Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Minotaur on March 04, 2000, 11:39:00 PM
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Regarding A/C performance, what does the term "Moment" mean?
Example:
- Moment of Inertia
- Moment of Stall
I am beggining to think that it means the A/C is trying to do something. All the force is present to do this something, but is not doing it yet.
Example:
- The moment of inertia for an aileron roll would mean that the ailerons have deflected, but the wing has not yet began to move.
Thanks in advance! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Mino
The Wrecking Crew
Trainer
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Uhhhh, don't think so, but I'll let the engineer types answer definitively.
When I built R/C planes, planes with long moments were the stable birds, and I always thought this term referred to how far the control surfaces were located from the CG.
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Mino it is a term used in wait and balance. Moment is a point or measurement where you would add or subtract weight to balance the airplane.
An example would be a nose heavy a/c you wiegh the plane and find it 100lbs heavy in the nose, you would compute how far aft you would have to put say a 10lb wieght to equal the 100lb heavy nose that would be your moment.
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Tommy (INDIAN) Toon
1st Aces High Trainer Crew
Home of The Allied Fighter Wing A.F.W.
A.F.W. Homepage (http://www.geocities.com/~tltoon)
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Indian-
That's it! How far any object is located from the CG, not the control surfaces. Thanx!
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To understand moment of inertia, you should first understand torque (Force * distance). The force part of that formula is the 'impulse' force or the change in momentum (mass * velocity) over a time interval. Velocity is determined by distance travelled in a time interval, so at any given time, you end up with
mass * distance * distance * C
where C is a constant based on the geometry of the object.
For something like a wing (rectangular plate), the formula for roll and yaw inertia is
1/12 * mass * (span^2 + chord^2)
For a fuselage (long thin rod), you could use
1/12 * mass * length^2 = pitch and yaw inertia
For a concentrated mass like landing gear or an engine, you could use
mass * radius^2
When you divide the applied torque force from ailerons/elevator/rudder by the moment of inertia, you get the angular acceleration (radians/sec/sec), all units in metric.
1 radian = 180/pi degrees (57.3)
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wells did it already explain it, but i think he forgot something to mention
The examples for the roll inertia are only correct if you roll it around itīs own centre of gravity.
If not, you have to add another function, the "steinerglied". That means, the distance of my mass to the centre of rotation is important.
A good example is the difference between a single engine fighter, and a twin engine fighter (P38). Letīs say the centre of gravity of the engine of a fw190 is exactly in the centre of rotation of the whole aircraft. If you describe the engine as a cylinder, you get for the roll inertia
Jx = mass*r*r/2 with r = radius of the cylinder
Now, the engine of the P38 is not in the centre of gravity for the whole AC, therefor you have to add something: mass*d*d
so the total roll inertia of an engine in a wing is
Jxī = mass*r*r/2 + mass*d*d
with d= distance between centre of gravity of your object to the centre of rotation of the whole system.
With d much bigger than r, you can easily sea how the added function dominates Jxī. Thatīs why i canīt believe the fast roll rate of a P38, at least not itīs roll acceleration. Donīt forget, you donīt have one heavy engine in your wings, you even have two of them! And itīs not a linear function of the distance d as it is for the torque of the aillerons, it is d*d (quadratic)
niklas
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Don't feel bad Herr Bovine, learning what a "moment" is and how its associated concepts work were probably the most difficult thing I had to learn in engineering school.
Once you figure it out, its easy to understand. But learning it originally was a real pain for me.
Of course maybe thats why I am an Electrical Engineer instead of a Mechanical or Aerospace. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Vermillion
**MOL**, Men of Leisure
(http://web.mountain.net/~arringto/pics/yak3.jpg)
[This message has been edited by Vermillion (edited 03-05-2000).]
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I believe i didnt read his post correctly but the discription I gave ddoes go for wight and balance.
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Tommy (INDIAN) Toon
1st Aces High Trainer Crew
Home of The Allied Fighter Wing A.F.W.
A.F.W. Homepage (http://www.geocities.com/~tltoon)
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Well I asked for it I guess. Abolutely more confused than ever now. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Abreviations:
- AC = Aerodynamic Center
- CG = Center of Gravity
- P-38 = The best plane that I can think of (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Tony LeVier, a Lockheed P-38 test pilot wrote:
The dive flaps did three things (All positive):
1) They produced a slight stalling moment.
<snip>
This has nothing to do with moving static weight around on the aircraft to adjust the CG. This is talking about forces that are happening dyanamically. IE: Forces that are adjusting the difference between the CG and the AC. In this case, re-positiong the AC forward so that it is once more in-line with the existing CG of the aircraft.
