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Reverse thrust

Started by Hardy Heinlin, Sat, 9 Apr 2011 15:50

Hardy Heinlin

Moin everyone,

two thoughts:

Does anybody have USB hardware with dedicated reverser levers? I mean separate sliders for forward thrust and separate sliders for reverse. I wonder if standard USB reverser levers, when at the idle stop, give at the USB output the maximum value. Using an USB slider as brake pedals, for instance, the slider gives the max value when its at the aft stop (where the user sits), i.e. actual USB brake pedal boxes have the pot installed so that fully forward pedal motion moves the slider fully backwards. Perhaps standard USB thrust reverser levers use a similar method? I expect it does not. I.e. slider fully back is max reverse thrust, not idle. I guess there's a basic USB design principle: Slider forward means vehicle gets faster. That would also agree with the brake pedal sliders.

If there's no standard, I need to add an extra checkbox on the Preferences page.

Here's just a thought experiment, because it cannot be tested in real life: Imagine a 744 parking on the NASA shuttle runway (very large). Set 90% N1 and enjoy the aircraft reaching 200 KIAS. Stop at the other end of the runway. Set 90% N1 REVERSE thrust. What IAS might the aircraft reach?

We know:
- The reverse thrust vector coming out of the reverser sleeves is not turned by 180° relative to the forward vector. It goes more sideways. Perhaps 135°, i.e. 45° offset to the heading.
- Inlet pressure decreases with increasing backward airspeed.

So, it will certainly not reach the same IAS as with forward thrust. But how much then? The half of it maybe. That would be ca. 100 KIAS. Just an approximation, of course.



Jeroen Hoppenbrouwers

Little bit of possibly useful info:

PPrune thread at :

"In these "high bypass" engines about 75% of the total thrust is from the large, low-pressure fan at the front of the engine. Because only the fan air is reversed, the high-pressure exhaust is always directed aft. Net result is about 40% of Takeoff thrust for full reverse."

"When standing still the maximum reverse thrust can be as low as 5 to 10% of the maximum forward thrust. At 150 knots it can be 20 to 30%, but that number decreases rapidly as the aircraft slows down."

Hardy Heinlin

40% comes close to my guessed 50% (at least in terms of force vector math).

But then the quote says 20 to 30% at 150 knots. Sounds like those 40% refer to a speed above 150 knots.

Edit: Anyway, with a negative airspeed the reverse net thrust most likely tends to zero. I guess, the limit is indeed rather in the range of -10 to -20 knots. Like in PS1.


What a great discussion.  Can I just throw out there: what's the maximum time a 747 can fly inverted before some kind of mechanical (not human) failure occurs?  By mechanical failure, I mean "failure of the mechanical systems to operate in the way that they are intended," meaning that disruptions in fuel flow (etc.) would count, but disruptions in food service would not.
Will /Chicago /USA

Hardy Heinlin

Good question, Will.

Re thrust reversers again: Just learned something new. The reversed exhaust gas generates an almost irrelevant force. (I have always wondered anyway how this thrust vector that goes nearly sideways could have such a big effect, compared to the remaining forward components). Now it's clear: In reverse configuration with high RPM, the braking force comes from the intake drag. When windmilling, the drag is low, when powered up, the drag is high.


Jeroen Hoppenbrouwers

It remains counter-intuitive to grasp, this inlet drag concept. When the engine is in reverse and at full (reverse) thrust setting, it swallows a couple of tons of air per second from the front. Although the exhaust may be going in any random direction and is not generating a useful thrust vector, the swallowing of tons of air still happens largely from the front. I understand that compressing air, therefore slowing it down, can be compared to having a huge drag chute in the engine. But would a real drag chute work better when you put a big fan in front of it, sucking air into the chute? Probably not, as this is exactly why having an onboard fan on a sailing boat does not buy you much. So this comparison likely isn't anywhere near reality.

Probably I'm being sent onto the garden path by my misperception of the magnitude of the swallowing thrust vector. But I know that static jet engines on high thrust settings do pull large objects in. Which means they also must generate forward-pulling 'thrust'.

May it be that the trick is that a static jet engine in reverse thrust configuration indeed tries to pull itself forward (and at the same time pushes itself backward given the reversers), but that with increasing forward speed it very quickly becomes a powered drag chute instead?


Hardy Heinlin

I feel what you feel regarding the "pulling" concept. But it's easy to drop this concept: Just imagine where this pulled air comes from. It does not only come from 12 o'clock. It comes from a sector between 9 and 3 o'clock. Not a straight air tube, but a whole air hemisphere in front of the intake. That is, also air from the side is sucked in. So, there are "pull" vectors also from the side. E.g. think of a microphone with a cardioid directionality, the membrane detects also pressure waves coming from the side, just not from behind. I think, the greater the angle range at the engines intake, the smaller the pulling force. With a complete sphere, the force would be zero.

