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Thrust line with CG

Asked by: 1428 views Aerodynamics

CG WITH THRUSTLINE AFFECTS  LONGITUDINAL STABILITY.

BUT, I DON'T UNDERSTAND THIS.

THRUSTLINE ABOVE CG - NOSE DOWN, TAIL UP

THRUSTLINE BELOW CG - NOSE UP, TAIL DOWN 

HOW WE CAN SAY COMMONSENSCALLY? 

10 Answers



  1. Nathan Parker on Mar 16, 2013

    Actually, your examples show how the thrust line affects *trim*, not longitudinal stability. Longitudinal stability refers to how pitching moments change with Angle of Attack changes. If you increase the AoA and the thrust line causes an increase in nose up pitching moments, then longitudinal stability is reduced.

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  2. Brian on Mar 23, 2013

    Not exactly Nathan. We’ve actually had some e-mails between us over this a long time ago. If you recall me asking you if it’s common to assume anything that causes a nose down pitch to be positive and a nose up pitch negative?

    Well if you read through aerodynamics for naval aviators, and a few other sources you’ve given me. John Andersons book and even Raymer’s book both note that a thrust line over above the CG is considered stabilizing. A thrust line below the CG is destabilizing. Each case refers to static longitudinal (pitch) stability). Raymer and AFNA comment on this being the reason for engines being canted downward. The goal is to cause changes in power to be less destabilizing. Explanation in the following paragraph.

    Further, a pusher engine is stabilizing and a tractor engine is destabilizing. This explains why tractor mounted engines need that nose down tilt. Since the tractor mount is destabilizing the engine is canted to put the thrust line higher over the CG to stabilize an unstable setup.

    Just like ground effect causes a nose down pitching moment. Also termed a pitch stabilizing situation.

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  3. Brian on Mar 23, 2013

    “Just like ground effect causes a nose down pitching moment. Also termed a pitch stabilizing situation.”

    Pitch STABILIZING…sry

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  4. Nathan Parker on Mar 24, 2013

    “John Andersons book and even Raymer’s book both note that a thrust line over above the CG is considered stabilizing.”

    I didn’t say it wasn’t, I said that the OP’s examples were trim examples. But your examples, too, are not examples of longitudinal stability. If I add thrust and the nose pitches down, how is this an example of a change in pitching moment with respect to change in AoA? This, instead, is an example of a change in pitching moment with thrust. Not the same thing.

    Likewise, the nose down moment in ground effect is not the same thing as increased stability. However, it is a **side effect of an increase in stability**. The actual increase in stability is caused by the change in the rate of downwash over the horizontal stabilizer; this change is produced by the reduction in the wingtip vortices in ground effect. When the aircraft first enters ground effect, the downwash will be reduced and the aircraft will generate an initial node down moment, but after that, it’s the rate of change that produces the stability change.

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  5. Nathan Parker on Mar 24, 2013

    FYI, the Raymer discussion of this topic occurs under the heading “velocity stability”, not “longitudinal stability”.

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  6. Brian on Mar 24, 2013

    ” If I add thrust and the nose pitches down, how is this an example of a change in pitching moment with respect to change in AoA?”

    AFNA “On the other hand, if the thrust line is ,located above the c.g., a negative moment is created and the effect is stabilizing.”

    I can give you similar quotes in Raymer. Can’t recall if I can from Anderson.

    Just check out page 259 bottom right “power effects” at http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

    Perhaps I’m misreading it, but it’s been bothering me since our e-mails. I find it quite confusing though. In everything I’ve read it sounds as though anything that causes a nose down pitch is considered stabilizing and vice versa. Wonder if Steve Pomroy will chime in with his knowledge?

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  7. Brian on Mar 24, 2013

    What I can’t find is something that confirms that the moment created needs to be with respect to a change in AOA. From my readings it seems any moment created by any part of the aircraft is considered with regards to stability. The tail stabilizing, the wing stabilizing if CG forward AC or destabilizing if the reverse is true, the fuselage destabilizing, for instance.

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  8. Nathan Parker on Mar 24, 2013

    “anything that causes a nose down pitch is considered stabilizing and vice versa.”

    Very much “no”. The very nature of stability in all contexts is that it is a **restoring** tendency given some other change that drives the system away from equilibrium.

    You’re making a mistake that I made early on….you’re dissecting the prose in the aerodynamics text looking for insights, much in the same way you might pick apart a religious text. Unfortunately, prose is sloppy and imprecise, and it doesn’t provide all the context information needed to properly interpret the text. You can’t only find truth in the mathematics. Longitudinal stability requires that dCm/dCl < 0.

    Let me quote Raymer (p. 486 in my edition):

    =============
    For static stability to be present, any change in angle of attack must generate moments that oppose the change. In other words, the derivative of pitch moment with respect to angle of attack [Eq. 16.8] must be negative.

    [...]

    Note that the wing pitching moment and thrust terms have dropped out, as they are essentially constant with respect to angle of attack.
    ================

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  9. Nathan Parker on Mar 24, 2013

    The “You can’t only find truth in the mathematics.” should read “You CAN only find truth in the mathematics…].

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  10. Brian on Apr 21, 2013

    “The “You can’t only find truth in the mathematics.” should read “You CAN only find truth in the mathematics…].”

    I’ll leave the search for truth there up to you. I’d suggest, however, that something is missing in your analysis given the profuse appearance of sentences claiming thrust line location as having an impact on stability. In other words, if every book I own on the topic makes some mention of thrust line relating to pitch stability then there must be something that mathematically supports this.

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