with lead ingots opposed to an empty one, what's easier? When you're empty it doesn't take that much force to move you, and a gust could make you exceed the structural limits of the airplane.
Site search of the CFI forums: http://forums.jetcareers.com/search.php?searchid=1938046&pp=25
This has been discussed at least 10 times in the last 6 months.
I am cautious about the inertia explanation, it doesn't really give the aerodynamic reason. The distance between the current flying AOA and the critical AOA is the why.
Inertia can help you remember it, but doesn't really have anything to do with the why.
This has been discussed at least 10 times in the last 6 months.
I am cautious about the inertia explanation, it doesn't really give the aerodynamic reason. The distance between the current flying AOA and the critical AOA is the why.
Inertia can help you remember it, but doesn't really have anything to do with the why.
Fly_Unity
Well-Known Member
I like Rod Malacho's way of explaining it. I Print his page for every student.
Maneuvering speed is based on the airplane being at gross weight. What happens when the airplane's weight decreases? The answer is: the maneuvering speed decreases.
Airplanes flown at weights below their gross weight require less lift for straight and level flight. Less lift means the airplane can be flown at a smaller angle of attack. In other words, an airplane at 2,500 pounds may require a 4.5 degree angle of attack at 110 knots to remain in level flight. Decreasing the weight to 1,800 pounds may require only a 3 degree angle of attack to remain in level flight at this speed.
With a speed of 110 knots, at this lower weight, a sudden and very strong gust could increase the angle of attack from 3 to 18 degrees. this produces six times the original lift for a force of 6Gs. This is way beyond the limit of a normal category airplane. At lighter weights, what can we do to keep from exceeding our example limit of 4Gs when in turbulence?
The answer is to slow the airplane down. At a slower speed (95 knots for example) a larger angle of attack (let's say 4.5 degrees) is necessary for level cruise flight at this lower weight.
At this speed, we can increase the angle of attack four times before the airplane stalls. Ninety-five knots becomes our new maneuvering speed if we want to limit ourselves to 4Gs. Thus, decreasing weight requires a decrease in the airplane's maneuvering speed.
Most of the newer Pilot's Operating Handbooks publish two or three different maneuvering speeds for variable weight conditions. If yours doesn't, try doing the following to compute a new one. For every 2% reduction in weight, reduce the max-weight maneuvering speed by 1%. In other words, if the gross weight decreases by 20%, reducethe max-gross weight maneuvering speed by 10%.
Maneuvering speed is based on the airplane being at gross weight. What happens when the airplane's weight decreases? The answer is: the maneuvering speed decreases.
Airplanes flown at weights below their gross weight require less lift for straight and level flight. Less lift means the airplane can be flown at a smaller angle of attack. In other words, an airplane at 2,500 pounds may require a 4.5 degree angle of attack at 110 knots to remain in level flight. Decreasing the weight to 1,800 pounds may require only a 3 degree angle of attack to remain in level flight at this speed.
With a speed of 110 knots, at this lower weight, a sudden and very strong gust could increase the angle of attack from 3 to 18 degrees. this produces six times the original lift for a force of 6Gs. This is way beyond the limit of a normal category airplane. At lighter weights, what can we do to keep from exceeding our example limit of 4Gs when in turbulence?
The answer is to slow the airplane down. At a slower speed (95 knots for example) a larger angle of attack (let's say 4.5 degrees) is necessary for level cruise flight at this lower weight.
At this speed, we can increase the angle of attack four times before the airplane stalls. Ninety-five knots becomes our new maneuvering speed if we want to limit ourselves to 4Gs. Thus, decreasing weight requires a decrease in the airplane's maneuvering speed.
Most of the newer Pilot's Operating Handbooks publish two or three different maneuvering speeds for variable weight conditions. If yours doesn't, try doing the following to compute a new one. For every 2% reduction in weight, reduce the max-weight maneuvering speed by 1%. In other words, if the gross weight decreases by 20%, reducethe max-gross weight maneuvering speed by 10%.
