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Reverse Thrust Braking?

Old 06-12-2018, 10:06 AM
  #1  
Suprawill1
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Default Reverse Thrust Braking?

Has anyone done this with an EDF? Wouldn't it be more practical than mechanical brakes? It would seem pretty easy to electronically reverse the motor wires to create this. I know the tubing isn't efficiently set up for reverse flow but it doesn't have to be just to create a reverse thrust.
I'm thinking that it could be set up to activate at closed throttle and add/subtract throttle trim to initiate. That way it won't be inadvertently activated while the EDF is in forward motion.
Your thoughts?

Will
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Old 06-12-2018, 12:37 PM
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solentlife
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Originally Posted by Suprawill1 View Post
Has anyone done this with an EDF? Wouldn't it be more practical than mechanical brakes? It would seem pretty easy to electronically reverse the motor wires to create this. I know the tubing isn't efficiently set up for reverse flow but it doesn't have to be just to create a reverse thrust.
I'm thinking that it could be set up to activate at closed throttle and add/subtract throttle trim to initiate. That way it won't be inadvertently activated while the EDF is in forward motion.
Your thoughts?

Will
Reversing the engine is not a good idea because if the rotor is not stopped when reverse power is supplied - your ESC will not appreciate the 'jolt' !! Brushless motors are like alternator when revolving with power applied ...

On youtube is a video of a guy who created 'buckets' like the real airliners on his EDF's ...



Various are trying the reverse engine trick - but without a decent air cooled boat / car ESC - you'd be struggling.
The car / boat ESC usually has a double tap reverse on the throttle ... it does not immediately engage reverse power. You need to pull back once - this produces BRAKE ... then release and pull back again ... this then applies limited power. This dual action basically designed to stop ESC being blown and also stopping inadvertent reverse action.

Nigel
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Old 06-12-2018, 01:42 PM
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JetPlaneFlyer
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There are a few aerobatic 3D drones use prop reversing to create reverse thrust. This is done in the ESC, not by swapping any wires.

At least for smaller motors this seems to work without any issue. I dare say larger motors with more spinning momentum in the prop/impeller might be a bit more of a challenge, but this should simply be a matter of timing the reversal to prevent a sudden 'jolt'

Last edited by JetPlaneFlyer; 06-12-2018 at 02:25 PM.
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Old 06-12-2018, 06:30 PM
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Not seen that one...but a lot of 3D planes I see "standing" on the nose utilize a pitch controlled prop...
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Old 06-13-2018, 01:05 AM
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quorneng
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note that in some out runner EDF motors the bell is only held in the correct position by the magnetic pull and the fab thrust so reversing the fan under any sort of power could cause something to rub.
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Old 06-13-2018, 03:04 AM
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solentlife
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Originally Posted by quorneng View Post
note that in some out runner EDF motors the bell is only held in the correct position by the magnetic pull and the fab thrust so reversing the fan under any sort of power could cause something to rub.
True ... all my EDF's in fact are like that ... the bell is only designed to run one way .. if it was reversed - the rotor would be pushing the bell of.

Nigel
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Old 06-13-2018, 04:42 AM
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dereckbc
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Variable Pitch with Reverse is entirely possible in both real and model aircraft. I do not see anyway possible to reverse a duct fan fan motor. It cannot be done with variable pitch that I know of, and reversing the motor is not likely due to the high RPM's required from Duct Fans. The mechanical and electrical loads would just be way to high not to mention the time it would take to go from spool down, reverse, then spool up in reverse.


Secondly a duct fan to be effective needs a large open input and tapered down on the output like a cone with the fan integrated into the fuselage. You cannot reverse that.



However with a duct fan you can vector thrust pretty easily within limits. Reverse I do not see happening.
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Old 06-13-2018, 05:53 AM
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solentlife
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Originally Posted by dereckbc View Post
Variable Pitch with Reverse is entirely possible in both real and model aircraft. I do not see anyway possible to reverse a duct fan fan motor. It cannot be done with variable pitch that I know of, and reversing the motor is not likely due to the high RPM's required from Duct Fans. The mechanical and electrical loads would just be way to high not to mention the time it would take to go from spool down, reverse, then spool up in reverse.


Secondly a duct fan to be effective needs a large open input and tapered down on the output like a cone with the fan integrated into the fuselage. You cannot reverse that.



However with a duct fan you can vector thrust pretty easily within limits. Reverse I do not see happening.
Hi Derek .. people do it though ... I have been watching youtube videos of EDF's reversed. The only thing I notice though is very few are followed up by flight tests. Most are bench runs. This then doesn't suffer the airflow induced rotor spin that would occur in flight when throttle closed.

One guy shows his model with reversed engine - but admits later that the ESC burned out. I reckon because of the brushless motor still spinning when reversed power applied.

The most successful I reckon was the MD80 model where the guy replicates the full size thrust director 'buckets' ... he actually can drive the model in reverse on the ground.



Interesting concept.

