Another Engine Ejector Question (DC8-10s)

[louross]

The engine ejector was there to create more thrust at "lower" speeds, but apparently the drag was equal to, sometimes, greater than the thrust created. So once that was known, why did they continue using them and why didn't Douglas design them out?
tkanks,
lr.

[aerofoto - HJG Admin]

"IF" .... there's any basis to that at all ....

Then "probably" because DC-8-11/12 through -40 production/deliveries (the only versions of these aircraft fitted with the Ejector/Translation Ring system .... to fascillitate the reverse thrust machanism as well as achieving "minimal" noise reduction on T/O and landing too) covered the 1958 through 1960 period only .... and by the stage this was drag issue was "proven" (if not already know during testing) DOUGLAS were already well into developing and marketing the DC-8-50 which was powered by an entirely different type of engine .... which didn't require the Ejector/Translatation Ring system .... and was also an aircraft DOUGLAS hoped would supersede earlier DC-8 versions.

I assume what had been produced by 1960 .... had been produced .... and DOUGLAS probably wanted to move on/forward.

In any case 4 of the pre DC-8-50 series aircraft were later upgraded to "SERIES 50" specification during the early 1960's.

All of the early DC-8 series (pre DC-8-50) suffered drag/underperformance related issues anyway .... but .... this was gradually resolved during development of the -20, -30, and -40 and which saw modifications retrofitted to the -10 and following pre -50 aircraft. Using the "NEW" HJG DC-8 FDE's (pand provided these simulations are flown correctly to start with ) one should get a fair idea of drag/performance related problems which afflicted the early DC-8's and how these aircraft were gradually improved through each successive development prior to the -50.

Further regarding the inflight deployment of both the Enjectors/Translation Rings .... the AOM for the DC-8-11/12 throuigh -43 aircraft states as follows ....

- Inflight Reverse (idle) Thrust = "AVAILABLE" (engine # and # 4 only and below 10,000 FT)

- Inflight Reverse (idle) MAX SPEED = 390 KIAS/MACH 0.88
- Inflight Reverse (idle) MIN SPEED = 200 KIAS

- Ejectors Extend (inflight) MAX SPEED = 390 KIAS/MACH 0.88
- Ejectors Retract (inflight) MAX SPEED = 200 KIAS

- Landing Gear Extention is authorized as a means of inflight decelaration at airspeeds below 250 KIAS

Mark C
AKL/NZ

[louross]

Thanks- that's all interesting!
lr.

[Herman]

Louros. This is the first time that I had heard of, that the DC-8 ejectors were there to "create more thrust at lower speeds".

Somehow that does not sound right to me. Could you please explain how you came to\obtained this information.?

Herman

[aerofoto - HJG Admin]

The engine Ejectors/Translation Rings have nothing to do with the generation of engine thrust .... apart from assistance reversing it (all 4 engines, or, any 2) to aid desceleration after landing.

Remember .... the use of inflight reverse, use on DC-8's, was "REVERSE IDLE" .... on engines # 2 and # 3 only .... and which could be engaged to improve the ROD, rather than deceleration rate (although it would undoubtedly impose a slowing effect to some extent), and it was used "only when circumstances warranted it" too.

It wasn't always used.

Mark C
AKL/NZ

[aerofoto - HJG Admin]

Further in regard to the the use of inflight reverse idle aspect of this DC-8 discussion ....

The DC-8 does not have flight spoilers (it has wing spoilers but they can't be used in flight) to assist with getting down quickly. Instead Douglas designed it with the ability to select idle reverse on engines 2 and 3 which is quite effective

Because the wing spoilers can't be used inflight on any of the DC-8's .... reverse idle (on the 2 inboard engines only) achieves what wing spoilers normally do on other aircraft .... assisting "going down" rather than slowing down.


Here's someone elses recollection/s .....

In the early 1980's I was on an Arrow Air DC-8 from NAS to MIA when the captain announced that we were a bit high on downwind to MIA9R and that the vibration that was soon to start was the thrust reversers on the two inboard engines

That old DC-8 shook and rattled when the reversers were set

The only reason why I diverge here in regard to engine thrust reversers is because the Ejectors/Translation Rings form "part of" the engine thrust reverser system on DC-8-11/12 through -40 aircraft.

Mark C
AKL/NZ

[louross]

hermainkreimes,
I belive that the ejector assembly was there to reduce noise and increase thrust at low speeds. I have no idea who told me that, but it probably came from a DC8 pilot at United back in the early 60s. All ther pilots I knew back then have probably died by now. However, I did hear a story or two that I later found out was either only a partial story to "sound good" or just flat at incorrect.
lr.

[aerofoto - HJG Admin]

I'd say probably just a "sound good" or just "flatly incorrect" story .... by virtue of the fact the Ejectors/Translation Rings aren't a sucking or squeezing part of the engines .... they don't prosuce thrust/power.

They had 2 functions only ....

(1) To assist with noise reduction during T/O and landing (though the extent of any noise reduction these, or even the daisy petal tubes, facilitated has always been "debatable").

(2) The inner part of the Ring/s also housed a pair of clam-shell type doors that rotated inwards/sideweays, to redirect engine thrust forwards and sideways after the Ejectors/Translation Rings were extended, when reverse was pulled on.

I'm not sure as to what extent this particular feature functioned (if it functioned differently to any extent at all) when "REVERSE IDLE" was applied .... and which, once again, implicates engines # 2 and # 3 only.

Mark C
AKL/NZ