Under "Roll Performance" Robert Shaw writes on Pages 412-413:
Aerodynamic roll controls operate by increasing lift on one side of the aircraft relative to that on the other, producing a rolling moment. When this condition occurs, a roll will commence, accelerate to a maximum value, and then stabilize at that rate. A stabilized roll rate is generated when a balancing or "dampning" moment is generated which offsets the torque of the roll controls. This dampning moment is produced primarily by lift differences between the two wings caused by one wing moving upward and the other downward, and is proportional to the roll stability of the aircraft. In general, the more stable a fighter is about the roll axis, the slower the roll rate will be.
This is what caused me to make my assumptive statements in the above post. I thought from what had I read that these forces were dymanic in nature and not static.
Under "Pitch Performance" Robert Shaw writes on page 417:
Pitch acceleration is dependent on control power and on the aircrafts pitch stability and its inertia. The moment of initia about the pitch axis is a function of a fighters weight and its distribution fore and aft about the CG. Increasing total aircraft weight or moving some of this weight farther from the CG either forward or aft tends to increase pitch inertia and reduce pitch acceleration. The position of the CG also has an effect. Aft CG positions usually increase pitch performance by reducing aircraft stability.
I suppose this could be called a pitching moment which is overcoming the moment of inertia, but I'm not sure.
In this section by Mr. Shaw also describes CG balance weight and total aircraft weight having an effect. The force of this static weight has the form of inertia that must be overcome by the dynamic forces applied by the control surfaces.
What I now infer from all this. A "Moment" describes a force acting upon the aircraft. This force is applied from a position outward from the CG and acts to control or change roll, yaw, pitch or speed of the aircraft.
These forces can be static, as with the case of adjusted CG weighting prior to flight and they can also by dynamic, as a result of forces created by the operation aircraft control functions. For example, rotational forces from rudders, ailerons and elevators as well as acceleration / deceleration forces from engine thrust, spoilers and air brakes.
Is this pretty much the idea? If so, then what the heck is a "Stalling Moment"?
My theory is that when the P-38 dive flap was rapidly deployed and the wing shape was changed almost instantly. IE: The dive flap was deployed in 1 second.
For the same reason stated above, this instantly effected the speed of the air passing over the lower surface of the wing. Until the AC stabilized farther forward on the wing, the wing was producing less lift.
As for as the wing was concerned, it had just made a rapid deceleration relative to the air flow around it. The wing was actually in a stalled condition for an instant.
This appearent deceleration force could be considered a moment, as it was effecting the longitudal axis.
Suppose this could be correct?
Sorry for the long one, I am a "Pit Bull" on this stuff sometimes. My education on the subject is very lacking and I did not understand the calculations presented.
Thanks in advance and for your patience! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Mino
The Wrecking Crew
Trainer
[This message has been edited by Minotaur (edited 03-06-2000).]
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niklas;
Concerning your post concerning "Roll Rate". Robert Shaw points out that it is "Roll Acceleration" that is important for agility.
Now, just considering "Roll Rate", refer to page 412-415 in Shaw's book. Particularly refer to page 414, that has 3 charts comparing airspeed to:
- Control force required by pilot
- Roll control deflection
- Stablized roll rate
All the charts show the relationship between unpowered and powered roll controls.
These charts show that in all cases once a specified airspeed is reached, the pilot no longer has the physical strength to maintain the required control surface deflection using unpowered controls. Roll rate past this point suffers significantly. Looking at the chart I am guessing that it drops to about 35% of the peak roll rate at maximum airspeed (Vmax).
The powered controls are not effected by airspeed, and maintain their full range of deflection. In fact, roll rate continues to increase as airspeed is increased.
Comparing the roll rate of unpowered controls to powered controls, they are equal up to around 50% Vmax. At Vmax the unpowered control roll rate has dropped to approximately 25% to that of powered. That is very significant.
The effect is quite dramatic, check it out.
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Mino
The Wrecking Crew
Trainer
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Mino-
But the concepts are all related. Shaw divides that information into rolling, pitching, etc. to describe the motion's direction and isolate the forces that need to be overcome. For instance, in a rolling moment you still are talking about overcoming the resistance of the weight outside of the centerline to movement.
The moment isn't the force; it is the mass and its position relative to the axis. The force is what is required to overcome the state of the mass around the axis. It require a certain amount of energy to move a certain size mass at rest. It also requires energy to overcome the inertia of an object in motion.
