Forum: Mishaps

Last edited by Roscoe on Nov 18, 2005 - 03:16 PM; edited 1 time in total

Wow, lot's of bad poop in this thread. let me correct some of it.

1) The jet wash problem does not come from the air entering the engine too fast. The inlet will take care of that (more later). The issue is that the jet wash is turbulent and older engines want the air flowing straight into it (the newer ones are more robust). Here's why...the engine compressor blades are simply airfoils. Turbulent air approaches the fan face enters at a different angle than designed, causing the angle of attack of the "airfoils" to exceed the stall angle and now the blade no longer functions...i.e. it no longer compresses the air and raises the pressure. This is called a compressor stall. Also consider that when the air ignites in the combustor, the pressure also goes up. Now, if the pressure behind the fan is reduced, the high pressure air in the combustor now has less resistance to going forward. Pressure ratios in engines are finally tuned...a drastic reduction of pressure forward and the hot air in the cumbustor suddenly flows back forward through the fan rather than aft through the turbine. Loud bangs, bright lights...can be quite exciting. Shocked

2) Inlet design. Two types: subsonic and supersonic. A subsonic inlet has but one function...to smooth and straighten the air (see above). A supersonic inlet has one more function...to slow the speed of the air to subsonic speeds. This is done in two ways. The Viper has a normal shock inlet. In other words, a shock wave 90 degrees to the flow is created at or near the lip of the inlet. Supersonic air going through a normal shock becomes subsonic...but in a very harsh manner, much like slowing down your car by driving through a wall. Thus normal shocks are very inefficient in terms of energy loss AND are at their optimum at only one Mach number. Subsequently, they tend to limit the maximum speed of the aircraft. However, they have no moving parts so they require no maintenance so they can't fail (other than delamination etc..) and are cheap.

A more efficient method of slowing the air down are oblique shocks where the shock wave is at an angle to the air flow. Supersonic air flowing through an oblique shock will still exit supersonic but less so, therefore multiple shocks are required to get the speed down to near Mach 1. The last one is by definition is a normal shock (only way to actually make the flow subsonic) but by then the speed is so close to Mach 1 that the inefficiencies are negligible. The angle of these oblique shocks relative to the air flow change with Mach number so they require variable inlets like ramps (Eagle, Tomcat) or translating spikes (SR-71, F-104) to manage the shocks. These systems however are complex, heavy, require maintenance, and can fail in flight. They are also very sensitive to flight condition and can be disrupted by turbulent air (i.e. the famous SR-71 "unstarts"). Finally, can you say $$?

One last point. The inlet is separated from the fuselage not for supersonic reasons but to keep the boundary layer air (slower moving air due to friction with the fuselage) out of the engine.

(edited for minor typos and for clarification)

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Kosmos wrote:

Cylon: No doubt it's thick.. But if a bird could go through the canopy, I still belive I could Smile

But a bird will only penetrate if hitting at 400 kts. The seat going up the rails is no where close to that speed when it reaches where the canopy would be. besides, the bird no longer resembles a bird post-penetration...

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

The only thing that stuck with me about that scene was Val saying "Maverick's in a flat spin... headed out to sea." in an apparent attempt to explain how they'd get from the desert to the ocean. If you're in a flat spin the only place you're headed is down!

Yea, that's my favorite example of a hollywood screw-up as well. Drives my wife nuts...everytime we see something stupid in a movie, I blurt out that line...one time somebody overheard me who understood where I was going and laughed so hard he spit out his jujube...then I was ROTFLMAO! Laughing

Roscoe, thank you for your explaination! It was really complete.

BTW, "how does an aircraft in a flat spin travel out to sea.... or is a Tomcat in a flat spin travelling like a "frisbee" or UFO??"

Adrian

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Quote:

how does an aircraft in a flat spin travel out to sea....

Because that's what the script calls for. Very Happy

Who knows, maybe its own inertia and wind carries it out to sea? Oh yeah, saw the blood running down his face, maybe died from plexiglass cuts?

avon1944 wrote:

BTW, "how does an aircraft in a flat spin travel out to sea.... or is a Tomcat in a flat spin travelling like a "frisbee" or UFO??"

Adrian

Thats's the point...it can't. Initially, it will continue in whatever vector it had when it entered the spin. However, spins are high drag events and it will quickly succumb to gravity and the vector will be straight down.

