Big fan reversers create a dam of air
Exactly my thoughts! Well written!
But what made me think like that?
The DC-10 has a more or less sideways-exhausting APU.
The opening is partly covered in-flight by a small door, which is some kind of a shield when the APU is running in-flight. It could be, for electric purposes but not for pneumatics (or the other way around, forgive me that I’ve forgotten that)!
Now running the APU brings a considerable penalty for take-off performance. If my memory serves me right the runway and climb limited weights are to be reduced by 4.7 “something”. Or even 6 ‘something’.
It may be 4.7 k pounds, but, as we were a metric operator, with metric numbers all over the MDC-J1030 FAA Approved Airplane Flight Manual (AFM), it could wel be 4.7 tonnes. On the other hand, I doubt every little AFM appendix was issued in a metric version as well.
Anyway, this detail many years ago prompted me to consider the powerful sideways exiting air as a sort of invisible speedbrake. Indeed an invisible ‘dam’ of air.
Now back to fan reversers.
Once this ‘deflected’ way of looking at so-called ‘reversed’ airflow is accepted, one can think of the plume of ‘reversed’, or, rather, deflected fan air as a drag device by itself. As if a giant drag chute is deployed around the engine. Enormous drag (and lift loss) at speed (remember the Lauda Air 767 that was not recovered above Thailand when one reverser deployed in-flight at FL 280 at M 0.76).
Much less dramatic drag at landing speeds, but quite useful during the speedy part of the landing roll, and very little remaining at taxying speed.
Note that the working area for the dragchute-like disturbed airflow is less when the airplane is rolling on the runway, since the runway surface is normally not ripped up in the process, and the wing provides a shield as well.
This also helps understanding the strange fact that the tail engine of a DC-10 yields much more reverse effect than one wing engine. There are very interesting graphs in either the Airplane Flight Manual OR the Flight Crew Operating Manual – Performance that show the amount of reverse thrust versus airspeed for various N1 settings. At landing speeds, the reverse effect of the No. 2 (tail) engine by itself amounts to 80% of the combined effect of both wing engines together.
The tail engine is high in the air, and its ‘virtual dragchute’ affects more air than the reverser of a wing engine that is partially shielded by the ground and the wing.
Also, the reverser air of the tail engine (composed of blocking doors in the normal fan exit and many individual cascade panels in the ring of louvres that is exposed by the fan reverser cowl transiting aft) deflect the fan air mainly upwards (but at an angle away from the rudder panels) and sideways (but at an angle aiming above the nearby elevators). Here the air is not really reversed forward!
The panels in the lower sectors of the reverser louvres direct the deflected air more or less straight down and sideways, but again not forward, in order to create “dragchute drag” without really reversing the flow direction since that would rip off the elevators).
This remarkable phenomenon came up one day when a DC-10 tried more reverse at low speed on a very slippery surface and the crew felt the airplane almost being thrust forward instead of really braked by the supposedly reversed air.
Indeed the performance graphs showed that going below some 30 knots airspeed the net reverse thrust of the No.2 engine changes into a moderate amount of forward thrust, because the hot air is still directed aft, while all the cold air is deflected more sideways and upwards/downwards than really ‘reversed’ to any great extent. This is interesting, since the very long intake could otherwise nicely enable reversing on the tail engine down to zero speed (in an emergency), which the wing engines may not do without protest because of re-ingestion of disturbed air at speeds well below 60 knots.
Note this is a phenomenon of the later operational variant with the technically awkward turbine reversers deactivated or removed: just fan air reversers operative. Also, at lower N1 speed the effect is much reduced.