Your rolling moment would therefore be described as a condition where the mass outside the centerline is rotating (doesn't have to be static to have moment) around the axis. To change this balance you need to introduce force (drag, gravity, reversing control throw, etc).
Wells, have I screwed this up? I see it, perhaps am not explaining it well enough?
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Mino, Iīm talking about accereration! But i didnīt compared powered to unpowered aillerons, i compared a single engine fighter to a twin engine fighter.
"The effect is quite dramatic, check it out"
I believe you, but have a look to the roll accereration of a twin engine design (P38) to an ideal single engine design with the SAME engine (mass). Now THIS effect is quite dramatic i tell you!
So letīs see how fast you can accelerate your engine(s), only the mass of the engines!
Letīs say again the engine is a kind of cylinder, with a radius r of 0.5meter (1.5ft).
the engines of a P38 had a distance to the middle of the aircraft of d= 8feet = 2.42meter
so the moment of inertia of the single engine design is
Isingle= mass*r*r/2 = 0.125*mass
for the twin engine concept
Itwin= 2*(mass*r*r/2 + mass*d*d)
= 2*mass(0.125 + 5.85)
= 11.96*mass
the ratio is: 11.96/ 0.125 = 95.7!!!!!!
95.7 - NOT 95,7%, factor 95,7 >>95700% !!
That would mean you need a rolling moment, produced by the aillerons, that is 95,7 times bigger to get the same accereration.
The rolling moment (torque) is F*x, with F the force of the aillerons multiplied with the half of the wingspan. Because you have to of them you can say M_aill=F*wingspan
Of course the P38 has a bigger wingspan. I give you double the wingspan. And bigger aillerons. Iīm generous and give you 4 times more force, produced by the aillerons.
M_aill= I* (dw/dt) with w= rollrate (rad/s)
dw/dt is the accereration of rollrate
(rad/s*s)
single engine:
dw/dt= Mr_single / Isingle
twin engine:
dw/dt= (8*Mr_single) / (95.4*Isingle)
= 0.083*(dw/dt) of the single engine design
only 8% of the accerlation compared to the ideal(!) single engine design in my example
But that was obviously not bad enough for Lockhead. They added also two fuselages after their engines. Donīt forget the fuel in the wings...
Now compare that to a 109- very light wings, single engine, now fuel in the wings , even no weapons... now you know why!
Do you remember the DO335 Pfeil? Twin engine fighter, but the engines not in the wings, but one after another, in the fuselage - Simple idea, great effect, great performance!
niklas
[This message has been edited by niklas (edited 03-06-2000).]
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A Moment is a TORQUE.
The Moment of Inertia is a parameter that gives the amount of torque required to accelerate an object at 1 radian/sec/sec about a given axis.
For linear motion we have:
Force = Mass * Acceleration
For rotary motion we have:
Torque = Moment_of_Inertia * Angular_Acceleration
Mino: Your guess is as good as mine regarding the definition of "stalling moment". It's not a physics or mechanical engineering term.
[This message has been edited by funked (edited 03-06-2000).]
[This message has been edited by funked (edited 03-06-2000).]
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oops, better leave this to the engineers. (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
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niklas;
Thanks! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Are you trying to tell me that the roll acceleration for the 190 was roughly 100 times that of the P-38?
Some how, even though I understood only 5% of what you were trying to describe, that does not seem right. Something is missing.
I will see if I can find some real numbers from the planes for comparison. I am sure we will find that the roll acceration for the 190 is much higher than the P-38. This is until an airspeed is reached where the powered controls of the P-38 have a sigificant effect over non-powered controls.
From my experience of the AH FM. Roll acceleration is indeed slow, but roll rate is awesome.
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funked;
Thanks!
Now I understand a moment has to do with torque. For the purpose of understanding. Isn't this torque created by static and dynamic forces applied outward and away from the CG, just as I have described?
If so then "A Moment of Stall" might have to due with changing the wings AoA due to a torque / moment applied along the pitch axis. Of course the though comes to mind that Tony Levier, was just really trying to stay that the wings stalled for just a moment. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Mino
The Wrecking Crew
Trainer
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Originally posted by wells:
To understand moment of inertia, you should first understand torque (Force * distance). The force part of that formula is the 'impulse' force or the change in momentum (mass * velocity) over a time interval. <snip>
Thanks!
This is starting to click now I hope.
To guess the meaning of a moment. I kind of now realize that a moment has to do with momentum. Just as momentum can be related to inertia, a moment can be related to the force, that is in the form of torque, that is effecting inertia.
