Variable geometry intakes

[Boomer]

are variable geometry intakes on fighter aircraft primarily there to reduce
ram drag or to manipulate the shock wave in the nacelle?

--



Curiosity killed the cat, and I'm gonna find out why!

[Orval Fairbairn]

In article >,
"Boomer" > wrote:

> are variable geometry intakes on fighter aircraft primarily there to reduce
> ram drag or to manipulate the shock wave in the nacelle?


Both -- The most efficient inlet geometry has the shock wave impinging
on the inlet lip; if it impinges inside, the inlet "swallows the shock,"
which leads to compressor stall.

[John Carrier]

> Both -- The most efficient inlet geometry has the shock wave impinging
> on the inlet lip; if it impinges inside, the inlet "swallows the shock,"
> which leads to compressor stall.

It isn't necessary that all shocks be external to the inlet, only that
stable subsonic air reach the compressor face. Some inlets are designed to
create several oblique shocks within the inlet prior to reaching a point
where an expansion of cross-sectional area (diffuser) creates a final normal
shock to decelerate the air.

R / John

[Kurt R. Todoroff]

>It isn't necessary that all shocks be external to the inlet, only that
>stable subsonic air reach the compressor face. Some inlets are designed to
>create several oblique shocks within the inlet prior to reaching a point
>where an expansion of cross-sectional area (diffuser) creates a final normal
>shock to decelerate the air.
>
>R / John

Amplifying on John's well written technical description, engine thrust is
directly proportional to air pressure at the engine face. The primary
objective of a variable geometry engine inlet is to effect a maximum pressure
recovery of the air prior to arrival at the engine compressor face. The shock
wave development that John describes, especially the final normal shock wave,
accomplishes this. The pronounced effect of an inlet system that fails to
articulate is quite amazing. While I have never had an inlet system fail
during functional check flights in the F-111, a couple of my colleagues have.
In one case, the central air data computer Mach signal failed to reach both
inlets. Their F-111F barely attained Mach 1.7 in Maximum Afterburner. The F
model had the largest engines in the fleet, and could attain Mach 1.1 in
Military power on the deck, and Mach 2.5 in less than Maximum Afterburner at
altitude. They brought the jet back, maintenance repaired the problem, and
they flew it again the next day. They hit Mach 2.5 without breaking a sweet.
I haven't heard only one inlet not articulating, however, I would imagine that
the first clue would be the pilot adding rudder into the "good" engine as the
Mach increased.

The other factors, such as flow smoothing and resistance to angle of attack
excusions, do not require a variable geometry configuration. A well designed
fixed geometry inlet can accomplish these objectives.





Kurt Todoroff


Markets, not mandates and mob rule.
Consent, not compulsion.

Remove "DELETEME" from my address to reply

[SteveM8597]

>The pronounced effect of an inlet system that fails to
>articulate is quite amazing. While I have never had an inlet system fail
>during functional check flights in the F-111, a couple of my colleagues have.
>
>In one case, the central air data computer Mach signal failed to reach both
>inlets. Their F-111F barely attained Mach 1.7 in Maximum Afterburner. The F
>model had the largest engines in the fleet, and could attain Mach 1.1 in
>Military power on the deck, and Mach 2.5 in less than Maximum Afterburner at
>altitude. They brought the jet back, maintenance repaired the problem, and
>they flew it again the next day. They hit Mach 2.5 without breaking a sweet.
>
>I haven't heard only one inlet not articulating, however, I would imagine
>that
>the first clue would be the pilot adding rudder into the "good" engine as the
>Mach increased.
>
>The other factors, such as flow smoothing and resistance to angle of attack
>excusions, do not require a variable geometry configuration. A well designed
>fixed geometry inlet can accomplish these objectives.
>
>
>
>
>
>Kurt Todoroff


Once in a while the variable inlet bellmouth rings on the F-4 at the
engine/inlet duct interface would fail to move at M 2.0+. The rings rotated
about 90 degrees or so as the ramps closed down to dump excess air at high
speed but didn't get that much use. Corrosion would cause the cable and pulley
system to corrode and not move freely.. When that happened the engine was very
stall susceptible. I had one happen when flying a Funcfional Check Flight.
Interesting experoence to compressor stall at M 2.3. Even with the centerline
thrust F-4 the bang and yaw pretty violent.

