speed and CG

[olasek]

Change of velocity shifts center of gravity??

Quote from the book Fate is The Hunter:

"Did you slow down when you first noticed the vibration? You did not because you had no fear of it. But if you had been the nervous type, if you slowed down, the center of gravity would have changed. That would have been quite enough to complete the process of unporting which had partially begun."

 

[FastEddieB]

I think that's poorly put, and untrue.

What it seems to be getting at is that the pitch angle change following a slowdown could affect the porting or unporting of the fuel. Or that the mere act of decelerating could affect it. Either one is certainly possible.

But CG is CG at any speed and any attitude. It's calculated, after all.

Even climbing or diving vertically, the CG remains stubbornly where it always was.

Maybe he's talking about the weight of the fuel moving within the tanks? That would affect CG, obviously.

 

[olasek]

OK, makes sense.

 

[jsstevens]

I think that's poorly put, and untrue.

What it seems to be getting at is that the pitch angle change following a slowdown could affect the porting or unporting of the fuel. Or that the mere act of decelerating could affect it. Either one is certainly possible.

But CG is CG at any speed and any attitude. It's calculated, after all.

Even climbing or diving vertically, the CG remains stubbornly where it always was.

Maybe he's talking about the weight of the fuel moving within the tanks? That would affect CG, obviously.

This specific reference is to the elevator on a DC-4 unporting causing flutter followed by an uncommanded and terminal dive. Nothing to do with fuel. I think he meant center of pressure on the tail plane rather than CG, but it was definitely not about fuel.

Apparently they lost DC-4 in a very short period of time from thus phenomenon. One attachment bolt would fall out of the hinge which seemed pretty innocuous given the number of bolts but it allowed the aerodynamic forces to push the elevator into a full dive and overpower the efforts of the flight crew to recover.

See this thread on pprune: http://www.pprune.org/archive/index.php/t-203324.html

And this on the list DC-4 : https://archive.org/stream/CABAAR19470530Eastern605/CAB-AAR 1947-05-30-Eastern-605_djvu.txt

John

Sent from my iPad using Tapatalk HD

 

[BigBadLou]

That's exactly what I thought that this was an incorrect reference to CG when they meant CP which obviously does change with speed.

Write to the editor and force another revision.

I wonder, what are editors paid for nowadays? Have you seen the drek that is printed nowadays? (ok, I'll shut up now)

 

[Jaime Metcher]

Thought I'd leave this here for anyone else searching because there seems to be a lot of confusion on this topic.

The aerodynamic unporting referred to by Gann is to do with a balanced elevator design with the hinge somewhat aft of the leading edge. Thus the leading edge moves up when the elevator is deflected down. If the leading edge of the elevator rises clear of the stabilizer, complete separation of flow over the upper elevator surface can ensue. In the case referred to by Gann, the missing hinge bolt was cited as a factor in allowing an abnormal elevator movement leading to unporting.

This much is clear from NACA report L-379 of 1942 (which unfortunately I can't link to here due to forum rules). The title is "Characteristics of Plain and Balanced Elevators on a Typical Pursuit Fuselage at Attitudes Simulating Normal-Flight and Spin Conditions".

As for the term "unporting", I'm speculating here, but I'd guess it's by analogy with the fuel tank situation, normal flow being interrupted by abnormal fluid movement - in this case, from the upper surface of the stabilizer through the gap between the stabilizer and elevator, and aggravated by the protruding elevator leading edge.

 

[Sac Arrow]

Unless the pitch axis is exactly coincident with the center of mass, then a change in pitch angle would in fact shift the center of gravity. Maybe not significantly, but it would be the case.

 

[flyingron]

The opposite however is true, an aft CG will give you a faster airspeed due to less drag (at least in conventional tailed aircraft).

 

[FastEddieB]

The opposite however is true, an aft CG will give you a faster airspeed due to less drag (at least in conventional tailed aircraft).

Easy to demonstrate...