Or to put it simpler, a moment is a force that effects momentum. Or possibly it is a rate of force change that effects a rate of momentum change.
Now we have the the term "Long Moments" to confuse me. Meaning that forces that are effecting torque are farther away from the CG. By the same token the weight that is effecting the moment is also farther away so the inertia changes.
Example:
Picture an ice skater spinning in one place. With their arms extended their rotation is slow. With their arms tightly alongside their body their rotation is fast. Moving their arms out, they slow. Moving their arms back in, they speed up.
The total energy or inertia, of their body and its motion has not changed. What has changed is the energy is simply positioned or stored farther or closer to their CG. To maintain the same energy state, the rotation must increase as the arms are pulled in and vice versa.
However in either case, it requires an equal amount of torque to effect a porportional change in inertia. What is different is the change in speed of rotation. The slower rotation would have less speed change than the higher rotation. IE: For the arms out position there would be a much less change in speed than for the arms in position.
Am I getting closer to this concept?
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Mino
The Wrecking Crew
Trainer
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Mino, i said if both aircraft have the same Torque produced by the aillerons, THEN yes. If you read my posting again, youīll see that the P38 can produce more Force with itīs aillerons.
I made a quick picture to explain my model. You can see that the engine in the wing is not only in a rotation, itīs also moving around in the space- THATīs the big difference that leads to the "steinerterm".
At the bottom i tried to demonstrate the difference between F,m,a and M(=F*x),J,u
Hope that helps.
good example with the ice-skater mino. Itīs a question of Energy, Energy=constant for this example.
When she pulls her arms out, J (moment of inertia) increases. If she spins with the same speed (compared to "arms in") , her Energy (safed in the rotation) would be much bigger now. But Energy=const therefor she must slow down.
Again, look how fast they accerlate when they make themself "small"- Believe me, the difference between "mass in the middle" and "mass far outside" is huge!
(http://www.stud.mw.tum.de/~sl1/traegheit.gif)
niklas
[This message has been edited by niklas (edited 03-07-2000).]
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Mino:
Here's another pair of equations for you:
Force = Rate of Change of Linear Momentum
Torque = Rate of Change of Angular Momentum
Niklas:
In the spinning skater example it is Angular Momentum that is conserved, not Kinetic Energy. Her kinetic energy increases by the amount of work required to pull her arms in. However since there is no external torque on her body, angular momentum remains unchanged. Assuming of course, an ideal frictionless environment.
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Mino heres a discription of inertia found on the web.
http://library.thinkquest.org/12632/motion/index.html (http://library.thinkquest.org/12632/motion/index.html)
check it out it might help.
Law of inertia
Abody that is in motion continues in motion with the same velocity (at constant speed and in a straight line), and body at rest continues at rest unless an unbalanced forc acts upon it. (Physics for Career Education)
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Tommy (INDIAN) Toon
1st Aces High Trainer Crew
Home of The Allied Fighter Wing A.F.W.
A.F.W. Homepage (http://www.geocities.com/~tltoon)
[This message has been edited by indian (edited 03-07-2000).]
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EEEEEEKKK, I'm getting flashbacks from Statics class. (http://bbs.hitechcreations.com/smf/Smileys/default/biggrin.gif) (Well actually. (http://bbs.hitechcreations.com/smf/Smileys/default/frown.gif) ) As far the the p38 goes, I believe that you need to take into account that by the engines are able to push air directly onto the control surfaces themselves. Not only that, only roll acceleration would be affected I believe. (This seems to be evident in the p38 in AH, it takes quite a while to get the thing really rolling.) Moment is a general term describing the tendancy of an object to rotate about an axis perpendicular to the axis of the moment arm. (Eek, another flashback. (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif) Ok, that's enough for me, I'm an EE, not a ME afterall. Now ... if anyone want to start a post about fun Electrical fundamentals stuff like Mutual Induction, or Bandpass filters, now that would be different. (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif) )
bloom25
THUNDERBIRDS
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Mino: I think you are correct that when he said "stalling moment" he meant a "momentary stall" or something like that.
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Going off the deep end here, I hope there is water in the pool! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Newton: An object at rest tends to stay at rest and and object in motion tends to stay in motion.
This statement describes the term "Inertia". IE: The property of matter that resists changes is motion
Mass is the measurement for the property of Inertia.
Force is the measurment of an effect that attempts to make a object move (For this dissussion)
Gravity is the attraction (pull) by the earth on an object. This is an expression of the force applied on the object directly toward the center of the earth.