Guy
Well-Known Member
Did she happen to say what she didn't like about your explanation or what her explanation was?
However, I will say in defense of anyone who has trouble grasping the idea, it isn't a principle that is intuitively obvious.
Just thought of this, would it work, wouldn't it, thoughts?
Consider a 5 pound lead ball and a scale. If you pick the ball up 2 inches and drop it, will it impact harder or softer (more or less force) than if you pick it up 5 inches and drop it? Compare that to the movement of AOA. Maybe as a visual way to explain it.
Still have to explain how the AOA is lower at a lighter weight, but I think that concept is easier to explain.
Consider a 5 pound lead ball and a scale. If you pick the ball up 2 inches and drop it, will it impact harder or softer (more or less force) than if you pick it up 5 inches and drop it? Compare that to the movement of AOA. Maybe as a visual way to explain it.
Still have to explain how the AOA is lower at a lighter weight, but I think that concept is easier to explain.
Here is one more for you.
Take a bowling ball (heavy airplane) and a ping pong ball (light airplane) Roll them down the lane where half way down there is a fan blowing across the lane (sudden gust). Which ball moves more.
But the Muchado explanation about the AOA with varied weights is golden.
Take a bowling ball (heavy airplane) and a ping pong ball (light airplane) Roll them down the lane where half way down there is a fan blowing across the lane (sudden gust). Which ball moves more.
But the Muchado explanation about the AOA with varied weights is golden.
moxiepilot
Well-Known Member
PFGiardino
Well-Known Member
This heavy object discussion really covers only half the discussion and deals with max turbulent airspeed (Vno) and a [15kt] gust "pushing" you over the load limit while above that speed.
Va being lower at lower weights deals with aerodynamics, and once I have more time to put my thoughts together (and am not at work), I'll post my explanation here.
Va being lower at lower weights deals with aerodynamics, and once I have more time to put my thoughts together (and am not at work), I'll post my explanation here.
tgrayson
New Member
Except that it doesn't. Note that maneuvering speed doesn't mean that a particular gust velocity (or control deflection) will not cause you to exceed load factor limits, it means that you are unable to have any gust or control deflection that will cause you to exceed load factor limits (see note). This cannot be explained without explaining how the stall speed changes.
Note: An airfoil stalling is only a limited means of overload protection. Lift doesn't go away and it's possible that you can have a vertical gust of such strength that even though it stalls the airfoil, the resulting load factor can still exceed structural limits.
Ooh. That's the example I use. Are you in my head, or were you one of my students?
No, not one of your students, however did pick that up while I was working on my CFI. It made sense to me.
I don't think we are on the same page? The AOA is lower for any given speed at a lighter weight than a heavier weight. I wasn't saying you could ignore explaining this.
My explanation was with regards to understanding "throw" so to speak. Meaning current AOA - critical would be similar to dropping a weight from a given height. Falling from a given height is relatively the same as a given number of degrees available to travel, before stall, in that they both vary the time for the given acceleration to take place. Does that make sense?
tgrayson
New Member
Not to me. The change in AoA produces a change in force; the bigger the AoA change, the bigger the force. In the case of the accelerating ball, the force is constant...m*9.8m/s^2.
Not the impact force though. Which to a laymen would suffice. Drop a ball on a scale from 6 inches will yield a smaller deflection on the scale than dropped from a foot. Moving a wing 5 degrees to reach stall would result in less g's than moving it 10 degrees. All physics aside this experiment would hold true, god here we go with absolutes, every time. Right?
tgrayson
New Member
You have to have an impact first.
The force generated then is the result of the interaction between the moving object and the object it impacts; the force that the moving object imparts is the Newton's 3rd Law reaction force of the stationary object's attempt to slow down the moving object.I don't see the corollary, myself, with the present subject.
pdxcfi
Flyin' Shoe
Easiest way to explain it...with an example. Lest you refer to Aero for Naval Aviators.
