[Alfredrogers Posts: 254 Status: Offline]

Hi Ian, I am sorry I have no photographs of the holes I cut in the bottom of my hulls directly under the lift fans.      I expected an increased flow of air to the segments (because of less resistance) dumping more air into the lower bag compartments from the plenum(at design speed ).   My results were as above but I wonder ,having read your article why the results were different especially when you mentioned that the plenum  holes were not absolutely necessary for lift, as pressure wastage is high.     Apologies if not understood.   Kind regards Alf                          PS      I keep thinking of the two cans, scales and hairdryer as originally conceived

[Kip]

Mike/ Chris,
I really do believe you are overthinking the problem.
The only problem is where did you put the jigsaw.
Kip

[Ian Brooks Posts: 3552 Status: Offline]

On Susurrus lift fans will always be operating at different speeds as each fan is paired with its respective thrust fan so the worst case is probably what is happening. Whilst I understand Kip's theory of cutting the sides to allow more air in I suspect we need to address the mismatched running of the fans first to increase the efficiency of the subordinate fan.

Given that the fans are identical, As long as you control the throttles so that the engine speeds are within a couple of hundred rpm of each other they should be sufficiently matched.

[MikeS Posts: 194 Status: Offline]

"If the two fans are not identical or run at different speeds, they will not achieve first lift at the same revs. In this case, the one that achieves first lift first will alway dominate, and the subordinate fan will always work at low efficiency. At the worst case, the subordinate fan may actually reverse flow. Its not that they are fighting each other as such, its just that they are not matched. "

On Susurrus lift fans will always be operating at different speeds as each fan is paired with its respective thrust fan so the worst case is probably what is happening. Whilst I understand Kip's theory of cutting the sides to allow more air in I suspect we need to address the mismatched running of the fans first to increase the efficiency of the subordinate fan.

[Ian Brooks Posts: 3552 Status: Offline]

Ian, I have two different sized hovercraft with variable speedmotors.   To improve the airflow and possibly the lift I drilled a 3 inch hole  through the plenum in each craft.  It improved the lift in the larger craft but reduced it in the smaller craft.   This was a bridge too far for myself so I re-fill the holes.   On reflection ,I think that the smaller one is matched very well with the motor the bigger one perhaps not.   Your thoughts would be appreciated.         kind regards Alf

Have you any photographs? I'm not sure I understand the description at the moment.
Ian

[Philip Posts: 2436 Status: Offline 12" above the Earth's surface WWW]

        just a  good point I lick to say now I run the lift engine at  2800 to 2900 rpm instead of 3400 .

That craft looked very good on the Medway, seemed to sit nice and level. It also was one of the quiter craft.

[tomtilley Posts: 568 Status: Offline]

                                                            Hi on my ts3 I have cut  2 .holes in the plaining service directly under the tip sbeed of lift fan the holes  are 4in x 6 inch and then fitted a very loosely  sheet of canvas over the hole so the air can get up and down on the front plaining service and still have all the feed holes to each segment  I have tested this mod out on the Medway  50th  last  year it work a treat  you may not do this mod on a dry craft  mine is wet .    Tom

        just a  good point I lick to say now I run the lift engine at  2800 to 2900 rpm instead of 3400 .

[tomtilley Posts: 568 Status: Offline]

Our other craft... will be segmented with the cusion predominantly directly fed through a large hole in the bottom of the hull. Additionally we'll still have air feed holes but they're only there to initially inflate the skirt to help get the craft out of boat mode and over hump. Hope it'll work...

                                                            Hi on my ts3 I have cut  2 .holes in the plaining service directly under the tip sbeed of lift fan the holes  are 4in x 6 inch and then fitted a very loosely  sheet of canvas over the hole so the air can get up and down on the front plaining service and still have all the feed holes to each segment  I have tested this mod out on the Medway  50th  last  year it work a treat  you may not do this mod on a dry craft  mine is wet .    Tom

[Lindsay Posts: 150 Status: Offline]

Twin thrust fans versus one thrust fan - The part of the fan that does the work is the fast part, say the outer third - this gives a better working area bias towards the twins than just disc area? - Racing craft used to favour twins but now favour singles (interestingly a few contra-rotating) - Big craft, commercial, military favour twins - Twins using separate engines give a useful differential thrust - Twins look cool? - Singles lighter and simpler  - Singles usually have fan tips further inboard - In the end you can justify which one suits the overall design best (or your mood at the time of commitment?). 

[rossfloyd Posts: 2025 Status: Offline]

My first craft had a segment skirt but open hull and worked fine

[Alfredrogers Posts: 254 Status: Offline]

Ian, I have two different sized hovercraft with variable speedmotors.   To improve the airflow and possibly the lift I drilled a 3 inch hole  through the plenum in each craft.  It improved the lift in the larger craft but reduced it in the smaller craft.   This was a bridge too far for myself so I re-fill the holes.   On reflection ,I think that the smaller one is matched very well with the motor the bigger one perhaps not.   Your thoughts would be appreciated.         kind regards Alf

[Chris Searle Posts: 124 Status: Offline]

Our other craft... will be segmented with the cusion predominantly directly fed through a large hole in the bottom of the hull. Additionally we'll still have air feed holes but they're only there to initially inflate the skirt to help get the craft out of boat mode and over hump. Hope it'll work...

[Warby Posts: 1043 Status: Offline]

So why oh why has no-one tried this? (Or have they) All those carefully cut holes around the periphery, potentially weakening the hull are a complete waste of time!! So now if the feed holes are eliminated, a way to introduce air in front of the anti-plough curtain/flap must be worked out? Another benefit with no feed holes equals masses of buoyancy just where needed!! It now seems obvious why opening up feed holes makes for a more efficient lift system!
I just wish I had a workshop to try out all these different ideas!! Will have to move house!

[Ian Brooks Posts: 3552 Status: Offline]

Ian, from your last thread are you saying that with a segmented skirt feed holes are not required? and the air simply fed into the underside of the craft

Yep!

They are there in the belief that an air jet is formed, that follows the profile of the segment to the ground contact point where it forms an air curtain to enhance the tip seal, like the early air curtain craft - this was the West single wall theory. Unfortunately the resulting turbulent jet has a maximum length of less than 5  diameters in free air and much less when attached to the segment wall. By the time it reaches the air gap it has spread and slowed considerably eliminating any air curtain effect it may otherwise have had.

[Ian Brooks Posts: 3552 Status: Offline]

While we are discussing fans, is there any gain thrust-wise having twin fans as apposed to a single fan. ie: two having an equivalent disc area to one larger fan. I know it will add weight and complexity but my thoughts are to reduce the 'Air draft' and fan tip speeds.

Neglecting weight and complexity, yes it's likely that a twin fan arrangement could provide a given fan area for a reduced tip speed, and that in turn may give less noise. However, that would need to be offset against the 3dB noise increase of having two similar sound sources. The air draft would of course be reduced.