Principles of Flight
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Aerodynamic Posers
Hi All
New to the Forum, with a couple of tech questions (for starters). Got my own thoughts on these but I'd like clarification from someone with a more solid understanding.
1. Why will the roll capability of an aircraft INCREASE with a DECREASE in Gross Weight? Roll capability is a function of AoA (i.e a lower AoA increases roll capability) but why?
2. Why will a swept wing INCREASE the amount of roll induced by rudder inputs?
Looking forward to your replies.
Rorschach
New to the Forum, with a couple of tech questions (for starters). Got my own thoughts on these but I'd like clarification from someone with a more solid understanding.
1. Why will the roll capability of an aircraft INCREASE with a DECREASE in Gross Weight? Roll capability is a function of AoA (i.e a lower AoA increases roll capability) but why?
2. Why will a swept wing INCREASE the amount of roll induced by rudder inputs?
Looking forward to your replies.
Rorschach
Why will the roll capability of an aircraft INCREASE with a DECREASE in Gross Weight? Roll capability is a function of AoA (i.e a lower AoA increases roll capability) but why?
roll rate is a function of wingspan and airspeed
2. Why will a swept wing INCREASE the amount of roll induced by rudder inputs?
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With sweep, there is also a shorter functional span, reducing radial moment for the same wing loading. An aspect ratio "decrease". By coincidence, dihedral resists the roll, due vertical load, anhedral increases it (rate). Also, empty tip tanks help, they are released prior to the "knife fight". (Olden days speak).As Rudder induces Yaw in the Airframe (per PA), a swept wing ( outboard) gains effective lift, frontally, increasing the rate of Roll.
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Pugilistic Animus & Bearfoil
Here are the bits from a manual that I'm specifically referring to:
Roll Rates
Speed brake usage, flap position, gross weight, altitude and hydraulic
system operation all affect roll rates. At the same airspeed with flaps down, roll capability increases withreduction in gross weight. In other words, roll rate capability is a function of angle of attack, and, for different gross weights at the same angle of attack, the roll rate is almost the same. Since all normal approaches to landings are made at approximately the same angle of attack regardless of weight, the pilot will probably not notice any
difference in roll capability on final approach. However you may notice
that the aircraft feels more responsive in roll when flying the fixed
speed schedule at light weights.
Yaw Control – Rudder
In comparison to the ailerons, the rudder is very powerful and, due to
the swept wing, small rudder movements cause considerable roll as a
secondary effect.
So Pugilistic Animus, I am talking about roll capability, which I interpret here as roll rate for a given control input. Might be wrong on this though. I just can't quite get my head round why a lower gross weight increases roll capability.
Bearfoil
Why wouldn't a straight, non-swept wing also gain more lift as the aircraft yaws? Doesn't it's outboard wing also speed up relative to the inboard wing as the aircraft yaws - and hence gain lift, inducing roll? Or indeed, why is this effect greater on a swept wing aircraft?
Here are the bits from a manual that I'm specifically referring to:
Roll Rates
Speed brake usage, flap position, gross weight, altitude and hydraulic
system operation all affect roll rates. At the same airspeed with flaps down, roll capability increases withreduction in gross weight. In other words, roll rate capability is a function of angle of attack, and, for different gross weights at the same angle of attack, the roll rate is almost the same. Since all normal approaches to landings are made at approximately the same angle of attack regardless of weight, the pilot will probably not notice any
difference in roll capability on final approach. However you may notice
that the aircraft feels more responsive in roll when flying the fixed
speed schedule at light weights.
Yaw Control – Rudder
In comparison to the ailerons, the rudder is very powerful and, due to
the swept wing, small rudder movements cause considerable roll as a
secondary effect.
So Pugilistic Animus, I am talking about roll capability, which I interpret here as roll rate for a given control input. Might be wrong on this though. I just can't quite get my head round why a lower gross weight increases roll capability.
Bearfoil
Why wouldn't a straight, non-swept wing also gain more lift as the aircraft yaws? Doesn't it's outboard wing also speed up relative to the inboard wing as the aircraft yaws - and hence gain lift, inducing roll? Or indeed, why is this effect greater on a swept wing aircraft?
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moment of inertia, perhaps?
In my humble opinion the increased roll authority is simply due to the reduced moment of inertia.
In general, the lesser mass the lesser moment of inertia about the longitudinal axis and therefore, the greater the roll rate the same ailerons/spoilers are capable of.
In general, the lesser mass the lesser moment of inertia about the longitudinal axis and therefore, the greater the roll rate the same ailerons/spoilers are capable of.