During early flight testingof the the F-16 with the F-110 engine in 85, we
had a test bird with the large inlet, IIRC, that the pilots called Thumper
because of banging in the inlet due to airflow. The engine was pretty stall
resistant with the electonic control but the pilots said the banging was enough
to bounce their feet off the rudder pedals.Apparently at some speeds and
configurations in this particilar aircraft, the shockwave would draw back into
the inlet. The banging was due to oilcanning of the sheetmetal from the
pressure drop across the shockwave as shown by additional instrumentation and
high speed photography.

[Boomer]

VERY interesting stuff guys thanks :-)
was wondering this: F-15 has nacelles that physically move externally,
whereas F-14 is fixxed externally yet they have somewhat similar performance
in the upper right of the envelope. How does the F-14 deal with reducing ram
drag ? It appears the F-15 can simly move it's nacelles down to restrict air
flow. I know F-14 has shock ramps inside to deal with the shock wave, but
what does it do about ram?

--



Curiosity killed the cat, and I'm gonna find out why!
"SteveM8597" > wrote in message
...
> >The pronounced effect of an inlet system that fails to
> >articulate is quite amazing. While I have never had an inlet system fail
> >during functional check flights in the F-111, a couple of my colleagues
have.
> >
> >In one case, the central air data computer Mach signal failed to reach
both
> >inlets. Their F-111F barely attained Mach 1.7 in Maximum Afterburner.
The F
> >model had the largest engines in the fleet, and could attain Mach 1.1 in
> >Military power on the deck, and Mach 2.5 in less than Maximum Afterburner
at
> >altitude. They brought the jet back, maintenance repaired the problem,
and
> >they flew it again the next day. They hit Mach 2.5 without breaking a
sweet.
> >
> >I haven't heard only one inlet not articulating, however, I would imagine
> >that
> >the first clue would be the pilot adding rudder into the "good" engine as
the
> >Mach increased.
> >
> >The other factors, such as flow smoothing and resistance to angle of
attack
> >excusions, do not require a variable geometry configuration. A well
designed
> >fixed geometry inlet can accomplish these objectives.
> >
> >
> >
> >
> >
> >Kurt Todoroff
>
>
> Once in a while the variable inlet bellmouth rings on the F-4 at the
> engine/inlet duct interface would fail to move at M 2.0+. The rings
rotated
> about 90 degrees or so as the ramps closed down to dump excess air at high
> speed but didn't get that much use. Corrosion would cause the cable and
pulley
> system to corrode and not move freely.. When that happened the engine was
very
> stall susceptible. I had one happen when flying a Funcfional Check
Flight.
> Interesting experoence to compressor stall at M 2.3. Even with the
centerline
> thrust F-4 the bang and yaw pretty violent.
>
> During early flight testingof the the F-16 with the F-110 engine in 85,
we
> had a test bird with the large inlet, IIRC, that the pilots called Thumper
> because of banging in the inlet due to airflow. The engine was pretty
stall
> resistant with the electonic control but the pilots said the banging was
enough
> to bounce their feet off the rudder pedals.Apparently at some speeds and
> configurations in this particilar aircraft, the shockwave would draw back
into
> the inlet. The banging was due to oilcanning of the sheetmetal from the
> pressure drop across the shockwave as shown by additional instrumentation
and
> high speed photography.
>

[John Carrier]

"Boomer" > wrote in message
...
> VERY interesting stuff guys thanks :-)
> was wondering this: F-15 has nacelles that physically move externally,
> whereas F-14 is fixxed externally yet they have somewhat similar
performance
> in the upper right of the envelope. How does the F-14 deal with reducing
ram
> drag ? It appears the F-15 can simly move it's nacelles down to restrict
air
> flow. I know F-14 has shock ramps inside to deal with the shock wave, but
> what does it do about ram?

This is a pretty good graphic of the system.
http://www.anft.net/f-14/f14-detail-airintake.htm

OBTW, the F-14A programming optimized performance at the upper end and
(theoretically) could outaccelerate an F15A above 1.6 or so. Ramp
reprogramming, elimination of the glove vanes, and some additional drag
counts make the F-14B (and moreso the D) slower above 1.6 than the old A
with significantly less thrust. OTOH, the B and D have their ramp
scheduling optimized for real world tactical requirements. They're awesome
in the transonic range. A clean F-14B/D can exceed both its NATOPS and
manufacturer's KIAS placard limits, even if they're barely mach 2 capable
(if that, I've heard 1.9).

