Inertia Coupling, what is it?
Ed Davisson
I'm a layman that has some passing interest in aviation. In watching a
documentary about the X planes, they mentioned the time when Chuck Yeager
reached too high a speed, in I believe the X-2. They referred to a term
"inertia coupling". What is "inertia coupling"? I've heard this term used
in the past in discussions involving supersonic flight. Would some kind
soul in this newsgroup please explain it to me? My newserver is notorious
for missing posts, so could you please email any answers to edp...@indy.net
Thanks in advance.
--
Ed Davisson
edp...@indy.net
Fishers, Indiana
BOYCOTT!
WRTV 6 Indianapolis, a McGraw-Hill Company,
until they quit trying to build captive
markets on the backs of satellite dish owners.
As free people, we have the right to decide
what programming is right for our household.
Not the government, not greedy corporations!
JCBowman
In laymen's terms, inertial coupling is movement about
an axis caused by movement on another axis. Gyroscopes
exhibit this effect. You can use a bicycle wheel spinning
in your hands to see this. If you try to wiggle the wheel
side to side it wants to twist out of your hand.
In aircraft, inertial coupling is usually a result of too much
mass (inertia) along the longitudinal axis and not enough
vertical tail area (typical on early supersonic fighters). There
is aerodynamic roll-yaw coupling which causes the movements
which agitate this inertial coupling mode.
I remember seeing a picture that used a dumbell to help explain
this. Imagine that you have a big weight at the nose and a big
weight at the tail, just like a dumbell. Say the aircraft pitches,
yaw, or rolls ever so slightly. The weights at the end start
spinning producing a gyroscope-like effect. Now, all you
have to do is add some disturbance like minor corrections
>From the pilot or from gusts and the dumbell starts rotating
about an axis perpendicular to the original direction. The
combination of the two rotations produces a wobble. If the
tail size isn't large enough to dampen out the roll-yaw
oscillations, the airplane can go out of control- ala Yeager.
The fact that this happens mostly at supersonic speeds leads
me to believe that the frequency of the modes is too quick for
the tail or pilot to apply the correct actions (this leads into
another discussion about pilot induced oscillations which led
to the first F-22 being totaled).
I'm not a dynamicist, more of a aerodynamicist and design
type, so I may have oversimplified. But I hope this helps.
Jason (jcbo...@aol.com)
Thanks
Jason (jcbo...@aol.com)
"Any sufficiently developed technology is indistinguishable
from magic."
- Arthur C. Clarke (I think)
Max Feil
Ed Davisson (edp...@indy.net) writes:
> I'm a layman that has some passing interest in aviation. In watching a
> documentary about the X planes, they mentioned the time when Chuck Yeager
> reached too high a speed, in I believe the X-2. They referred to a term
> "inertia coupling". What is "inertia coupling"? I've heard this term used
> in the past in discussions involving supersonic flight. Would some kind
At the risk of being verbose, here is a thread on this topic I saved from
sci.aeronautics about 3 years ago.
Regards,
Max
--
Max Feil | Email: m...@unicad.com
UniCAD (CANADA) Ltd. |------------------------------------------------------
CAD Software Development| I'm an "optimist wannabe".
Ottawa, Ontario, Canada.|
---forwarded articles--->
From: sha...@c-17igp.wpafb.af.mil (Civ Daniel G. Sharpes, ASC/YCEF, DSN785-3341)
Newsgroups: sci.aeronautics
Subject: Inertial Coupling & Mach Tuck
Date: 2 Feb 1993 13:06:58 -0600
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> I've been following the show "X-Planes" on the discovery channel the
> last several weeks. For the most part, it has been quite enjoyable,
> but they keep mentioning a phrase without defining it: "inertial
> coupling". Apparently, this was one of the biggest headaches to early
> supersonic flight and the cause of several fatal accidents.
> Unfortunately, they never define what the mechanism is for this
> effect. I've looked in John Anderson's books, but haven't found
> anything on it.
There's a very nice discussion of 'inertial coupling' or 'roll
coupling' (depending on your reference) in _Aerodynamics for Nasal
Radiators_ (uh, Naval Aviators, sorry), page 315:
"The long, slender, high-density fuselage with short, thin wings
produces a roll inertia which is quite small in comparison to the pitch
and yaw inertia. These characteristics are typical of the modern
airplane configuration. The more conventional low speed airplane may
have a wingspan greater than the fuelage length. This type of
configuration produces a releatively large roll inertia."
