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Post  roddie Tue Aug 27, 2013 12:36 pm

Hi all, I've often wondered why elec. powered airplane ESC's aren't offered with "reverse"...

Before you all have me committed... here's my reasoning; Ever see a full scale C130 equipped with STOL technology, reverse it's prop pitches while landing at an airshow? It's impressive. Suppose you're "landing" your plane, and need to slow down real quick; because of something in the way (another model, spectator, whatever...) pilot error, or... landing too fast or "deep" on a short runway. You may even be able to avert a mid-air... by "putting on the brakes" if your "high enough" to recover from the flat spin it would "likely" induce.

Your thoughts??? (don't worry, I can handle sarcasm) Smile 

Roger
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Post  duke.johnson Tue Aug 27, 2013 12:56 pm

They use stuff like this on the Electric 4D foamies. They can fly backwards just as easy as forwads.

Airplane ESC question... V-pitc10
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Post  roddie Tue Aug 27, 2013 1:14 pm

duke.johnson wrote:They use stuff like this on the Electric 4D foamies.  They can fly backwards just as easy as forwads.

Airplane ESC question... V-pitc10
Well "there you have it" Duke... That set-up "must" require reverse on the ESC. Looks like it works on a centrifugal force/counter-balancing for pitch-shift principal... kinda'...

I'll bet it costs more $$ than a Cox Grey 6 x 3...Smile
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Post  ian1954 Tue Aug 27, 2013 1:26 pm

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Post  duke.johnson Tue Aug 27, 2013 2:16 pm


I don't know much about them. But I believe they just use another servo to change the pitch of the prop like a RC Heli or the real life Beaver or Otter float planes in Alaska.  And I know I enjoy watching the 4D planes dance to music.  Go to YouTube and watch a couple videos.  I'm sure you'll enjoy it.
 
Sweet! Posted my first YouTube video
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Post  kevbo Tue Aug 27, 2013 2:48 pm

Edit: The info below is correct for brushed motors, which are pretty much obsolete these days. see my followup post downthread.

Well it is an electrical design issue.  A unidirectional control needs one switching device, which is typically several MOSFET transistors in parallel and a diode, or another transistor switch for improved effiency if running low voltage batteries.  A reversing esc needs four switches and four diodes. Google "H-bridge" for example circuits.  You could do it with a relay but it would need very beefy contacts.


Last edited by kevbo on Thu Aug 29, 2013 9:25 am; edited 1 time in total
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Post  batjac Tue Aug 27, 2013 3:09 pm

roddie wrote:
duke.johnson wrote:They use stuff like this on the Electric 4D foamies.  They can fly backwards just as easy as forwads.

Airplane ESC question... V-pitc10
Well "there you have it" Duke... That set-up "must" require reverse on the ESC. Looks like it works on a centrifugal force/counter-balancing for pitch-shift principal... kinda'...

I'll bet it costs more $$ than a Cox Grey 6 x 3...Smile
All it is is a model helicopter tail rotor unit. I remember a few years ago when they first started using them on the indoor foamies. You just replace the electric motor shaft with the hollow tail rotor shaft of a helicopter and run the pushrod all the way through to change the prop pitch.

The Pitchy Mark
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Post  duke.johnson Tue Aug 27, 2013 3:16 pm

Very Happy I knew one of the smart RC guys would help me out.Very Happy 
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Post  pkrankow Tue Aug 27, 2013 3:19 pm

Cars and boats use reversing ESCs.  My airplane ESCs can be set to "forward" or "reverse" in the logic, but you cannot switch during operation.

The 3 phase brushless motor, which is what most of the motors people are using are, switches direction by having the timing altered.  Alternately reversing any 2 leads reverses the direction.

So, why no airplane ESCs that have reverse?  Nobody really needs them since actual reversing engines are, as far as I am aware, unheard of in real aviation.

Thrust reversal via a number of means including variable pitch propellers is a different story.  

Now, I CAN think of some situations when having reverse thrust available via reversing the propeller rotation would be an advantage.  I know reversing the rotation of a fixed pitch prop does not impart "good" thrust but a much weaker thrust.  I don't know how it would be implemented in a "safe" manner.

Phil
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Post  roddie Tue Aug 27, 2013 6:26 pm

duke.johnson wrote:Very Happy I knew one of the smart RC guys would help me out.Very Happy 
Duke, That vid was amazing! Thanks!

After reading through these posts, and checking out Ian's link for the prop/motor combo... it became more clear how it works. Your 1st pic doesn't show a "hollow" motor shaft for the control rod to slide through.

and I don't get out much...


Last edited by roddie on Tue Aug 27, 2013 6:38 pm; edited 1 time in total (Reason for editing : add'l. comment)
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Post  roddie Tue Aug 27, 2013 7:12 pm

pkrankow wrote:Cars and boats use reversing ESCs.  My airplane ESCs can be set to "forward" or "reverse" in the logic, but you cannot switch during operation.

The 3 phase brushless motor, which is what most of the motors people are using are, switches direction by having the timing altered.  Alternately reversing any 2 leads reverses the direction.

So, why no airplane ESCs that have reverse?  Nobody really needs them since actual reversing engines are, as far as I am aware, unheard of in real aviation.

Thrust reversal via a number of means including variable pitch propellers is a different story.  

Now, I CAN think of some situations when having reverse thrust available via reversing the propeller rotation would be an advantage.  I know reversing the rotation of a fixed pitch prop does not impart "good" thrust but a much weaker thrust.  I don't know how it would be implemented in a "safe" manner.