I guess the compressed air at the intake functions like a "wall of air". As the wall gets denser, less air can flow through it. In forward thrust configuration, the drag may be just as high, but the exhaust vector is then a lot greater and compensates this drag, resulting in a positive net thrust, even though it's small compared to the overall thrust.





This insight, that braking force is not genererated by reversal of the thrust, is so great. I have wondered for years, why thrust reverse makes so much noise without any noticable effect on braking...  :lol:



i wonder if the older clamshells/bucket type reverser did more.... probably not..



Phil Bunch

I am also having trouble visualizing thrust reversing.  Best I can do is to imagine a large diameter disk where each engine is located.  I guess that's a lot of square feet of drag if it is mostly just a non-momentum-transferring air sucking gadget!

Almost as bad as understanding lift without primarily depending on the Bernoulli effect!
Best wishes,

Phil Bunch

Hardy Heinlin

Am I right if I think there are no USB thrust reverser levers at all on the market? Just buttons, forward throttles, sometimes flaps, speedbrakes, and for the props throttle/pitch/mixture combos?



Pierre Theillere

Hi Hardy!

I'm not really sure how this one handles thrust reverse levers:
It's probably the dream for every future PSx user...
For now, I'm sticking with my SafeLine Jet console (with 4 independant throttles, and only 2 reverse levers: Torrence also owns same hardware) that is plugged into a serial port (via USB -> RS232 converter) that emulates keypresses. Reversers can only send 2 positions: "reverse idle", and "max reverse". But it was OK with PS13.
I may investigate a bit further regarding the price and axis specifications for that FlightDeckSolution hardware... that's likely to enable a Windows-less (only Linux and OSX) flight deck, if their hardware can be recognized a multi-axis USB joystick!
Pierre, LFPG


Michel Vandaele

The normal joystick and trottle providers certainly don't have it. I'm wondering if the beautiful B744 trottle unit setup  ( )  is not providing this.
Maybe a good idea to contact Peter once regarding your question.
Many greetings
Board member  FSCB
EBOS Scenery Designteam
My B744 project

Jeroen Hoppenbrouwers

Quote from: skinnot sure how this is connected ...
I know that one by heart, I wrote the PS1 drivers for it. The reverse thrust levers are plain switches.



Hi guys,

I have the motorized version of the FDS B747 throttles mentioned above.  The reverse levers are via slider pots just like the throttles.  Fully functional, even the stab trim indiators.




C 17 going backwards with reversers.... how does that work?



Blake H

Hi Guys,

To give you an idea how my reverse will work is 1 pot 1/5 is reverse the other 4/5 is 0% to 100% travel AFT to FWD of the GO throttle levers. :) At flight idle (throttle AFT) the pot is at 60 degrees the reverse will lever will pull the pot to 0 degrees Max reverse. The reverse is stowed back to 60 degrees the throttle is moved FWD from full AFT the solenoid lock the reverse in, the pot goes to a max of 300 degrees.

Other solution is 2 pots one for reverse one for GO throttle :) with a switch in the AFT position that switches from reverse to GO throttle  :). 60 degree cross over with one axis.

Or 2 axis with GO throttle :) and reverse throttle lever.

Hardy Heinlin

Hi Blake,

I think we have two levels of precision:

1. USB stuff from the computer shop

2. Original Boeing throttles from the desert, or similar high tech pot/sensor stuff.


For option 1: The resolution is rather low, I think it's only 255 units per pot. USB sets the limit here. If USB is used for a combined forward and reverse lever all-in-one pot, then even fewer units are left for reverse and forward thrust control. I recommend to use two separete pots if USB is used for the PSX/hardware interface.

For option 2: For such a precise control, the PSX/hardware interface should use TCP/IP. For this option PSX uses an internal throttle range of -8925 to +5000 units. That is a resolution of 0.01°. In this case, no problem to put everything on one pot.



Jeroen Hoppenbrouwers

Additional info: for nearly everything with the possible exception of the flight yoke and rudder pedals, I would gravitate towards using dedicated software in between the hardware and PSX. Although PSX itself can take a lot of input devices, there will always be a need for extra software, and it is technically much easier to distribute the workload over multiple machines.

Hardy's argument about the USB pot resolution may be relevant. But if you use dedicated hardware (not the game joystick interfaces) you could increase the pot resolution to more than 8 bit. Still, 60% of throw of a normal pot isn't likely to be accurate enough unless you deploy a high-resolution long-throw pot like those used in professional audio mixing consoles. Pots are mechanical -- they produce jitter and noise. Real hardware in aircraft does not often use pots.

This is a pot (which needs an interface board, see the other links) that gets converted to 1000 units, already 4 times better than a typical USB joystick. But I have my doubts about such a small pot being stable enough. Anybody has experience?