"Inertia coupling can be illustrated by considering the mass of the
airplane to be concentrated in two elements, one representing the mass
ahead of the c.g. and one representing the mass behind the c.g. There
are two principal axis systems to consider: (1) the aerodynamic or wind
axis is through the c.g. in the relative wind direction, and (2) the
inertia axis is through the c.g. in the direction of the two element
masses."
When the plane is at zero angle of attack, there is no inertia (or roll)
coupling. When you go to an AOA and the pilot rolls the aircraft,
centrifugal forces are created that cause a pitching moment. This
causes the AOA to increase and unless the pilot takes out roll stick,
AOA will get very high very fast. When the Blue Angel F-18's do their
high-speed rolls, they have to first put the plane at zero AOA. I saw an
out-the-cockpit video of a pilot who almost had his F-18 at zero alpha.
The plane reached 60 degrees alpha in less than two seconds and if the
pilot hadn't been doing departure flight tests the week before, it's
doubtful he'd have recovered the aircraft (or even survived).
>
> The only effect that I've seen that might be what they are referring to
> is when, at high speeds (beyond the sonic point), the airflow over a
> wing begins to seperate from the surface. Since the airflow is no
> longer being deflected downward behind the wing, the horiz. stabilizer
> sees an increased alpha and pitches the plane over. But I thought this
> happened at much slower speeds (M=0.8) and was understood by this
> time (as several WWII planes ran into this problem).
This is what happens when a shock wave forms on the airfoil at about 65%
to 70% of the chord. The airflow behind the shock separates and you lose
control effectiveness. It was a problem on the P-38 and a few others (the
P-47 too, I believe). It's something the Guy in Back has to be aware of
on F-4 Phantoms. I've heard that aircraft has a nasty tendency to drop
the nose rather quickly when coming through the transonic speed range.
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
+ _____________ +
+ Dan | "It's not a real plane +
+ Sharpes | unless you can stand +
+ _|_ up inside it" +
+ \____________/===\____________/ +
+ (*) (*)( . )(*) (*) +
+ \___/ +
+ +
+ Opinions expressed are mine alone, and not another's +
+++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Newsgroups: sci.aeronautics
From: ma...@kelvin.seas.Virginia.EDU (Michael Allen Waters)
Subject: Re: Inertial Coupling & Mach Tuck
Message-ID: <1993Feb3.1...@murdoch.acc.Virginia.EDU>
Sender: use...@murdoch.acc.Virginia.EDU
Organization: University of Virginia
References: <930202190...@c-17igp.wpafb.af.mil>
Date: Wed, 3 Feb 1993 19:10:57 GMT
Lines: 23
There are some really nice equations in a book by
Nelson out of Notre Dame called Stability and Control of
Aerospace Vehicles (I think). Inertial coupling is simply when
some of the terms in the dynamic equations of motion are no
longer negligible. These are the products of the roll, pitch
and yaw velocities with the prodcts on inertia of the
aircraft. Inertial coupling plays a roll whenever these
products become large. The Me-262 was a great example of
this. Those big engines with the spinning turbines created
alot of gyroscopic effects that only became important when you
tried to pitch and yaw at the same time. The key is that at
any angle of attack when you roll, make sure that you DO
NOT pitch or yaw at the same time. In planes like the F-18,
this is built into the SAS for most flight conditions. At high
angle of attack, the danger is that it is more difficult to
roll without pitch or yaw. If you look at the equations, it is
a little easier to understand.
Mike
--
_______________________________________________________________________
Duj tIvoqtaH!
'Always trust your instincts.'
