[Warby Posts: 1043 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

[Warby Posts: 1043 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.

[Ian Brooks Posts: 3551 Status: Offline]

Ian, ref section on where to place the lift fan, and where to introduce the air. Many years ago (about 40!!!!) I recall it being said the best place was at the front. The reasoning was- If the HC was being driven into a "Short steep sea" there was a possibility that a wave passing under the HC could contact the hull and block off air to the front of the cushion as it moves from front to back, reducing lift at the front. I could see this as a possibility with a Sev where the air is introduced at the rear but not with a segmented skirt where you have feed holes at the front. Just a thought!!

This is just one of the myths that have persisted for decades and held back the development of light hovercraft! There are a number of 'almost plausible' reasons which are often cited for delivering air to the front of the cushion, non of which stand up to close examinatiion. Quite apart from anything else, we have a decade of experience with Sevs that tell us that the postulated issue does not happen! The Severn produces many sets of sharp short wavelength standing waves that are the ultimate short steep sea but even these don't promote the postulated effect.

It seems likely that these myths are rooted in a lack of understanding of the plough-in mechanism, leading to numerous theories of what happens during plough-in and several ultimately unsuccessful attempts at solutions. Fortunately plough in is now well understood in terms of pitch stability with proven solutions for most craft

As an aside, the rationale for the plenum and feed holes of most segment skirted craft appears to be from something called the 'AA West Single Wall Theory' from the late 60's, a notion that is quickly debunked by anyone familiar with submerged jet theory. Basically, you just don't need the plenum or the feedholes and yet they persist to this day!

[Lindsay Posts: 150 Status: Offline]

'Like' for Tech Talk 5: Lift Fans (Ian must be related to Adrian Newey and professor Brian Cox!)

[John Robertson Posts: 5136 Status: Offline WWW]

Tech name is "wave pumping".  I've never experienced it.  If the cushion system is designed properly (i.e with enough margin to keep the hull out of the water) then it simply shouldn't happen.

What happens is the bow skirt impacts the wave front and the momentary increases in cushion pressure instantly pushes the nose up thus maintaining the hover-height (a high bow is essential in preventing the wave front overcoming the skirt and landing on top of the hull - dumping a ton of water onto the nose will almost definitely overcome the cushion!).  The issue is then becomes having enough cushion volume flow available to maintain that gap whilst passing over large wave troughs that the skirt is unable to conform to.  If you start hearing/feeling water contact on the hull underside then you've reached the limit of the cushion capability and it's time to turn for home!

Sev cushions have a secondary front compartment feed from the skirt which will re-pressurise the compartment (takes a second or two though).  It seems to be designed to provide some degree of recovery should the front compartment be compromised.  It provides a way to raise the nose should the surface not allow the partition/divider skirt to rise high enough to provide the "normal" compartment feed.  And. thirdly, it back-feeds the front/bow "tubes" should the bow skirt get a big hit and the front compartment pressure rapidly rises (i.e it momentarily stiffens the skirt if you hit something big).

[Warby Posts: 1043 Status: Offline]

Ian, ref section on where to place the lift fan, and where to introduce the air. Many years ago (about 40!!!!) I recall it being said the best place was at the front. The reasoning was- If the HC was being driven into a "Short steep sea" there was a possibility that a wave passing under the HC could contact the hull and block off air to the front of the cushion as it moves from front to back, reducing lift at the front. I could see this as a possibility with a Sev where the air is introduced at the rear but not with a segmented skirt where you have feed holes at the front. Just a thought!!

[Ian Brooks Posts: 3551 Status: Offline]

Tech Corner 5: Lift Fans

Given the conversations about lift fans I thought it might be interesting to discuss the basics of how a lift fan works, and how it interacts with the cushion, and lastly what effect ducts and lift holes have.

Lift system requirements

Its worth just thinking for a moment about the lift system and what it has to do. This is two things:

• Create enough cushion pressure to overcome the craft weight
• Provide enough lift air flow to make up for the loss of lift air through the airgap under the skirt

First lift

As the fan spins, if there is no air flow (ie the craft has not lifted and the skirt is sealing against the ground) it creates a pressure increase from inlet to outlet. The faster the fan goes, the greater pressure it will create. At low revs there is not enough pressure increase to cause the craft to lift, and nothing happens other than the skirts begin to fill out a bit. At a certain speed, the fan can create enough pressure to lift the craft. This is called first lift, and is the point at which the craft just begins to lift off the ground.

With the fan operating at first lift speed, all the fan can do is create the pressure. There is no power left over to provide any flow, and the fan will be operating in pressure mode only, there will be no flow to lubricate the cushion and the craft isn't really hovering properly.

