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Crossunder
18th Sep 2004, 14:49
For a jet aircraft, the effect of reverse thrust is greater at high speeds than at relatively low speeds, say 60-80kts. But how does this work for turboprop aircraft? The other way around? During a RTO close to V1 - is "discing" more effective than full reverse until the ground speed is reduced to below, say, 50-60kts? The question came up during my OPC recently. The aircraft in question is the Dash-8.

oldebloke
18th Sep 2004, 19:54
Some of the turbo props(metro3 case)have a graph based on 'Full reverse'stop distance..I'm not sure if the Dash8 has JAA info in it's perf Data(it should as Canada has adopted JAA as a norm)...JAA info gives 'credit' for reverse(distance)although the gen thought is that anything above reverse idle,in the eng/out case/rejected T/Off,would provide adverse YAW???
Canada's' coeficient of friction' runway charts afford 'credit' for symmetrical discing/reverse(canada's AIP):ok:

411A
19th Sep 2004, 02:43
Dunno about the Dash 8, but in the Lockheed Electra it didn't seem to make all that much difference, high speed or low.
In both cases, equally effective.
Likewise for all large 4-engine piston aircraft (1649 Constellation and DC-6, specifically).

Centaurus
19th Sep 2004, 06:58
Many years ago I did a high speed abort in Viscount 800 just at V1 after multiple bird strikes (we later found 23 dead seagulls on the runway). The Viscount did not have reverse thrust propellers but a ground fine position which gave impressive drag.

During normal landings the props were close to the fine pitch stop and the ground fine selection was accompanied by a distinct increase in noise which, when coupled with noticeable decelleration, indicated that you were in ground fine.

When the throttles were rapidly closed at the start of the abort I was surprised at the lack of immediate sound of ground fine and also lack of decelleration for the first few seconds. In fact I thought that ground fine had somehow failed to engage. Ground fine had engaged however, it was just that it took some time for the pitch of the props to change from high speed take off angle down to where the ground fine angle bit in. My guess about 5-7 seconds which seemed like an eternity.

Old Smokey
19th Sep 2004, 07:02
Crossunder,

In a Rejected Takeoff, Every little bit helps, and Reverse Thrust, be it on a Jet or a Turbo-Prop, only supplies a little bit of retardation. The brakes (aided by ground spoilers to make them more effective) do the lion's share of the work, with Reverse Thrust contributing a little bit. There is an extant pilot myth that Reverse Thrust during an RTO is so veeerrry important, probably because all of the roaring and snorting involved makes them feel as though they're doing something so very proactive. Reverse thrust, be it on a jet or a turbo-prop, is very inefficient in terms of the effort involved for a small return, if we could swivel the engines 180 and apply full takeoff thrust in the reverse direction, the brakes would still win (maybe not on a DC3, but we've gone a bit beyond that).

411A says -
Dunno about the Dash 8, but in the Lockheed Electra it didn't seem to make all that much difference, high speed or low.
I agree with that for all aeroplanes I've flown, prop or jet (but the roaring and snorting feels good). 411A also says "In both cases, equally effective" - Presumable he refers to high speed / low speed as both cases, and I disagree, Reverse thrust for the turbo-prop is slightly more effective at lower speeds, but only slightly.

If you consider the use (or non-use) of Reverse Thrust during a RTO for the turbo-prop or jet, there are basically 3 engine / thrust conditions -

(1) Thrust / Power to Idle (Certification) > The residual forward idle thrust is a 'negative' to the retardation process, and you will be doing yourself a great favour if you can remove it.

(2) Reverse Thrust to Idle (Open the buckets) / Propellers to 'discing' > Removal of the residual forward thrust greatly assists the retardation process. 'Discing' propellers add to the retardation process in applying significant drag at high speeds, and negligible drag approaching zero at low speeds.

(3) Apply Reverse Thrust. Most effective in a jet at high speeds, most effective in a turbo-prop at low speeds, but effective at all speeds in supplying a little bit of extra retardation.

In all 3 cases, item (2) - Cancelling the residual forward thrust is the biggest favour you can do for yourself, item (3) - applying reverse thrust gives you a little bit more.

For WET runways, maximum use of reverse thrust above aquaplaning speed (typically not too far below V1) is your best friend. Below that speed, brakes are again 'the king of the hill'.

For ICY / Slick runways, Reverse thrust is in it's element and this is the only time that they may be of major over-all significance, but only if applied symetrically. For an RTO following engine failure on Icy / Slick runways, 'discing' may be your only feasible option if directional control is to be maintained (Remember that the failed engine is also 'discing').

Oh! - Your original question, Reverse thrust is more effective than 'discing' at ALL speeds.

The following posting slots are reserved for those regularly landing where 1500M of runway is required, but they're operating on 3000M/4000M runways with their "Reverse Thrust beats Brakes" arguments.

Crossunder
19th Sep 2004, 14:42
Thanks for the answers guys! 'Twas as I suspected... ;-)
We operate out of very short fields (860-1199m), and the disc vs. full reverse discussion pops up from time-to-time. Useful to get some outside points of view; we don't have any pilots with flight testing experience in our company(except those who've slid off the runway...).

Cheers!

Old Smokey
20th Sep 2004, 07:36
Crossunder,

If I'd known that you're operaating from such short runways I would have changed the emphasis from Every little bit helps to EVERY LITTLE BIT IS NECESSARY !

Happy (short) landings,

Smokey