Newsgroups: sci.aeronautics
From: jo...@neon.dfrf.nasa.gov (Joe Pahle)
Subject: Re: Inertial Coupling
Message-ID: <1993Feb3.2...@news.dfrf.nasa.gov>
Sender: ne...@news.dfrf.nasa.gov (Usenet news)
Reply-To: jo...@neon.dfrf.nasa.gov
Organization: NASA Ames-Dryden FRF, Edwards, CA
References: <16B6A70...@VM1.CC.UAKRON.EDU>
Date: Wed, 3 Feb 1993 20:30:14 GMT
Lines: 78
In article <16B6A70...@VM1.CC.UAKRON.EDU>, D2...@VM1.CC.UAKRON.EDU writes:
|> Can we go into the Inertial Coupling thing a bit deeper? Are you saying that
|> the F-18 cannot be rolled when the angle of attack is greater than 0? Something
|> does not sound right here. I can understand centrifugal force causing a slight
|> pitchup when rolling at high alpha and low airspeed, but it seems that this
|> would be very detrimental in a fighter engaging in aerial combat. It also
|> seems to me that the Blue Angels pop the nose (ie. momentary increase in
|> alpha) just before starting a roll (4 point, 8 point, 3 rolls etc.). Is it
|> possible that the alpha reduction is programmed into the flight control
|> system? I'd love to hear the method they use to adjust the gain on the
|> stabilators during a typical roll sequence.
|> Thanks!
|>
|> Kraig Krumm
|>
Let me start out with ...
THE F/A-18 IS NOT RESTRICTED TO JUST ROLLS AT ZERO ALPHA!!
Inertia, Kinematic, and Gyroscopic coupling are all cross-axis
problems that can bite at different times, in different flight
regimes. Inertial coupling is a function of inertial geometry, i.e.,
we do not fly uniform spheres, but long-slender bodies. Satellites have
inertial coupling problems as well. If you look at the equations of
motion, the inertial part of, say, pitch acceleration is (in matlab-ese...)
%
% calculate inertial component of the pitch angular acceleration
% about the y body axis (rad/sec**2)
%
qdoti = ((izz - ixx) ./ iyy) .* pr .* rr + ...
(ixz ./ iyy) .* (rr.*rr - pr.*pr) + ...
(ixy ./ iyy) .* (pdotr + qr.*rr) + ...
(iyz ./ iyy) .* (rdotr - pr.*qr);
where:
% ixx - rolling moment of inertia (slug-ft^2)
% iyy - pitching moment of inertia (slug-ft^2)
% izz - yawing moment of inertia (slug-ft^2)
% ixz - rolling - yawing product of inertia (slug-ft^2)
% pdotr - roll angular acceleration (rad/sec^2)
% qdotr - pitch angular acceleration (rad/sec^2)
% rdotr - yaw angular acceleration (rad/sec^2)
% pr - roll rate (rad/sec)
% qr - pitch rate (rad/sec)
% rr - yaw rate (rad/sec)
%
Ignoring the smaller cross-axis terms:
qdoti =~ ((izz - ixx) ./ iyy) .* pr .* rr ;
if the yawing inertia is different in magnitude than the roll inertia (remember
long-slender ...) then roll rate * yaw rate couples into pitch acceleration !!
Now take a fast roll at moderate alpha. Assume that you are
commanding stability-axis roll-rate , i.e. you command p*cos(alp) +
r*sin(alp). You are actually commanding a pitch acceleration as well !!
Most modern control laws have inertial coupling feedbacks to
counter this effect in the pitch axis and the similar effect in the
yaw axis.
The Blue Angels departure was inertial coupling at very low alpha. There was some kinematic
coupling as well ...
--
Joseph W. Pahle
F-18 HARV flight controls
phone: 805 258-3185
e-mail: jo...@neon.dfrf.nasa.gov
UUCP: ames!neon.dfrf.nasa.gov!joep
Comment: disclaimers attract lawyers ... here boy !
NASA Ames Dryden Flight Research Facility, Edwards,CA
Newsgroups: sci.aeronautics
From: bow...@rigel.dfrf.nasa.gov (Al Bowers)
Subject: Re: Inertial Coupling & Mach Tuck
In-Reply-To: sha...@c-17igp.wpafb.af.mil's message of 4 Feb 1993 14:28:30 -0600
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References: 16B6A70...@VM1.CC.UAKRON.EDU <930204202...@c-17igp.wpafb.af.mil>
Date: Fri, 5 Feb 1993 19:35:11 GMT
Lines: 65
In article <930204202...@c-17igp.wpafb.af.mil> sha...@c-17igp.wpafb.af.mil (Civ Daniel G. Sharpes) writes:
>In sci.aeronautics you write:
>>Can we go into the Inertial Coupling thing a bit deeper? Are you
>>saying that the F-18 cannot be rolled when the angle of attack is
>>greater than 0? Something does not sound right here. I can understand
>>centrifugal force causing a slight pitchup when rolling at high alpha
>>and low airspeed, but it seems that this would be very detrimental in
>>a fighter engaging in aerial combat.