Design lift

Once the fan speeds up beyond design lift, it is capable of creating more pressure than is needed for the cushion. However, if the cushin pressure rises any, it just lifts the craft up and opens up an airgap under the skirt, allowing this "extra" pressure to leak away. This regulates the pressure under the craft, and the extra power that the fan has goes to creat flow instead of pressure. Once this extra flow is enough to create a big enough air gap, that is to lubricate the skirt well, the craft is hovering properly. We call this design lift.

Design lift is the most important parameter in the lift system. On a very smooth surface, this might be achieved at lower revs and on a rough surface or scrubby vegetation it may be achieved at higher revs. When we calculate design lift, we make an assumption about "average" conditions and apply a safety factor to allow for non-ideal conditions. The assumption boils down to a 20mm air gap, with the safety factor normally taken as about 1.5-1.8. This is what you see in the performance calculator.

The two stage lift concept

What this means is that there are two distinct stages in the lift process to understand

• Pressure mode: Create pressure in the lift system until first lift is achieved. There is no (significant) flow in this stage
• Flow mode: Create flow through the lift system until there is enough clear air gap to reduce friction and allow the craft to move - design lift.

Below first lift the fan is in pressure mode, above first lift it moves into flow mode.

Upstream of the fan

So its now obvious that the create design lift, we have to have an airflow through the fan. To get this, the fan must be operating above a critical speed (first lift) AND it must be able to get enough air in. Sounds obvious really! But there are some key bits that are often overlooked.

The air that goes through the fan is literally "sucked" in by the fan. Air can only move from a high pressure to a low pressure EXCEPT immediately across the fan. So the pressure immediately in front of the fan is at a lower pressure than atmospere, and thats why the air in front of the fan rushes towards the fan. Its just pressure. Remember that at a given speed, the fan can only create a given pressure rise - and this INCLUDES the suction part at the front. So, if your fan has to suck air through slots or ducts, the suction effectively comes off the pressure that can be raised on the high pressure side. If you fan has to suck hard to gets its air, its output pressure is reduced.

So its not that a fan cannot work when "buried" in bodywork BBV style, its just that it will work with reduced efficiency. Take the bonnet off a BBV3 and see how much the lift improves - its very significant.

For efficient fan operation. make sure that the upstream air inlet path is always of increasing area when compared to the fan - it must never be less area than the fan. Ideally, the inlet path area should be at least 50% greater than the fan area by a distance of 1 fan diameter in front of the fan.

Downstream of the fan

Once the fan has created some pressure, and assuming that we're above first lift and there is some flow, then that air has to move from the fan into the cushion. Remember that air only moves from high pressure to low pressure -if air is to travel through the duct, the air downstrea (in the cusion) must be lower than the pressure upstream (just behind the fan). This is wasted pressure, and wasted power.

If the air has to go through complex ducting and small lift holes, then the restriction is high and the pressure wastage is also high. On the other hand, if the ducting is short and of large size, the pressure wastage is low. A typical integrated craft with a plenum ducting air into the cushion via many small holes wastes about half the lift air pressure at design lift, in contrast delivering the air directly into the cushion wastes almost no pressure.

Ideally all the lift air should be delivered immediately and directly into the cushion.

Is there a best place to feed air into the cushion?

Some people will say that the lift air should be introduced to the cushion at the front of the craft. Reasons given for this are usually something to do with reducing plough-in or something vague about "track laying with air". Thats all just hogwash. It doesn't matter where the air is put into the cushion, it will distribute itself equally through the cushion immediately once in there. The only exception to this is when a compartmented cushion system is used to ensure pitch stability, when the air should be delivered to each compartment in accordance with the design of the system. The Sev system, for example, requires that the air is delivered to the rear compartment first, but the antil-plough flap requires that air is delivered to both compartments at the same time for reasons beyond the scope of this article.

Twin fans

So what if twin fans are used? Will they fight each other?

If the two fans are identical and run at the same speed, they will both have the same first lift and transition from pressure mode to flow mode at the same time. In this case, the twin fans will have exactly the same first lift as a single fan, but above first lift they will create twice the flow, provided the upstream inlet area is big enough and not cluttered. To work out the approach area, think of each fan as having a solid wall between it and its partner fan.
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.

Matching the fans means not only the same fan, but the same upstream and downstream areas.

Further work

Theres a lot more to lift fans than I've discussed here, such as what is the optimum blade angle and why, fan stalling, etc. But that will have to wait for another installment, we can talk about this part first then go onto the more advanced stuff.