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It would seem to me that some confusion is due to sloppy (imprecise) language. The book quotation clearly speaks about roll rate (or intends to), which context is missing from your (Jane's?) OP. For same airspeed reduction of gross weight means lower AoA. I don't understand the book's claim that this increases roll rate. I think the author is confusing roll rate with roll acceleration (the rate of increase of roll rate), and making the tacit assumption that the weight is mainly in the wing, and then reduction of weight means lower inertia in the roll axis.
regards,
HN39
regards,
HN39
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Rorschach(DoH)
If looking at Plan view, the straight wings in Yaw lose frontal exposure at a reasonably "equal rate". The Lead wing is roughly equivalent in lift to the Trailing wing as Yaw increases.
With Sweep, the Lead wing is gaining effective frontal exposure as the Trailing wing loses it. This can be dramatic and cause inversion of the airframe with additional Yaw. One wing is 'extending' as the other "tucks". Additionally, the trailing wing is progressively "Blanked" by the fuselage further reducing its lift.
This is expressed as 'acceleration of rate', as Hazelnuts has stated. Roll rate has to do with velocity about the Longitudinal axis, and is reliant on concentration of mass about the axis of the airframe, among other factors.
Yaw produces an acceleration of wing speed in the outer wing, yes, but more important is the attendant and relative aspect ratios of the 'new' and changing frontal areas of the respective leading edges. Yaw is almost always not to be wished, hence the term "adverse". It produces a "skid", an unaerodynamic feature that greatly increases drag, some of it morphing into discrepant lift, hence Roll.
If looking at Plan view, the straight wings in Yaw lose frontal exposure at a reasonably "equal rate". The Lead wing is roughly equivalent in lift to the Trailing wing as Yaw increases.
With Sweep, the Lead wing is gaining effective frontal exposure as the Trailing wing loses it. This can be dramatic and cause inversion of the airframe with additional Yaw. One wing is 'extending' as the other "tucks". Additionally, the trailing wing is progressively "Blanked" by the fuselage further reducing its lift.
This is expressed as 'acceleration of rate', as Hazelnuts has stated. Roll rate has to do with velocity about the Longitudinal axis, and is reliant on concentration of mass about the axis of the airframe, among other factors.
Yaw produces an acceleration of wing speed in the outer wing, yes, but more important is the attendant and relative aspect ratios of the 'new' and changing frontal areas of the respective leading edges. Yaw is almost always not to be wished, hence the term "adverse". It produces a "skid", an unaerodynamic feature that greatly increases drag, some of it morphing into discrepant lift, hence Roll.
Last edited by bearfoil; 18th Jan 2011 at 14:11.
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I think I understand now why it says roll rate is function of angle of attack (during approach). It has to do with moment of inertia and dynamic pressure.
As you know, force is to mass and linear acceleration what moment is to moment of inertia and angular acceleration. Moments about points (or an axis) are created by forces. In this case we are dealing with rolling moments (about the longitudinal axis) created by the differential lift force in the ailerons.
On the one hand, for a constant speed, the less the weight, the less moment of inertia about the longitudinal axis, which means that the ailerons can create greater roll rates. This relation is inversely proportional.
On the other hand, for the same angle of attack (approach AoA) the less the weight the less the dynamic pressure. The less the dynamic pressure, the less the lift ailerons can create. This relation is directly proportional.
Both effects cancel each other and therefore roll rates due to aileron deflection will be constant for a given AoA regardless of the weight.
As you know, force is to mass and linear acceleration what moment is to moment of inertia and angular acceleration. Moments about points (or an axis) are created by forces. In this case we are dealing with rolling moments (about the longitudinal axis) created by the differential lift force in the ailerons.
On the one hand, for a constant speed, the less the weight, the less moment of inertia about the longitudinal axis, which means that the ailerons can create greater roll rates. This relation is inversely proportional.
On the other hand, for the same angle of attack (approach AoA) the less the weight the less the dynamic pressure. The less the dynamic pressure, the less the lift ailerons can create. This relation is directly proportional.
Both effects cancel each other and therefore roll rates due to aileron deflection will be constant for a given AoA regardless of the weight.
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Microburst2002;
I agree with large parts of what you wrote but, at the risk of sounding pedantic, I would like to add two distinctions:
Moments of inertia are not directly proportional to weight because they depend on mass distribution. If the mass is mainly carried in the fuselage, variation of gross weight will have little effect on rolling moment of inertia. If, on the other hand, the wing contains a large mass of fuel in its tanks, then reduction of that mass has a large effect on the moment of inertia about the longitudinal axis.
