Thrust Reversers
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Just an aerodynamic thought from a guy who has never had reversers on his jet:
Doesn't the efflux from the deployed reversers act as a dam to block a lot of the airflow around the the engine pods and create a major change in the flow field around the aircraft? In essence, not only do you have some reverse thrust, you also have much more drag from an effectively larger, less aerodynamic aircraft. For that reason the reversers will be more effective at higher speeds. The greater the efflux from the engine, the larger the aerodynamic dam being formed around the engine and the greater the aerodynamic drag.
Doesn't the efflux from the deployed reversers act as a dam to block a lot of the airflow around the the engine pods and create a major change in the flow field around the aircraft? In essence, not only do you have some reverse thrust, you also have much more drag from an effectively larger, less aerodynamic aircraft. For that reason the reversers will be more effective at higher speeds. The greater the efflux from the engine, the larger the aerodynamic dam being formed around the engine and the greater the aerodynamic drag.
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.
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.
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Machinbird & Plumb Bob,
Good posts!
To add some information, when the outer shroud translates aft, blocker doors close off downstream flow path and cascade boxes (we called them egg crates) are exposed through which the fan flow exits in a forward direction. The smooth annular surface over which the flow turns outward into the cascades is called the Dagmar. Therefore, the fan flow first turned radially outward where it approaches the cascades. The cascade boxes are comprised of turbine-stator turning vanes which accelerate the flow to the minimum section at the exit. Most of the cascade boxes have radial outward exhaust in a direction 40 to 50 degrees axial. Some boxes provide other than radial outflow by skewing of the turning vanes or by virtue of axial vanes in an arrangement that resembles an egg crate.
Each cascade is special as to location, whether it is to be used on a lefthand or righthand engine, or a center engine as on a DC-10 or MD-11. It requires close teamwork with the aircraft manufacturer to establish the proper parameters of the cascade design in relation to the surrounding features.
Reverse thrust of the engine system alone results from three sources. The unaffected core engine thrust is more than overcome by the fan flow exhausting forward and by the fan and core flow ram drag. The ram drag is the largest of of the forces.
Reversers deployed after touchdown are generally used to speeds just above the speed at which reverse flow is re-ingested or cross ingested so as to affect engine operation. The range of speed is generally 120 knots down to 60 knots. In this speed regime, the aircraft drag is a large retarding force. However the reverse flow shrouds a part of the aircraft and changes the aircraft drag. This is why close coupling between the aircraft manufacturer and the thrust reversal system manufacturer is very important to gain the most efficient overall reversal system.
TD
Good posts!
To add some information, when the outer shroud translates aft, blocker doors close off downstream flow path and cascade boxes (we called them egg crates) are exposed through which the fan flow exits in a forward direction. The smooth annular surface over which the flow turns outward into the cascades is called the Dagmar. Therefore, the fan flow first turned radially outward where it approaches the cascades. The cascade boxes are comprised of turbine-stator turning vanes which accelerate the flow to the minimum section at the exit. Most of the cascade boxes have radial outward exhaust in a direction 40 to 50 degrees axial. Some boxes provide other than radial outflow by skewing of the turning vanes or by virtue of axial vanes in an arrangement that resembles an egg crate.
Each cascade is special as to location, whether it is to be used on a lefthand or righthand engine, or a center engine as on a DC-10 or MD-11. It requires close teamwork with the aircraft manufacturer to establish the proper parameters of the cascade design in relation to the surrounding features.
Reverse thrust of the engine system alone results from three sources. The unaffected core engine thrust is more than overcome by the fan flow exhausting forward and by the fan and core flow ram drag. The ram drag is the largest of of the forces.
Reversers deployed after touchdown are generally used to speeds just above the speed at which reverse flow is re-ingested or cross ingested so as to affect engine operation. The range of speed is generally 120 knots down to 60 knots. In this speed regime, the aircraft drag is a large retarding force. However the reverse flow shrouds a part of the aircraft and changes the aircraft drag. This is why close coupling between the aircraft manufacturer and the thrust reversal system manufacturer is very important to gain the most efficient overall reversal system.
TD
An aircraft can be stopped with reversers alone
The reason being if you cancel the reverse quickly when stopped, forward thrust caused by the engines winding down (no brakes, remember) can cause you to start moving again.