R / John

[Dave Kearton]

"John Carrier" > wrote in message
...
|
|
| OBTW, the F-14A programming optimized performance at the upper end and
| (theoretically) could outaccelerate an F15A above 1.6 or so. Ramp
| reprogramming, elimination of the glove vanes, and some additional drag
| counts make the F-14B (and moreso the D) slower above 1.6 than the old A
| with significantly less thrust. OTOH, the B and D have their ramp
| scheduling optimized for real world tactical requirements. They're
awesome
| in the transonic range. A clean F-14B/D can exceed both its NATOPS and
| manufacturer's KIAS placard limits, even if they're barely mach 2 capable
| (if that, I've heard 1.9).
|
| R / John
|



How relevant is Mach 2+ performance these days - how relevant was it at all
?


I can imagine high speed being useful when intercepting the odd Foxbat or
two, but otherwise - how often would you require such high speeds ?





Cheers


Dave Kearton

[Kurt R. Todoroff]

<< How relevant is Mach 2+ performance these days - how relevant was it at all
? >><BR><BR>

This is the same pedantic question that I've heard for twenty-nine years.

These velocities are a consequence of meeting specific excess power
requirements (P_s). A positive P_s allows an aircraft to accelerate (gain
velocity), sustain G, or climb in altitude, or any of these three. P_s does
not come free. P_s is computed as:

(Thrust - Drag) * Velocity / Weight

A natural consequence of a fighter's design is speed. The design is a result
of tradeoffs. The fact that fighters rarely exercise their supersonic
capabilities is not relevent. By reducing the thrust of the engines to limit
the aircraft speed to M-1.5 or M-1.0, the aircraft's performance is other
realms is sharply limited as well.

Since high speed is a natural consequence of a fighter's design, the USAF and
USN have taken advantage of it.




Kurt Todoroff


Markets, not mandates and mob rule.
Consent, not compulsion.

[Boomer]

Thanks for the link and Tom stats JC :-)

--



Curiosity killed the cat, and I'm gonna find out why!
"Kurt R. Todoroff" > wrote in message
...
> << How relevant is Mach 2+ performance these days - how relevant was it at
all
> ? >><BR><BR>
>
> This is the same pedantic question that I've heard for twenty-nine years.
>
> These velocities are a consequence of meeting specific excess power
> requirements (P_s). A positive P_s allows an aircraft to accelerate (gain
> velocity), sustain G, or climb in altitude, or any of these three. P_s
does
> not come free. P_s is computed as:
>
> (Thrust - Drag) * Velocity / Weight
>
> A natural consequence of a fighter's design is speed. The design is a
result
> of tradeoffs. The fact that fighters rarely exercise their supersonic
> capabilities is not relevent. By reducing the thrust of the engines to
limit
> the aircraft speed to M-1.5 or M-1.0, the aircraft's performance is other
> realms is sharply limited as well.
>
> Since high speed is a natural consequence of a fighter's design, the USAF
and
> USN have taken advantage of it.
>
>
>
>
> Kurt Todoroff
>
>
> Markets, not mandates and mob rule.
> Consent, not compulsion.

[John Carrier]

> How relevant is Mach 2+ performance these days - how relevant was it at
all
> ?

It was never tactically relevant.

>
> I can imagine high speed being useful when intercepting the odd Foxbat or
> two, but otherwise - how often would you require such high speeds ?
>

An F-4 could theoretically reach launch parameters for a mach 3, 70,000'+
target doing about 1.4 at 36,000. Biggest problem was controlling target
aspect in the horizontal. More speed would have helped some there.

Speed can be useful in minimizing raid penetration and increasing AA missile
LARs. It's also very useful when leaving hostile territory, albeit that's
typically at mid altitudes where Q vice mach is the controlling factor.

R / John

[John Carrier]

> This is the same pedantic question that I've heard for twenty-nine years.
>
> These velocities are a consequence of meeting specific excess power
> requirements (P_s). A positive P_s allows an aircraft to accelerate (gain
> velocity), sustain G, or climb in altitude, or any of these three. P_s
does
> not come free. P_s is computed as:
>
> (Thrust - Drag) * Velocity / Weight
>
> A natural consequence of a fighter's design is speed. The design is a
result
> of tradeoffs. The fact that fighters rarely exercise their supersonic
> capabilities is not relevent. By reducing the thrust of the engines to
limit
> the aircraft speed to M-1.5 or M-1.0, the aircraft's performance is other
> realms is sharply limited as well.
>
> Since high speed is a natural consequence of a fighter's design, the USAF
and
> USN have taken advantage of it.

