Blackbird
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Maximum DC-8 Mid-Air Thrust Reverser Speed

Fri Aug 31, 2007 11:59 am

The DC-8 from what I was told can extend the reversers at a maximum airspeed of 390 kts. Was the fact that the reversers were mounted in an aft-translating ejector that made them able to activate at such high speeds? I mean could a thrust reverser design of a 707-type be activated safely at such a high speed without the risk of sustaining damage?

Also, while at it...

1.) The 390 kt speed listed, does that mean just at sea-level reading on the gauge (The DC-8's speed gauges are not particularly reliable at high altitude with IAS readings that seem to creep up and up and up, particularly when close to VMO)

2.) What's the maximum mach number you can use the reversers at in mid-air safely?

3.) Can the B-707's reversers use full reversing power, or like the CV-880 desgn can only use 50 percent power for reversing?


Andrea Kent
 
Blackbird
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sat Sep 01, 2007 9:38 am

Nobody?

Andrea Kent
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SFOMB67
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sat Sep 01, 2007 11:27 am

Quoting Blackbird (Thread starter):
Was the fact that the reversers were mounted in an aft-translating ejector that made them able to activate at such high speeds? I

When the DC8's were re-enginned into the 70 series, they also replaced the t/r's with cascade vane type reversers, although i don't know the speed at which they cold be used.
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Viscount724
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sat Sep 01, 2007 2:11 pm

Quoting Sfomb67 (Reply 2):
When the DC8's were re-enginned into the 70 series, they also replaced the t/r's with cascade vane type reversers, although i don't know the speed at which they cold be used.

If not mistaken the DC-8-50 and -61 series also had cascade type thrust reversers, while the -62 and -63 had target-type (clamshell) reversers.
 
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sat Sep 01, 2007 8:01 pm

Quoting Blackbird (Thread starter):
1.) The 390 kt speed listed, does that mean just at sea-level reading on the gauge (The DC-8's speed gauges are not particularly reliable at high altitude with IAS readings that seem to creep up and up and up, particularly when close to VMO)

2.) What's the maximum mach number you can use the reversers at in mid-air safely?

3.) Can the B-707's reversers use full reversing power, or like the CV-880 desgn can only use 50 percent power for reversing?

1.) If the limit is 390 then that must be IAS. The crew have no indication of "sea level airspeed".

2.) If no Mach limit is given then only an airspeed limit applies.

3.) Most engines have reduced maximum power settings in reverse, automatically applied by the FCU. 50% limit would be lower than the norm. The Convair engines had an aft fan IIRC, which might have been a limiting factor.
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Blackbird
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sat Sep 01, 2007 11:28 pm

The DC-8 to my knowledge could achieve MCP on the inboards, and somewhere from zero to 32 pct MCP on the outboards... to my knowledge way above 50 percent while in the air to my knowledge... but why would you use reduced power in reverse?

BTW: Convair 880 was a turbojet...


Andrea Kent
 
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sat Sep 01, 2007 11:43 pm

Quoting Blackbird (Reply 5):
Convair 880 was a turbojet...

Sorry, I confused it with the Convair 990 while trying to think what might differentiate it.
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n8076u
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sun Sep 02, 2007 3:35 am

For the DC-8 (10 to 70 series are all the same), maximum speed for extending thrust brakes is 390 kts. or Mach .88, which happens to be the aircraft's Vmo/Mmo.
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Blackbird
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sun Sep 02, 2007 7:16 am

I thoght it's VMO was like 340 kts indicated.

Andrea Kent
 
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Sun Sep 02, 2007 10:36 am

Blackbird: As my first real post, I'll answer a portion of your question. The DC-8 could deploy it's inboard reversers at any speed. It has no speed/altitude input, it is strictly mechanical off of the nose gear torque links. Using the same mechanism underneath the throttle pedestal the outboard thrust reversers were locked out. Believe it or not I have been on test flights where a misrig of the cables off the torque links allowed all four engines to go into reverse(it was a test flight check), talking about scary, it felt like we had come to a complete stop in the air. Hope this explains some. Karl
 
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RE: Maximum DC-8 Mid-Air Thrust Reverser Speed

Mon Sep 03, 2007 3:23 am

Were the reversers as effective, more effective, less effective than the B-707's speedbrakes?

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