Find smooth air, set power and trim. Note the airspeed. Now slide your seat back. The plane will try to climb, and you’ll have to apply forward pressure to maintain level flight. Trim, stabilize and then note the airspeed increase.

The reason is that the wings need to support the aircraft weight plus the downforce on the tail, with the associated increased induced drag. Less tail downforce with a more rearward CG means less weight for the wings to support, and so less induced drag.

As an aside, this thread has been resurrected from 2014!

 

[Sac Arrow]

The opposite however is true, an aft CG will give you a faster airspeed due to less drag (at least in conventional tailed aircraft).

I think the same should be true in a canard, if that is what you are comparing it to.

 

[flyingron]

I think the same should be true in a canard, if that is what you are comparing it to.

I'm not sure it has much in the canard. First, unlike a conventional tail, the canard is also flying (upward). Also, the CG is petty far aft to begin with in most canards.

 

[Sac Arrow]

I'm not sure it has much in the canard. First, unlike a conventional tail, the canard is also flying (upward). Also, the CG is petty far aft to begin with in most canards.

Right, but the point I'm making is that if you move the CG forward, you would need to increase the angle of attack of the canard to compensate, which would increase drag. Same as a conventional tail, just operating in the opposite direction.

 

[Cap'n Jack]

Right, but the point I'm making is that if you move the CG forward, you would need to increase the angle of attack of the canard to compensate, which would increase drag. Same as a conventional tail, just operating in the opposite direction.

I agree with your statement above. But if the canard is "taking more of the load", does that not mean the angle of attack of the wing can be reduced, and reduce it's drag, at least to some point? I can't say what that point might be.

 

[Sac Arrow]

I agree with your statement above. But if the canard is "taking more of the load", does that not mean the angle of attack of the wing can be reduced, and reduce it's drag, at least to some point? I can't say what that point might be.

The answer is yes, but the angle of attack on the main wing changes very little in comparison to the control surface. But you do bring up a good point - a forward CG would therefore be a less adverse condition than with a conventional tail. Good question, actually.

It is my understanding, however, that although in theory a canard should be more efficient than a conventional tail because both wings are positively loaded, that in practice, the main wing has to be oversized for what it needs to be, to ensure the front wing stalls first, adding drag back in.

 

[nauga]

It is my understanding, however, that although in theory a canard should be more efficient than a conventional tail because both wings are positively loaded...

That's not the theory, that's conjecture from people who don't understand the theory.

Nauga,
who lets the most efficient surfaces do the lifting.

 

[Crashnburn]

The answer is yes, but the angle of attack on the main wing changes very little in comparison to the control surface. But you do bring up a good point - a forward CG would therefore be a less adverse condition than with a conventional tail. Good question, actually.

It is my understanding, however, that although in theory a canard should be more efficient than a conventional tail because both wings are positively loaded, that in practice, the main wing has to be oversized for what it needs to be, to ensure the front wing stalls first, adding drag back in.

Actually, stalling has nothing to do, directly, with the size of the surface, just AOA. In a typical canard, the front wing has a few degrees more incidence than the main wing, to ensure it stalls before the main wing. It is true, the bigger the lifting surface, the less AOA it needs (everything else unchanged, of course) to generate the same amount of lift.

 

[flyingron]

The answer is yes, but the angle of attack on the main wing changes very little in comparison to the control surface. But you do bring up a good point - a forward CG would therefore be a less adverse condition than with a conventional tail. Good question, actually.

It is my understanding, however, that although in theory a canard should be more efficient than a conventional tail because both wings are positively loaded, that in practice, the main wing has to be oversized for what it needs to be, to ensure the front wing stalls first, adding drag back in.

It also contributes to a longer ground run because you got to get a whole lot of airflow over the canard to lift it enough to rotate.

I don't disagree that a canard probably does better as the CG goes back, but I don't think the performance shift is as dramatic as with a conventional tail as you're just moving the CG round on the main wing