Implying that very high speed is a free benefit of high thrust. The F-16
has more thrust but is slower than the F-104, but it's more capable in many
ways. It's not a function of reducing thrust, but rather a function of
optimizing the design for mission-related functions. Mach 2 speed isn't one
of those functions and has been deemphasized (ala F-14B versus F-14A). At
the other extreme, the (highly specialized) SR-71 is a legitimate 3.2 cruise
airplane, yet is severely Q limited.

Probably the best example is the F-18, which has excellent performance
subsonic but rapidly runs into a brick wall above the number (highly
configuration dependent). IMO, too much high speed performance was
sacrificed (high indicated airspeeds are illusive as well), or more
correctly the drag was never really designed out of the F-17, its prototype.
Despite its shortcomings, its a pretty capable airplane, even if it's slower
than many earlier jets with half the thrust.

R / John

[Ed Rasimus]

On Sun, 11 Apr 2004 06:11:25 -0500, "John Carrier" >
wrote:

>> How relevant is Mach 2+ performance these days - how relevant was it at
>all
>
>It was never tactically relevant.

Agreed. But, high speed was a big advertising/PR point during the
Century Series days. I flew two aircraft that were definitely Mach 2
capable, but in 23 years of tactical aviation never went M-2 once.

The parallel might be the horsepower of your sporty car--while the car
might be capable of 155+ MPH, it really won't be done by 99.99% of all
owners. The corollary benefit of good acceleration between 30-75 MPH
is what most users will take advantage of.
>
>>
>> I can imagine high speed being useful when intercepting the odd Foxbat or
>> two, but otherwise - how often would you require such high speeds ?
>>
>
>An F-4 could theoretically reach launch parameters for a mach 3, 70,000'+
>target doing about 1.4 at 36,000. Biggest problem was controlling target
>aspect in the horizontal. More speed would have helped some there.

When the Foxbat was the rage, we often practiced "snap-up" intercepts
in the F-4 and, as you indicate they were extremely critical regarding
geometry. The key was getting as close to head-on as possible so as to
be at R-max in your pitch-up. At the high closing speed the interval
between R-max and R-min was brief and waiting to pull until within
range meant the target would be past you before you could fire. Any
angular displacement horizontally would drastically compound the
problem. Virtually impossible to pull enough lead.
>
>Speed can be useful in minimizing raid penetration and increasing AA missile
>LARs. It's also very useful when leaving hostile territory, albeit that's
>typically at mid altitudes where Q vice mach is the controlling factor.

Once again, you're spot on. Speed in knots is clearly life. Speed in
Mach is propaganda.


Ed Rasimus
Fighter Pilot (USAF-Ret)
"When Thunder Rolled"
Smithsonian Institution Press
ISBN #1-58834-103-8

[John Carrier]

> When the Foxbat was the rage, we often practiced "snap-up" intercepts
> in the F-4 and, as you indicate they were extremely critical regarding
> geometry. The key was getting as close to head-on as possible so as to
> be at R-max in your pitch-up.

How true. I've run simulations where target aspect just got away (and it
wasn't much to begin with). And even with horizontal geometry solved being
a bit late for the pitch one would never get the nose up fast enough to
center the dot. With Sparrow, it was imperative the missile get off in
medium altitude autopilot gain and with a lead-collision geometry wired at
launch. Any major inflight guidance corrections by the missile would drive
PsubK from fair to non-existent.

R / John

[SteveM8597]

>When the Foxbat was the rage, we often practiced "snap-up" intercepts
>> in the F-4 and, as you indicate they were extremely critical regarding
>> geometry. The key was getting as close to head-on as possible so as to
>> be at R-max in your pitch-up.
>
>How true. I've run simulations where target aspect just got away (and it
>wasn't much to begin with). And even with horizontal geometry solved being
>a bit late for the pitch one would never get the nose up fast enough to
>center the dot. With Sparrow, it was imperative the missile get off in
>medium altitude autopilot gain and with a lead-collision geometry wired at
>launch. Any major inflight guidance corrections by the missile would drive
>PsubK from fair to non-existent.
>
>R / John
>
>
>
>