Speaking of Top Gun...I love the mission where he "busts the hard deck" of 10K ft. During that flight they were playing chicken with the mountains. Last time I looked there were no peaks over 10K in the airspace around Miramar...

Great flying cinematography but the script blew...(I have to admit though I have the DVD although I was greatly disappointed in the quality of the transfer)

What is funny about TG is that one of the tech advisor's ( a navy aviator ) said he never heard about a pilot "turning in his wings" so to speak.

Yet that is exactly what happened on a Kennedy cruise off of Lebanon in '83 after the C.O. of an F-14 squadron got the guys together after 2 mishaps and asked if anyone felt they didn't have what it took, do the right thing. Later there was one taker to that request.

Anybody here ever done a flat spin in a jet? I've only done 'em in a Cap-10. Kind of fun. Better than inverted spins or outside snap rolls.

In a fully developed flat spin in the F-14 the pilot was at the end of the whip and would be experiencing 4.5-5.5g eyeballs out. If you didn't lock your harness before the g's built up you'd be munching on glare shield hoping your RIO could reach one of the ejection handles because it wasn't going to be you. Unfun.

It's funny. For such a technically terrible movie the spin scene was the only thing remotely based in reality. The TF-30's were stall prone pig's that were mounted 5 1/2 feet off of aircraft centerline. If one stalled while you were maneuvering, things got interesting very quick if you didn't quit maneuvering and regain control. If you didn't, by the time the jet had spun twice it was going flat and your controls didn't have the authority (no air over them) to stop it. The canopy would "float" above the falling (not Frisbee'ing) Tomcat, which was why the procedure for ejection would have the RIO jettison the canopy FIRST, and then initiate ejection. The theory being that this would give the extra second for the airstream to catch the canopy and get it out if the way. That canopy weighed hundreds of pounds..if you hit it...and a few RIO's paid with their lives to develop that procedure.

NATOPS and -1's are written in blood.

Interesting story Elp. I guess that he must've been asleep during Vietnam then, as there were quite a few pilots who turned in their wings, because they couldn't hack it.

One of the other fmr. TOPGUN instructors who speaks on the DVD commentary (Mike Galpin, IIRC), said that a guy did that during a cruise he was on. The guy came into his CO's office in the morning, and turned in his wings. He stated that the guy was on the COD and gone that afternoon.

Just curious...if a pilot does this, do they also grant him a resignation and Honorable Discharge, or do they transfer him to a desk assignment? I never knew any pilots that did this. I knew some who were RIF'ed, but none who turned in their wings.

Beers and MiGs were made to be pounded!

Kosmos wrote:

There is no way you can crash into your own canopy, not matter how hard you try. And even if you should make it somehow, the ACES seat is higher then my head/helmet (this goes for all seats that I have ever seen). So it would crash into (or through) the canopy, and protect my head (if needed).

Actually it is possible to hit the canopy. In the movie the F14 enters a flat spin, which under the best of sercumstances is hard to recover from, even for experienced test pilots. It would have been harder for maverick to reach the ejection handles than for goose because the centrifical force is greater as you move away from the pivot point (centre of the aircraft). So Goose ejects first. In a flat spin there is no forward airspeed the aircraft is rotating horizontally and falling. This creates a low pressure zone over the top of the aircraft and when the canopy is blown, it hangs around in that pressure zone instead of catching a slip straem and thats how goose hit it.

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Pedantic correction: It's technically centripetal force Maverick is feeling, not centrifugal force. It depends on your frame of reference. From his it would be centripetal.

And Kosmos' point is that doesn't the ACES seat extend above the pilot's helmet? That (I'd think) would absorb some of the impact... But as others have pointed out this was something F-14s actually had happen (aside from the heading out to sea part Rolling Eyes

)

Guysmiley wrote:

And Kosmos' point is that doesn't the ACES seat extend above the pilot's helmet? That (I'd think) would absorb some of the impact...

The top of the ACES ejection seat does, in fact, protrude above the helmet level. But this may not provide complete protection to your head -the 'headrest' section of the seat is the highest part and there is nothing directly above your helmet to protect it (as opposed to some of the MB seats with the 'face curtain' seat iniatiators.)

At the same time the canopy would not, in all likelihood, be sitting in the same orientation as it was on the jet. It is blown off by rockets at the forward edge, so it would probably be tumbling. I had been told by people in the Tomcat community (back during the 'Top Gun' timeframe) that the movie scene was based upon an actual incident.

Mike

Forum: Mishaps

Top Gun movie - Jet wash/Flat spin