Ian

[Alfredrogers Posts: 254 Status: Offline]

Hi Ian, it was with great interest and pleasure that I read your last two articles on roll et cetera and propellers ,all in a nutshell as they say. Using my very unconventional and perhaps controversial electric HC I have experienced all you have written about by trial and error.(The hard way but you learn more by your mistakes than getting it right first time). I built my HC to fit my physical ability and also limited space. In the process I noted that by removing the cowling from my twin 50 cm propellers that it reduced the noise level (Energy )and improved the thrust(clean air) and because it has electric thrust I can Rev The propellers to a very very high speed giving initial thrust(it's quite quiet really).    I have no idea how fast it would actually go as my radio control unit has a limited range and I worry that it may go out of range never to be seen again.     Finally my lift fan ,I have tried all sorts but the best fans for me have a very close fitting rim containing the air in the plenum better. I still have a long way to go but I enjoy it.      Look forward to any more of your articles.  Regards Alf

[edwadee Posts: 156 Status: Offline เรารักในหลวง]

Having just reread my post I should point out that all references to prop size mean
swept area and not diameter .

[edwadee Posts: 156 Status: Offline เรารักในหลวง]

." If you go to a larger duct with 2 times the exit area it will halve the exit speed, the power needed to shift a given amount of air reduces to a quarter, but (and this is the key bit) the thrust only reduces to half. To get the original thrust back you can double the air flow rate, which of course doubles the new power - but 2 times a quarter is only a half! You now have the same thrust but for half the power. I'll say it again!! Double the prop/fan area and you get the same thrust for half the power! "

I have been crunching the numbers on this and I believe there is a fundamental mistake.
If you double the prop area -and keep the same airflow rate-(I presume by altering the speed or pitch ? ) you halve the exit air speed.
This does indeed give half the thrust for a quarter of the power.
If you then double the air flow rate you double the thrust and the power needed.
-But if you double the air flow would this not double the exit air speed (for the same size prop ) ?
So you would end up with double the prop size, double the thrust at double the power.
Or you double the prop area again to increase the air flow rate thus ending up with the original thrust at half the power but
the prop area is 4 times the original not  double.

If you increase the 2x prop air flow by approx. 40 % and thus the exit air speed by 40 % you get the original thrust at about
70 % of the power. Or you can increase by about 70 % to get the original power giving you 40 % extra thrust as  Ian said in a different
part of his post.

This prop size ,power, pitch thing is something I've been trying to get my head around for a while now so feel free to
let me know if I'm getting it all wrong.

[Nick Flint Posts: 4865 Status: Offline]

Shpeal chicken at work ED--?



Remembering to factor in the "consequences"  often depletes the gains. 


I E -Petrol station flowers sometimes don't have the desired effects,,,  "IM FINE" , through tight lips is good evidence that you must not count on assumed brownie points gained.


Same goes for Thrust-

 

[edwadee Posts: 156 Status: Offline เรารักในหลวง]

My bad.
You get the Same thrust for half the power
Still worth having.

[Ian Brooks Posts: 3551 Status: Offline]

Very interesting.  However:
"Double the prop area and you get twice the thrust for half the power"

Ah! Slight mis-quote there, it's good but not that good!!! What is said was "if you double the fan area you get 40% more thrust for the same power".

The main reason we don't use physically huge props is practicality, a 10ft prop just couldn't be used Ina reasonable sized light hovercraft. Think of the thrust line if nothing else, all that nose-down moment would need to be resisted by something. But it's no coincidence that SRN4 had the largest propellers ever made - at 26ft!!

The Guy Martin speed attempt result could have been predicted by any decent Aeronautical Engineer with a day or so of work - I used our performance calculator and predicted within 10mph of the final result. Sadly, (with some notable exceptions) there are few Engineers in the hovercraft world, but lots of cut-and-try mixed in with dollops of folklore.

Anyway not too much time this morning so I'll do a fuller answer later.

[edwadee Posts: 156 Status: Offline เรารักในหลวง]

Very interesting.  However:
"Double the prop area and you get twice the thrust for half the power"
So why isn't everybody running a 10' prop off a 5 hp engine ?
There must be some trade off other than physical size problems.
My understanding is that you must reduce the pitch as you increase
the diameter. Thus you increase static thrust at the expense of top speed.

Although the bigger question should probably be  Why didn't Guy Martin
talk to you first ?? 

[tomtilley Posts: 568 Status: Offline]

 
Hi Ian  just a thought  on some racing crafts on the duct side  it is tapered to about 3 to 5 % or  so it tapers down to make more air pressure leaving the duct .     
                                                                  Tom