>Some good points. It's sustained roll rate at high AOAs that causes the
>trouble. After going about 270 degrees through a roll, the airplane has
>seen a lot of centrifugal force and the masses forward and aft of the
>c.g. are start causing the nose to rise. In combat, continuously
>rolling does get the guy behind you in front of you - it just makes you
>dizzy and him mad ;). This kind of maneuver is pure airshow stuff.
Actually, if you examine the equations of motion, it is quite apparent
that the big determainent if inertial coupling for a given
configuration is the cross-product of body axis roll rate and body
axis yaw rate (this is sometimes refered to as "wind axis roll rate").
And as for the continuous roll in an ACM environment, this is a very
good defensive maneuver. See Saburo Sakai's "Samurai" account of
flying from Iwo Jima in a Mitsubishi A6M5 Zero Model 52 against US
Navy F6F Hellcats. The RAF used a similar tactic with Spitfires
against attacking Messerschmitt Bf 109s (the Spit rolled faster).
That any fighter pilot would not take an advantage, even a trivial
one, is not good tactics.
>>seems to me that the Blue Angels pop the nose (ie. momentary increase in
>>alpha) just before starting a roll (4 point, 8 point, 3 rolls etc.). Is it
>>possible that the alpha reduction is programmed into the flight control
>>system? I'd love to hear the method they use to adjust the gain on the
>>stabilators during a typical roll sequence.
> The F-18 is a unique aircraft in roll - the plane is so flexible that
>the flight control system commands movement from every control surface.
>Al Bowers from the F-18 HARV program can comment on this better than I,
>though. (Take it, Al!)
There are several F-18 HARV people out here on this group. Mary
Shafer is _very_ aware of work on the program (even if she is not
officially assigned to the HARV), and the lead controls engineer, Joe
Pahle, is here as well.
Most modern flight controls take account of inertial coupling. I know
of no digital flight control system that does not (though I am not
well versed in FCS).
A simple demo of what is happeneing. Take a text book. Imagine the
inertias of that text book and relate it to an aircraft. Place a
rubberband around the textbook (to keep it closed while you do this).
Now, toss it into the air and flip it as though it is rolling. Note
that this is a stable mode. Now try it in yaw. Again a stable mode
is evident. But try to get it to flip purely in pitch. Note how the
book flips around. given enough room, the motion will stabilize in
the yaw axis (usually the motion tends to the lowest energy state).
this is due to inertia coupling. Try it.
--
Al Bowers bow...@rigel.dfrf.nasa.gov Alfa GTV6 Ducati MHR
NASA F-18 High Alpha Research Vehicle, Lead Aero DoD #900
NASA, Dryden Flight Research Facility, Edwards, California
Newsgroups: sci.aeronautics
From: lor...@avistic.demon.co.uk (Lorcan Mongey)
Reply-To: lor...@avistic.demon.co.uk
Subject: Inertial coupling, what is it?
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Date: Sat, 6 Feb 1993 19:53:46 +0000
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In article <1kkfto...@gaia.ucs.orst.edu> jo...@oce.orst.edu (John A.
Gregor) writes:
> but they keep mentioning a phrase without defining it: "inertial
> coupling". Apparently, this was one of the biggest headaches to early
> supersonic flight and the cause of several fatal accidents.
> Unfortunately, they never define what the mechanism is for this
Rotational inertis is to do with how the aircraft mass is distributed
around the axis of rotation. If all the mass is gathered close to the
axis then the inertia is low - if it is distributed far from the axis
then the inertia is high. For stability and control purposes we consider
the inertia about the longitudinal axis (rolling) and the lateral axis
(pitching) and use them in the equations of motion to predict the
aircraft's response to control inputs or externally applied aerodynamic
inputs.
There is, however, another type of rotational inertia and that is
cross-coupled between the longitudinal and lateral axes. This is less
intuitive but represents how far mass is distributed from both axes.
Classical aircraft had small values of inertial coupling (because most
of the mass was distributed along the longitudinal and lateral axes -
the fuselage and wing) and so its influence was generally neglected but
around the time of the F-100 it had become large enough to be important.
I suppose that swept wings moved some wing mass from the lateral axis
and placed it where its inertial cross-coupling contribution was large.
Once it was understood what was happening the effect was included in
stability & control analyses and the problem was solved.