Maneuvering speed:rawk:
In aviation, maneuvering speed is the highest speed at which full deflection of the controls about any one axis are guaranteed not to overstress the airframe. At or below this speed, the controls may be moved to their limits. Above this speed, moving the controls to their limits may overstress the airframe and potentially cause a structural failure. It is normally designated as VA in flight manuals, but is not typically shown on most airspeed indicators.
With full elevator deflection at maneuvering speed, an aerodynamic stall will occur, reducing or eliminating lift forces before damage can occur to the aircraft.[1] To increase lift of a given wing, the angle of attack, air density, or the airspeed must be increased. The wing of an aircraft stalls at a specific angle of attack, regardless of airspeed. However, the higher the airspeed, the more lift the wing is capable of producing, and at a certain airspeed it is capable of producing more lift than it can support structurally. The declared maneuvering speed is based on the aircraft's maximum gross weight. At lower weights, maneuvering speed is always lower.
Hypothetically, if an aircraft were flying at a weight equal to its maximum structural load, it would be flying at both stall speed for that weight and maneuvering speed, with no excess angle of attack and lift available to accelerate the aircraft upward. At lower weights, and the same air speed and air density, the aircraft would be flying at a lower angle of attack, well below of stalling condition, and therefore with an excess lift available which could not be structurally supported. Therefore, as gross weight is decreased, maneuvering speed also decreases.
The maneuvering speed decreases as the aircraft's weight decreases from maximum takeoff weight because the effects of the aerodynamic forces become more pronounced as its weight decreases. The flight manuals for some aircraft (such as the Piper Cherokee) specify the design maneuvering speeds for weights below the maximum takeoff weight but sometimes it is left to the pilot to calculate. Using a "Rule of Thumb", the reduction in VA will be half the percentage reduction in aircraft weight. For example if, with only one person on board, weight is 16% below maximum takeoff weight, then VA is reduced by 8%. [2]
Maneuvering speed:rawk:
In aviation, maneuvering speed is the highest speed at which full deflection of the controls about any one axis are guaranteed not to overstress the airframe. At or below this speed, the controls may be moved to their limits. Above this speed, moving the controls to their limits may overstress the airframe and potentially cause a structural failure. It is normally designated as VA in flight manuals, but is not typically shown on most airspeed indicators.
With full elevator deflection at maneuvering speed, an aerodynamic stall will occur, reducing or eliminating lift forces before damage can occur to the aircraft.[1] To increase lift of a given wing, the angle of attack, air density, or the airspeed must be increased. The wing of an aircraft stalls at a specific angle of attack, regardless of airspeed. However, the higher the airspeed, the more lift the wing is capable of producing, and at a certain airspeed it is capable of producing more lift than it can support structurally. The declared maneuvering speed is based on the aircraft's maximum gross weight. At lower weights, maneuvering speed is always lower.
Hypothetically, if an aircraft were flying at a weight equal to its maximum structural load, it would be flying at both stall speed for that weight and maneuvering speed, with no excess angle of attack and lift available to accelerate the aircraft upward. At lower weights, and the same air speed and air density, the aircraft would be flying at a lower angle of attack, well below of stalling condition, and therefore with an excess lift available which could not be structurally supported. Therefore, as gross weight is decreased, maneuvering speed also decreases.
The maneuvering speed decreases as the aircraft's weight decreases from maximum takeoff weight because the effects of the aerodynamic forces become more pronounced as its weight decreases. The flight manuals for some aircraft (such as the Piper Cherokee) specify the design maneuvering speeds for weights below the maximum takeoff weight but sometimes it is left to the pilot to calculate. Using a "Rule of Thumb", the reduction in VA will be half the percentage reduction in aircraft weight. For example if, with only one person on board, weight is 16% below maximum takeoff weight, then VA is reduced by 8%. [2]
Technically correct, sure. Easiest way? Only to select individuals.
I would bet if you read this passage to 100 private pilots, at least half of them would look at you like you were out of your mind. Of the half that didn't, only half of them would actually have a good basic understanding.