A Moment of ???

[Minotaur]

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!  


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Mino
The Wrecking Crew
Trainer

[Kieren]

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.

[indian]

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

[Kieren]

Indian-

That's it! How far any object is located from the CG, not the control surfaces. Thanx!

[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.  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)

 
 

[niklas]

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

[Vermillion]

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.  

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Vermillion
**MOL**, Men of Leisure
 
 

[This message has been edited by Vermillion (edited 03-05-2000).]

[indian]

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

[Minotaur]

Well I asked for it I guess.  Abolutely more confused than ever now.      

Abreviations:

• AC = Aerodynamic Center
• CG = Center of Gravity
• P-38 = The best plane that I can think of    

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!      

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Mino
The Wrecking Crew
Trainer


[This message has been edited by Minotaur (edited 03-06-2000).]

[Minotaur]

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

[Kieren]

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?

[niklas]

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).]

[funked]

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).]

[Kieren]

oops, better leave this to the engineers.  

[Minotaur]

niklas;

Thanks!  

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.  

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Mino
The Wrecking Crew
Trainer