Lorcan
From: sha...@c-17igp.wpafb.af.mil (Civ Daniel G. Sharpes, ASC/YCEF, DSN785-3341)
Newsgroups: sci.aeronautics
Subject: Roll Rates and Inertial Coupling
Date: 9 Feb 1993 09:31:37 -0600
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Al Bowers and I have had an interesting side conversation about roll
rates and roll coupling. Here's the text:
Al writes:
>And as for the continuous roll in an ACM environment, this is a very
>good defensive maneuver. See Saburo Sakai's "Samurai" account of
>flying from Iwo Jima in a Mitsubishi A6M5 Zero Model 52 against US
>Navy F6F Hellcats. The RAF used a similar tactic with Spitfires
>against attacking Messerschmitt Bf 109s (the Spit rolled faster).
>That any fighter pilot would not take an advantage, even a trivial
>one, is not good tactics.
I stand corrected - continuous roll rates aren't just airshow
stunts! Al Bowers reply got me digging in my references for more
info. Here's what I found in Robert L Shaw's _Fighter Combat Tactics
and Maneuvering_ (Naval Institute Press, 1985), pages 30 and 31:
"One further useful defensive maneuver against a near dead-stern
attack is a continuous rolling turn ... This tactic also may be
referred to as a high-G barrel roll and it is most effective when the
attacker is at close range with high overtake. It is not recommended
if the attacker enjoys a substantial turn advantage over the defender
since, if the shooter can controlhis overtake, he may still be camped
at the defender's six o'clock after completion ofthe maneuver."
"A modification of this maneuver has also proven useful under some
circumstances. When the attacker is near six o'clock with little
closure and inferior roll and acceleration performance, the defender
can use a continuous low-G barrel roll. The aircraft is rolled in one
direction just fast enough to prevent the attacker from matching wing
positions, and a small load factor is maintained to produce a
spiraling, "corkscrew" flight path. This maneuver spoils the
attacker's aim until the defender can dive and accelerate out of
range using full power."
He concludes this section by saying, "The guns defense tactics
described here are designed first to defeat the gun itself, then to
defeat the gun platform, and, finally, to defeat or complicate the
tasks of the gunsight and the attacking pilot. The objectives are
first, to deny any shot, second, to deny a good shot opportunity, and
third, to make even a poor shot as difficult as possible."
Al responds:
>Exactly. This was the strength of the Spit in WWI as well as the
>Zero, and later, the P-61 (the highest roll rate of all Allied
>fighters in WWII). most people don't think of the Black Widow in this
>light, but it was very effective, and you can bet your bottom dollar
>that P-61 pilots knew it and used it! :-)
Ray Whitford discusses roll performance requirements in _Design for
Air Combat_ (Jane's Publishing Inc, 1987), on page 95 and concludes
with this proviso: "Too high a roll rate can lead to inertial coupling
problems, which in any case place a limit on the number of consecutive
rolls an aircraft is allowed to perform." (Which gets us back where
we started!)
Al replies:
>Ray is a pretty nice guy, great to talk to. But he is right, and if
>compensation is built into the control system, the limit is determined
>by the control power limit. So the higher the alpha the lower the
>limit (determined by control power). But control power goes _down_ at
>high alpha, not up! So what's left? Bigger/more control surfaces?
>(F-4 CCV, AFTI F-16 CCV, X-29A, F-15 STOL MTD, etc :-) Or thrust
>vectoring? Or both? :-)
>
>I think you can see where this is headed. Welcome to the _REAL_
>world...
>
>Regards!
>Al
--
Max Feil
ah...@freenet.carleton.ca
Ottawa, Canada.
Richard N. Rea
In article <rddDxz...@netcom.com> ah...@FreeNet.Carleton.CA (Max Feil) writes:
>From: ah...@FreeNet.Carleton.CA (Max Feil)
>Subject: Re: Inertia Coupling, what is it?
>Date: Thu, 19 Sep 1996 13:48:48 GMT
>Ed Davisson (edp...@indy.net) writes:
>> I'm a layman that has some passing interest in aviation. In watching a
>> documentary about the X planes, they mentioned the time when Chuck Yeager
>> reached too high a speed, in I believe the X-2. They referred to a term
>> "inertia coupling". What is "inertia coupling"? I've heard this term used
>> in the past in discussions involving supersonic flight. Would some kind
Not related to the subject, but wasn't it the X-1A?