Secondly, I would prefer to distinguish more clearly between angular acceleration (usually expressed in deg/sec/sec) and roll rate (deg/sec). At zero roll rate, angular acceleration is, as you write, rolling moment from aileron deflections divided by rolling moment of inertia. The angular acceleration produces an increasing roll rate, which means that one wing moves down, that downward velocity increases the AoA of that wing, and hence its lift. Together with the opposite effect on the other wing, that produces a rolling moment opposing that from the ailerons. At some point these moments will cancel each other and roll rate will not increase any further. At constant roll rate the moment of inertia is irrelevant.
regards,
HN39
I agree with large parts of what you wrote but, at the risk of sounding pedantic, I would like to add two distinctions:
Moments of inertia are not directly proportional to weight because they depend on mass distribution. If the mass is mainly carried in the fuselage, variation of gross weight will have little effect on rolling moment of inertia. If, on the other hand, the wing contains a large mass of fuel in its tanks, then reduction of that mass has a large effect on the moment of inertia about the longitudinal axis.
Secondly, I would prefer to distinguish more clearly between angular acceleration (usually expressed in deg/sec/sec) and roll rate (deg/sec). At zero roll rate, angular acceleration is, as you write, rolling moment from aileron deflections divided by rolling moment of inertia. The angular acceleration produces an increasing roll rate, which means that one wing moves down, that downward velocity increases the AoA of that wing, and hence its lift. Together with the opposite effect on the other wing, that produces a rolling moment opposing that from the ailerons. At some point these moments will cancel each other and roll rate will not increase any further. At constant roll rate the moment of inertia is irrelevant.
regards,
HN39
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HZ39
Yes, the moment of inertia is proportional to mass... And to distance from rotation axis squared, too. All other factors constant, a weight reduction means a reduced moment of inertia. Moreover, fuel burn tends to redistribute weight towards the longitudinal axis, further reducing moment of inertia.
As the wings roll, other forces appear that finally cancel the moment and a steady roll rate is reached. I don't know those roll opposing forces in detail, but seems like they depend exclusively on the roll rate itself, like Drag depends on the airplane's airspeed. Therefore maximum roll rate should be constant regardless of airplane's mass. Moment of inertia would only affect the time to achieve that maximum roll rate. Am I right?
Yes, the moment of inertia is proportional to mass... And to distance from rotation axis squared, too. All other factors constant, a weight reduction means a reduced moment of inertia. Moreover, fuel burn tends to redistribute weight towards the longitudinal axis, further reducing moment of inertia.
As the wings roll, other forces appear that finally cancel the moment and a steady roll rate is reached. I don't know those roll opposing forces in detail, but seems like they depend exclusively on the roll rate itself, like Drag depends on the airplane's airspeed. Therefore maximum roll rate should be constant regardless of airplane's mass. Moment of inertia would only affect the time to achieve that maximum roll rate. Am I right?
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So a plane with a huge span, swept-wings, big ailerons on the tips, and most of the mass in the center of the plane, and on the tips with nothing in the middle would get the best roll rate 
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So a plane with a huge span, swept-wings, big ailerons on the tips, and most of the mass in the center of the plane, and on the tips with nothing in the middle would get the best roll rate
Best roll rate planes have their mass in the center, powerful ailerons, short wings and having hydraulic boost doesn't hurt a bit. Examples T-38 and A-4 Skyhawk. They'll bang your head off the canopy if you aren't paying close attention.
Now in the big iron stuff, with load distributed along the fuselage, increased AOA causes the fuselage to get out of line with the actual roll axis and that increases roll inertia and should decrease roll rate acceleration.
I believe that they may mean that an increase in normal incremental wing loading with aileron deflection is greater at heavier weights due to the additional loads imparted by turning superimposed on the already higher loading due to heavier weights, but there could also be a hinge moment limit wrt to mechanical forces at play...I don't really get their explanation--- a little long-winded...
this is more important
skip to 3:30...
then watch the whole clip...
and I'm supposedly a drummer too but like all others except Gene Krupa, I'm too just beating Tin Lizzie
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A long time ago l needed to know how effiicient a petrol engine would be with normal aspiration.
It centred around how much fluid could pass the inlet valves.
I`ll put it another way.
If you take a five gallon can and turn it upside down, with airways in the bottom/top - stay with me - and take the cap away altogether it will empty in x seconds.
How close would you need to hold the top to slow the flow?
l reckon slightly under one quarter of the diameter of the opening.
Which is bad news for boy racers.
It centred around how much fluid could pass the inlet valves.
I`ll put it another way.
If you take a five gallon can and turn it upside down, with airways in the bottom/top - stay with me - and take the cap away altogether it will empty in x seconds.
How close would you need to hold the top to slow the flow?
l reckon slightly under one quarter of the diameter of the opening.
Which is bad news for boy racers.
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From the same special
although I'll say that with Gene you don't listen or watch the drums you experience them-a mastery of rudimentary pattern interpretation heretofore un-excelled just like the L-1011 Tristar
come join the discussion
http://www.pprune.org/jet-blast/3634...-our-time.html