The reason being if you cancel the reverse quickly when stopped, forward thrust caused by the engines winding down (no brakes, remember) can cause you to start moving again.
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Plumbob Machinbird
I'm having a problem understanding the mechanism which would make your air-dam theory slow the aircraft.
Any force acting on the air blown forward would blow the air backward. How would this be transmitted to the airframe?
pattern-is-full
Newton is turning in his grave.....
If you could increase the thrust of an engine by fitting it backwards and then bouncing it off something then fighters would have done it long ago.
I'm having a problem understanding the mechanism which would make your air-dam theory slow the aircraft.
Any force acting on the air blown forward would blow the air backward. How would this be transmitted to the airframe?
pattern-is-full
Newton is turning in his grave.....
If you could increase the thrust of an engine by fitting it backwards and then bouncing it off something then fighters would have done it long ago.
the exhaust was also pushing backwards on the clamshell as it was redirected, thus pushing backwards on the whole airplane (by way of the nuts and bolts that held the clamshell to the airframe).
I would like to see the internal thrust balance proof of that or even some sort of real life test in an out-of-the-way run-up stand.
Actually, a fan blowing air at a sailboat sail would cause the boat to move. They say a sailboat on a close reach "makes its own wind" -- with a fan toward the bow, it would literally be true.
And if you put the fan at the stern blowing forward, the system would act like a jet engine with a thrust reverser (only facing the other way, of course). At least I think the curvature of the sail would deflect enough flow aft to generate net forward thrust.
And if you put the fan at the stern blowing forward, the system would act like a jet engine with a thrust reverser (only facing the other way, of course). At least I think the curvature of the sail would deflect enough flow aft to generate net forward thrust.
The boats don't 'make their own wind', so much as the relative wind vector increases as the boat speed increases allowing the boat to accelerate until drag stabilises the speed.
And as you've no doubt spotted, in your case the 'wind' is generated by accelerating a mass of air by a fan mounted on the boat so you wouldn't have an increase in the relative wind. Nice try though.
And as you've no doubt spotted, in your case the 'wind' is generated by accelerating a mass of air by a fan mounted on the boat so you wouldn't have an increase in the relative wind. Nice try though.
Journey Man --
I was being a little cute -- by "make its own wind" in this context, I just meant that the fan would in fact fill the sail and cause the boat to move. But once the boat started moving, the fan would be accelerating air that was already moving aft relative to the sail. So I think the apparent wind velocity at the sail would increase as the boat started moving faster.
As you point out, of course, the additional thrust as boat velocity increases would be offset by the increased drag through the water, as well as by the fact that the apparent wind direction would move further toward the bow of the boat.
Needless to say, all this applies only if the fan is facing somewhat aft to begin with -- and it might well be more efficient in the end to lower the sail and just have a fan boat.
I was being a little cute -- by "make its own wind" in this context, I just meant that the fan would in fact fill the sail and cause the boat to move. But once the boat started moving, the fan would be accelerating air that was already moving aft relative to the sail. So I think the apparent wind velocity at the sail would increase as the boat started moving faster.
As you point out, of course, the additional thrust as boat velocity increases would be offset by the increased drag through the water, as well as by the fact that the apparent wind direction would move further toward the bow of the boat.
Needless to say, all this applies only if the fan is facing somewhat aft to begin with -- and it might well be more efficient in the end to lower the sail and just have a fan boat.
I was being a little cute -- by "make its own wind" in this context, I just meant that the fan would in fact fill the sail and cause the boat to move. But once the boat started moving, the fan would be accelerating air that was already moving aft relative to the sail. So I think the apparent wind velocity at the sail would increase as the boat started moving faster.
This is all scaring me. Please tell me none of you guys are engineers?
What about the effect of reversers on braking performance?
Presumably any air dam effect would disrupt flow over the wings, tending to destroy residual lift and putting more weight on the wheels. And to the extent the reversers deflect engine efflux upward, this would create downward thrust, also increasing the effective weight on the wheels.
Presumably any air dam effect would disrupt flow over the wings, tending to destroy residual lift and putting more weight on the wheels. And to the extent the reversers deflect engine efflux upward, this would create downward thrust, also increasing the effective weight on the wheels.