Nigel
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Old 06-13-2018, 07:22 AM
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Suprawill1
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Thanks for your thoughts.
My outlook is that after the EDF aircraft has landed, the throttle stick is usually in an idle or stop position. At that point, an electronic reversal would be engaged by a separate transmitter function ie a throttle trim or an aux switch. The throttle would then be engaged after forward thrust has ceased, eliminating a sudden reversal jolt.
At that point, I wouldn't be too concerned about thrust efficiency because it doesn't have to be as critical as forward thrust just for braking. That of course is contingent on the duct work of that particular aircraft.
When coming to an end of a runway, any help would be appreciated if mechanical brakes weren't available.
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Old 06-13-2018, 07:36 PM
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dereckbc
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Originally Posted by solentlife View Post
One guy shows his model with reversed engine - but admits later that the ESC burned out. I reckon because of the brushless motor still spinning when reversed power applied.
Perhaps I phrased it poorly. Is it possible to reverse a Fan? Yes it is, but not practical. As the OP admitted burned up his controller. Now for show on ground operations it could be done to say make the plane back up simply because you have the element of time to let fan come to a stop or near stop before reversing in the other direction.

Perhaps a better idea for an air brake is what real planes use like Spoilers, Slats, and Flaps. As myself and many have learned the hard way, the easiest way to bur up a controller or motor is to use to big of a prop, or easier yet jam the prop so it cannot turn, apply power, and watch the magic smoke appear.
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Old 06-13-2018, 07:45 PM
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Originally Posted by solentlife View Post
One guy shows his model with reversed engine - but admits later that the ESC burned out. I reckon because of the brushless motor still spinning when reversed power applied.

Perhaps I phrased it wrong. Is it possible to reverse a Fan? Yes it is, but not practical. As the OP admitted burned up his controller. Now for show on ground operations it could be done to say make the plane back up simply because you have the element of time to let fan come to a stop or near stop before reversing in the other direction.



Perhaps a better idea for an air brake is what real planes use like Spoilers, Slats, and Flaps.



As myself and many have learned the hard way, the easiest way to bur up a controller or motor is to use to big of a prop, or easier yet jam the prop so it cannot turn, apply power, and watch the magic smoke appear.
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Old 06-13-2018, 08:27 PM
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Originally Posted by solentlife View Post
The car / boat ESC usually has a double tap reverse on the throttle ... it does not immediately engage reverse power. You need to pull back once - this produces BRAKE ... then release and pull back again ... this then applies limited power. This dual action basically designed to stop ESC being blown and also stopping inadvertent reverse action.

Nigel

That seems to be a most practical application.
My ESC applies brake when fully throttled back. The blade stops immediately. I would then program my transmitter to apply reverse polarity only in this braked position. I may be able to get this done without having to purchase a new ESC. (for boat/car)
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Old 06-13-2018, 11:36 PM
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Best to redirect the discharge, as Nigel suggests.
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Old 06-14-2018, 01:50 AM
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Suprawill1
If as you suggest you apply reverse thrust only after the plane has touched down (like full size!) and the fan has been braked to a halt then provided the EDF unit is happy running backwards there will be no problem although the fan efficiency will be very poor and so will be the actual reverse thrust created.
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Old 06-14-2018, 02:18 AM
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Would reversing the airfoil possible cause over revving the esc? How do escs react to maxing the rpms? I know when I reverse the motors in my quads to flip after crash, them props really spin fast! Then again those escs are flashed with specific firmware (BLHeli) to perform that.

Also, as JetPlaneFLyer mentioned above, some quad software/escs are designed to reverse for 3D maneuvers. Never tried that myself.
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Old 06-14-2018, 02:21 AM
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Originally Posted by Suprawill1 View Post
Thanks for your thoughts.
My outlook is that after the EDF aircraft has landed, the throttle stick is usually in an idle or stop position. At that point, an electronic reversal would be engaged by a separate transmitter function ie a throttle trim or an aux switch. The throttle would then be engaged after forward thrust has ceased, eliminating a sudden reversal jolt.
At that point, I wouldn't be too concerned about thrust efficiency because it doesn't have to be as critical as forward thrust just for braking. That of course is contingent on the duct work of that particular aircraft.
When coming to an end of a runway, any help would be appreciated if mechanical brakes weren't available.

I had a special ESC made for seaplane use. that had a reverse function. You had to completely stop the motor then flip a 2 position switch to change.


Please note, I said I "HAD" the ESC.
The damn thing reversed itself in midair and exploded, LITERALLY !


I will NOT try that again and strongly advise against it.
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Old 06-14-2018, 05:58 AM
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solentlife
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Originally Posted by quorneng View Post
Suprawill1
If as you suggest you apply reverse thrust only after the plane has touched down (like full size!) and the fan has been braked to a halt then provided the EDF unit is happy running backwards there will be no problem although the fan efficiency will be very poor and so will be the actual reverse thrust created.
Well - in full size - the thrust in reverse is a lot less than forward thrust. Its only there to provide braking.
The MD80 series of aircraft were unusual in that they had a reasonable amount of thrust and could actually drive themselves backwards at quite a rate. It was a trick they had for some of the 'lesser' bus-stop airfields.
Most aircraft could move backwards from standing start with the reversed thrust - but only at snails pace. It was also discouraged.

Nigel
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Old 06-14-2018, 02:39 PM
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quorneng
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With all the ESCs I have used they are quite happy to run the motor without any load at all which results in the 'no load' current.
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