Weight is the measurement of an objects mass under the force of gravity.
To make an object move a force has to be applied to it. This force can be applied from only one direction, or from multiple directions. Opposing forces will cancel out each and a "Net Force" will be applied to make the object move.
Example:
- A rope "Tug-of-War". The center point moves in the direction of greatest force.
- An aircraft wing produces an upward force called lift. If the aircraft is flying level this force equals the force of gravity and the forces cancel out. The net force is zero
Momentum describes the mass of an object while it is motion. An object moving in a a straight line is said to be in "Linear Momentum". An object moving in rotational motion is said to be in "Rotational Momentum".
Momentum is the measurment of the mass of an object in motion. It is the combonation of mass the object at rest and the net force that was required to put to the object into motion. Basically this a way to say that the mass of the object is now changed. The change in this mass is equal to the force that effected change in the motion. IE: Applied force = {Mass in motion) - (Mass at rest)
Torque (http://www.physics.uoguelph.ca/tutorials/torque/Q.torque.intro.html) is the measurement of the force that causes an object to rotate on an axis called a "Pivot Point".
Moment Arm is the measurement of distance between the pivot piont and how far away from the pivot point the force is acting. IE: The distance would be the radius of the circle
Moment of Intertia is the measurement that describes how much mass is effectively at the distance for which the force of the moment arm is acting. IE: Effective mass at the radius
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Now to get back to airplanes. (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Airplanes rotate on 3 axis. Because we are dealing with a rotation, in comes our topic about "Moment".
From my discussion, I will consider the airplanes CG to be the pivot piont, relating to how torque is applied.
Concerning balancing the aircraft for pitch. This really means you are equalizing two forces. Weight in the nose and weight in the tale, which are torque forces applied straight down.
This means that there is actually two moment arm's in play here. One fore and one aft of the CG. The length of these two moment arms is different and the weight at the radius is not the same.
What puts the airplane in balance is that each respective torque, which is force in form of weight, exactly opposes the other one. There is no net torque and the plane is in balance.
As there are two moment arms fore and aft for pitch rotation, there are also two for yaw and roll. Roll being the more important one for this discussion as roll directly effects the agility of the airplane.
For discussion I will only discuss one wing or just one of the moment arms associated about the roll axis.
To make the aircraft roll, torque must be applied on the roll axis. This torque is applied as the result of a force being applied by the deflection of an aileron. This force attempts to make the wing move in one direction.
What attempts to prevent this movement is the effective mass at the point of the wing where the force is applied or the Moment of Inertia.
Once the aircraft begins rolling it now has "Rotational Momentum", which is the sum of force that was applied and the moment of intertia.
To stop the rotation, a force must be applied in the opposite direction. This force must equal the mass of the rotational momentum.
Therefore, airplanes with shorter moment arms and / or less moment of inertia roll faster, with an equal amount of torque aplied. This is because the moment of inertia is less and requires less force to start rotation. Additionally, less force is required to be applied to oppose rotational momentum, to stop rotation.
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Whew, did I get it? (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
Mino
[This message has been edited by Minotaur (edited 03-08-2000).]
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More roll torque = faster final roll rate and faster roll acceleration.
less moment of roll inertia = faster roll acceleration but same final roll rate.
All moments of inertia for roll, also count for yaw. All moments of inertia for pitch, also count for yaw. You can tell that the greatest moment of inertia is about the yaw axis, while the fin/rudder combo are much smaller than the hstab/elevator for the same 'moment'. Result = airplane is more sluggish in yaw than in pitch. So why not just put a bigger fin/rudder on?
Good question!
If the lift from the rudder/fin exceeds that of the body/fuselage (they lift opposite to each other), then you start getting really weird yawing motions. Take, for example, the pilot applies left rudder. The nose will yaw left (at first) and then start yawing right, requiring left wing drop to maintain heading! Not natural at all. So generally, fins are kept smaller and fuselage area is increased to provide the necessary directional stability, while meeting a specific sideslipping requirement. The added resistence of the fuselage to yawing motion makes for even less response and smaller yaw angles to be achieved.
[This message has been edited by wells (edited 03-08-2000).]
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Geeez guys, i'm just here to have fun, not earn a degree!!!!!!!
You guys are pickin this apart like a turkey carcas the day after thanksgiving!!!!!!!
Very informative tho, thnx
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niklas;
Thanks! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif) Those diagrams really helped me out.
Something dawned on me. I think that you are using the wrong r value.
