Reversing on Cushioncraft CC7 - RC Groups
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Feb 07, 2017, 08:12 AM
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Reversing on Cushioncraft CC7


While drawing plans to build a Cushioncraft CC7 model, I'm trying to figure out how reversing the trust from the centrifugal fans was done on it. It has 2 fans on a horizontal axis. If seen from starboard, the fan rotates clockwise & the flow goes under it & exist at the rear. To go backwards, the flow is directed to go up around the fan from the back & exists at outlet on the top facing forward. Same fan also supplies air in to the cushion. I'm struggling to understand how the deflector in the chamber looks like & how it tilts to divert the air in either channels. All I have are many exterior photos, & low-res cutaway drawing which vaguely shows a some deflector but it's still unclear. Can someone shine a light on it or perhaps have better GA plans or can simply draw how that place looks like?
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Feb 07, 2017, 08:14 PM
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Cutaway of CC7


This Cutaway may help you out..
Feb 08, 2017, 04:13 AM
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This post has some good info in it, worth a look.

Edit: Doh! That's for the CC2, not the CC7. Details matter
Last edited by FiftySlicks; Feb 09, 2017 at 12:13 PM.
Feb 08, 2017, 04:32 AM
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That's the cutaway drawing I mentioned & it's unclear how that deflector looks like in order to divert the flow. I kept drawing it variously trying to figure out, but it still doesn't look right to me.
Feb 08, 2017, 09:31 AM
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I see the snow shovel looking flow deflector on the fwd thrust exhaust, but it isn't evident where that is diverting flow to. I feel like that's for cushion pressure? Unless there is another diverter for rwd thrust, and they move in conjunction with each other? (rwd thrust diverter is blocking rwd thrust exhaust while fwd thrust exhast is open, and vise versa)

Do let us know what you come up with, please.
Feb 09, 2017, 01:36 AM
serial scratchbuilder
Granted's Avatar
The rear flow is blocked by the buckets in front of the rudder vanes and the shutters at the fore/top of the fan housing are opened for reverse thrust.
Feb 09, 2017, 02:33 AM
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Alright & now try to figure out how the flow goes to the top & how the deflector is positioned so the flow won't go up. It's easy on words, but trying to draw it makes issues - all channels should be with smooth turns & deflector should be simple in shape & in use.
Feb 09, 2017, 06:09 AM
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So far this is what I think. There's a (green) deflector that tils around a (purple) point. In "light green" position it diverts the flow smoothly around the duct to the upper outlet. In "dark green" position it closes that upper opening in the channel, so the flow goes to the back. Some 50% or less of air is taken from the fan & it goes to cushion. It looks simple, but the whole fan duct doesn't look like how I drew in red, compared to the cutaway pic. So I don't know.
Feb 09, 2017, 08:11 AM
serial scratchbuilder
Granted's Avatar
This is what I see. I believe you're thinking about this wrong. You're not trying to maintain flow, you want to build pressure.
Feb 09, 2017, 08:15 AM
serial scratchbuilder
Granted's Avatar
If the craft is powered up and sitting on its inflated cushion, but not moving and allowing air to escape the cushion, theoretically there would be zero airflow past the fans as they maintained the pressure needed to keep the bag inflated and the craft supported.
Feb 09, 2017, 10:00 AM
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Your drawing looks quite close to the cutaway & to how I was thinking as well. But why I don't like it & kept thinking frther is:
1) During forward motion - only part of air that's between the 2 orange lines provides thrust, while more air is supplied to the cushion because of the duct shape. Some air simply goes nowhere & is trapped between the fan & duct, creating lots of vortexes while trying to escape the duct throught the bottom openings. Not optimal in my opinion, especially if you compare to how the duct looks like & how the flows goes in ordinary centrifula fans for ventilation.
2) During reverse - closed bucket makes air there trapped creating many vortexes.
Feb 09, 2017, 10:04 AM
serial scratchbuilder
Granted's Avatar
I think you are trying to fix problems that don't exist or don't matter.
Feb 09, 2017, 12:20 PM
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I had thought that about the buckets on both fwd and reverse thrust ducts, but couldn't see them in the cutaway drawings so I didn't comment on it. That's where my mind went first. I would do 2 independant servos (1 for fwd bucket and 1 for reverse bucket) that way you could hook control them independantly with a switch/curve/throttle input, etc.

I agree with Granted. The vorticies in question are only a fraction of the the power of the pressure/airflow generated by the fans. IE it just won't impact performance of the craft in a significant way, IMO. I might try to improve airflow on the 2nd generation model but I wouldn't be overly concerned on this one.
Feb 10, 2017, 04:47 AM
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Quote:
Originally Posted by Granted
I think you are trying to fix problems that don't exist or don't matter.
The thing is, the arangement should be in a way it'd be possible to use whole flow from the fan than pick only some part of it while the rest is constanlty "trapped" (or creating pressure) between the fan & duct rather than suplying it to cushion or out of the craft.
Feb 10, 2017, 01:05 PM
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With both buckets blocking thrust ports, all the airflow goes to the skirt/lift.

I'm assuming that the buckets can be partially open allowing you to control the forward (or reverse) thrust proportionally? So, opening the bucket %50 gets you %50 fwd thrust, %25 for %25, etc?

I'm not quite sure what your concern is?


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