Phil
I figured that by using an ESC with reverse... it might be useful to slow the airplane down; primarily on the "ground" after touchdown (too hot a landing speed/deep approach/crash avoidance etc.)

I guess it's something to be tested. I don't imagine that applying a "slight" (preset amt.)of reverse rotation/thrust to an airplane that is rolling forward on the ground would cause a problem... but you never know. Results could be very different with trike vs. tail-dragger gear.

Thanks for commenting
Roger
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Post  pkrankow Wed Aug 28, 2013 7:31 am

Ground braking and steering twin engine water planes are the primary thoughts on using reverse that I had too. I suspect that modern computer radios have the capability of safely controlling an ESC with reverse for these uses, but I don't have a modern computer radio, I have an older radio that is barely a computer.

Phil
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Post  roddie Wed Aug 28, 2013 10:18 am

pkrankow wrote:Ground braking and steering twin engine water planes are the primary thoughts on using reverse that I had too.  I suspect that modern computer radios have the capability of safely controlling an ESC with reverse for these uses, but I don't have a modern computer radio, I have an older radio that is barely a computer.

Phil
Hey Phil, Back in 1993; I bought a 2 ch. Futaba "Magnum Sport FP-2PB" surface radio (pistol type... still have it... still works) It was avail. with an ESC and single std. servo; which is what I have. I bought it while building a Dumas "Short Stuff" (18" built-up version V-hull) boat with Graupner motor/hdwe.

The Futaba ESC is a MC210CB with BEC. and reverse. It's very easy to set/limit the amount of reverse; with the "neutral trimmer". You naturally don't want water coming over the transom while running a-stern. I used it in several watercraft. Set-up directions illustrate both; a "trigger" and "gimbal" control. You could not go to reverse unless the system sensed a neutral position for a second or so. It worked very well. Here's someone's youtube vid...



I suspect "any" car/boat ESC w/rev.; currently on the market, could be configured for an airplane's elec. motor to produce "limited" reverse if desired. The difference would be in an "aircraft" radio's "throttle stick" design (no neutral position)... which probably "wouldn't" work with the MC210CB ESC.  

Here's a Radio question I've been wondering about... if using 2.4GHz spread spectrum systems; does the FCC still prohibit surface radios for use in aircraft... and vise versa?
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Post  pkrankow Wed Aug 28, 2013 10:32 am

I don't know about the surface vs air radio bit with modern radios. I know on the old crystal sets the frequencies were designated differently, and using for the wrong purpose could cause problems with interference if there were others around.

I wonder if a newer computer radio can have one of the switches bound to make the throttle channel go reverse of neutral or forward of neutral?

Phil
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Post  microflitedude Wed Aug 28, 2013 11:55 am

The electric variable pitch props have a push rod running through the hollowed motor shaft connected to a servo. A toggle switch on your radio actuates the servo, which pulls the nose in reversing the propeller's pitch.

Amazing stuff to see in person.
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Post  dinsdale Thu Aug 29, 2013 7:27 am

roddie wrote:Hi all, I've often wondered why elec. powered airplane ESC's aren't offered with "reverse"...

Before you all have me committed... here's my reasoning; Ever see a full scale C130 equipped with STOL technology, reverse it's prop pitches while landing at an airshow? It's impressive. Suppose you're "landing" your plane, and need to slow down real quick; because of something in the way (another model, spectator, whatever...) pilot error, or... landing too fast or "deep" on a short runway. You may even be able to avert a mid-air... by "putting on the brakes" if your "high enough" to recover from the flat spin it would "likely" induce.

Your thoughts??? (don't worry, I can handle sarcasm) Smile 

Roger
Just to be a little pedantic Rolling Eyes  C130s couldn't go into reverse whilst airborne - lock-out micros on the gear. Caribous, on the other hand, could - a standard 40 flap STOL landing called for full reverse at 15' above the ground. Man alive!! Do they ever BANG onto the ground! First time I did one I actually looked back at the gear, fully expecting the struts to be poking out the top of the wings.
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Post  kevbo Thu Aug 29, 2013 9:20 am

It is absolutely correct that a modern ESC for brushless motor would need minimal logic changes to reverse. My previous comment was based on the brushed motors I used 20 odd years ago when I messed with electrics.

It should be possible to swap two motor connections with a DPDT relay controlled by another channel, or have a servo flip a DPDT switch. It will need contacts rated for the full load current of the motor. Such are not small, light, or cheap.
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Post  roddie Sun Sep 01, 2013 7:24 pm

kevbo wrote:It is absolutely correct that a modern ESC for brushless motor would need minimal logic changes to reverse.  My previous comment was based on the brushed motors I used 20 odd years ago when I messed with electrics.

It should be possible to swap two motor connections with a DPDT relay controlled by another channel, or have a servo flip a DPDT switch.  It will need contacts rated for the full load current of the motor.  Such are not small, light, or cheap.
Thanks for clarifying that Kev, I am learning a lot, since joining this forum. You people are all very friendly and eager to help and answer questions; which is of great value to me.

Roger
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Post  WingingIt74 Sun Sep 01, 2013 8:46 pm

You can reverse a brushless motor by swapping any two motor leads.
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Post  roddie Sun Sep 01, 2013 8:56 pm

WingingIt74 wrote:You can reverse a brushless motor by swapping any two motor leads.
What I wanted to "try"... can't be done in a "practical" way; using an aircraft radio; as there is no provision to set a "neutral" position on the throttle channel; like there is with a surface radio.

The "reversible pitch" prop would be the way to go.
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