The torque application point is at the end of the moment arm. This value should be at the mean center of the ailerons, I believe.
Throw these numbers into your calculations.
- For the P-38 with a wingspan of 52 feet, use an r value of 23 feet.
- For the 190A-8 with a wingspan of 34.5 feet, use an r value of 15 feet.
Maybe fudge the torque up a hair for the P-38 due to larger sized ailerons, say 125% of the torque for the 190. But, not fudging this might approximatly account for the larger wing mass of the P-38 as compared to the 190.
Additionally do two more calculations just for fun, to compare powered and unpowered ailerons. Use the same r values as above.
Assuming that at airspeed Vp, which is about 50% Vmax, the aileron torque is equal between the 190 and P-38. Do one calculation showing roll acceleration at Vp.
Assuming that at airspeed Vmax aileron torque for the P-38 is 200% of Vp, and for the 190 it is 35% of Vp. Do another calculation showing roll acceleration at Vmax.
Thanks again! (http://bbs.hitechcreations.com/smf/Smileys/default/smile.gif)
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Mino
The Wrecking Crew
Trainer
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Mino, I think you have the idea now. I could go through and nitpick your language but I think you get it.
One thing:
Linear momentum is mass times velocity.
Angular (rotational) momentum is moment of inertia times angular velocity. Angular velocities are things like roll rates and pitch rates.
Also as to these calculations for the P-38 and the Fw 190, I think Wells has already done them. (http://bbs.hitechcreations.com/smf/Smileys/default/wink.gif)
[This message has been edited by funked (edited 03-08-2000).]
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mino, r is not the moment arm of the ailleron, itīs the radius of my engine model. I need it for the calculation of my moment of inertia. I added now the moment arm and the force of an ailleron in the picture, have a look again.
wells, do you agree, that if an aircraft has a wingspan twice as big compared to another aircraft, and has a Force produced by the aillerons twice as big (compared to the other one), both have same maximum roll rate?
Of course the bigger one has more inital roll torque, but the wingtip is moving around twice as fast compared to the smaller aircraft (when both have the same rollrate) - so drag increases twice as fast, and the Force of your aillerons decreases twice as fast, right?
niklas
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Mino you can go to Nasa's site they have a paper thier if you search it right on the p38 .
But glad to see you found the answer Its been so long sence I went to school on this stuff forgot how to discribe it. But the moment arm is what I was getting at. They actualy figure this stuff out by putting plane on a pendulum (old way that is) ne wway they calculate it. thats info is on nasa web site also.
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Tommy (INDIAN) Toon
1st Aces High Trainer Crew
Home of The Allied Fighter Wing A.F.W.
A.F.W. Homepage (http://www.geocities.com/~tltoon)
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wells, do you agree, that if an aircraft has a wingspan twice as big
compared to another aircraft, and has a Force produced by the
aillerons twice as big (compared to the other one), both have same
maximum roll rate?
Yes, that would be the case. The reason the roll rate is the same though, is because the counter-torque (the angle of attack produced by the rotation of the wing) is also located further out (sqrt of 1/3 span). For the most part, the ailerons are of the same proportion on most planes (about 10% of the wing area), so the maximum roll rate is predominantly a function of wingspan and forward speed. A plane with twice the wingspan, travelling twice the speed of another plane will have the same maximum roll rate, given equal aileron proportions.
Take a P-51 and a P-38 for example, wingspans are 37' and 52'. For any given speed (within the stick force limits for full deflection), the P-38 should roll about 70% as quicly as the P-51, give or take a small amount.
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Originally posted by indian:
Mino you can go to Nasa's site they have a paper thier if you search it right on the p38 .
But glad to see you found the answer Its been so long sence I went to school on this stuff forgot how to discribe it. But the moment arm is what I was getting at. They actualy figure this stuff out by putting plane on a pendulum (old way that is) ne wway they calculate it. thats info is on nasa web site also.
Can you point me in the right area, the NASA is huge. Thanks!
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Mino
The Wrecking Crew
Trainer
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Here you go mino. I thought I downloaded it but cant find it now. Use NACA fulltext uncheck all others.
http://techreports.larc.nasa.gov/cgi-bin/NTRS (http://techreports.larc.nasa.gov/cgi-bin/NTRS)
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Tommy (INDIAN) Toon
1st Aces High Trainer Crew
Home of The Allied Fighter Wing A.F.W.
A.F.W. Homepage (http://www.geocities.com/~tltoon)
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Thanks Indian!
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Mino
The Wrecking Crew
Trainer