Getting scrambled off air defense alert in Korea (Kunsan and Taegu), and
sometimes nothern Japan (Misawa) in the early 70s against high speed targets
was not all that unusual. The tracks were at 50k'+, south or eastbound
headings, usually near the NE corner of S Korea by the DMZ. Presumably
Foxbats. The tracks were usually M 1.5+, always out of N Korea and were, in
all likelihood, just probing of our tactics and reaction times, or else their
own quick reaction alerts protecting Bears transiting the Sea of Japan. GCI put
us on a max performance TO and burner climb to 25k', directly head on with the
track We continued the climb to 35k', started the pull at M 1.2 in a shallow
dive, took the Judy as we started the pull.. The bogeys never came near the
DMZ and almost always broke off just after we started the snapup maneuver.. I
am a little hazy on the details of the intercept geometry but I recall the
setups looked do-able up until we/they broke it off off. Seemed to happen once
a month or so.

Generally on AD alert out of Kunsan you had a good probability of getting
scrambled so it was interesting duty. You didn't often get put against a
potentially hostile track, though.. I suspect we got scrambled to protect
intell assets (EC-121, 130, or Navy ship) operating in the Sea of Japan or the
Yellow Sea as we usually got vectored to an orbit. Usual load was 4 x Aim-7 and
4 x Aim-9 though we carried the gun if the mission was predesignated to protect
drones and or destroy them if the controller lost the link.

We practiced snapup attacks with F-4 targets but at much lower altitudes. An
F-4 target could harely fly at 45k'.with three bags at mil power. I don't
recall those being a particlar challenge.so long as you got head-on early and
only had to snap 10k' to fire. I guess if you train for it several times a
week you got the hang of it..

[WaltBJ]

(SteveM8597) wrote in message >...
> >When the Foxbat was the rage, we often practiced "snap-up" intercepts
> >> in the F-4 and, as you indicate they were extremely critical regarding
> >> geometry. The key was getting as close to head-on as possible so as to
> >> be at R-max in your pitch-up.
> >
>Prior to transferring to TAC and the F4 I had gone to ADC's
Interceptor Weapons School in the F102A. I also had executed quite a
few passes against a M2.0 B58 - right down the throat. Once in the F4
program I and a couple Gibs explored snapups against a Foxbat using
the F4 simulator. We found that a flyup rather than a snapup was more
successful. In addition we used super-elevation, keeping the steering
dot below dead center of the scope, so that the missile would
approximate a zero-G trajectory. The real-life problem would be
getting into a 180 intercept and M2+ far enough out in front to climb
on up to launch position. Here is where a fan of converging
interceptors would maximize Pk. (Doctrine would be to launch
everything - all the missiles - radars and all-aspect IRs.)
Walt BJ

[Dweezil Dwarftosser]

John Carrier wrote:
>
> > When the Foxbat was the rage, we often practiced "snap-up" intercepts
> > in the F-4 and, as you indicate they were extremely critical regarding
> > geometry. The key was getting as close to head-on as possible so as to
> > be at R-max in your pitch-up.
>
> How true. I've run simulations where target aspect just got away (and it
> wasn't much to begin with). And even with horizontal geometry solved being
> a bit late for the pitch one would never get the nose up fast enough to
> center the dot. With Sparrow, it was imperative the missile get off in
> medium altitude autopilot gain and with a lead-collision geometry wired at
> launch.

It would still SWAB (Switch-after-boost), though.
With later missiles, your chances actually improved firing
from level (just at/inside Rmax); no snap-up equals no need
to recenter the dot; you're already there.

> Any major inflight guidance corrections by the missile would drive
> PsubK from fair to non-existent.

Agreed. Our simulator (WCS maintenance; APM-307) flew all
profiles with the aircraft tricked into thinking it was
flying at 44,000 ft, though. It was the one variable we
couldn't change without modifying the hardware...

[John Carrier]

SNIP

>We found that a flyup rather than a snapup was more
> successful. In addition we used super-elevation, keeping the steering
> dot below dead center of the scope, so that the missile would
> approximate a zero-G trajectory. The real-life problem would be
> getting into a 180 intercept and M2+ far enough out in front to climb
> on up to launch position. Here is where a fan of converging
> interceptors would maximize Pk. (Doctrine would be to launch
> everything - all the missiles - radars and all-aspect IRs.)
> Walt BJ

Works as long as launch altitude is below AIM-7 threshold for autopilot high
gain.

R / John