Tech Corner 3: Understanding Fans and PropellorsFan v. Propeller ... Now there's a subject that caused many debates over the years! Why do bigger fans create more thrust? And why do smaller fans make a faster craft? And is that even true?
And whilst we're on the subject, why did the Guy Martin hovercraft speed record attempt fail? The main reason is simple - the wrong fan, no matter how much much power he had, with that fan his speed was always limited to about the 85mph mark - all very predictable. Let's get to grips with it.
Eh?
So I just said that bigger fans make more thrust and that smaller fans make craft go faster - that, surely, is nonsense? Actually, no, both statements are true and that's one of the reasons why this debate rages on, and just to add to the confusion few people get the difference between fast round a racetrack and fast down the straight, which are not the same thing. But, the good news is that the truth is actually quite simple when you get down to it. No fancy maths needed, all you have to do us realign your common sense and it should come clear.
What type of thrust?There is static thrust and dynamic (moving) thrust. They're different! Well, obviously thrust is thrust, but what you measure when static using a spring balance will be different to what you measure (if you could) whilst moving along at speed. Let's explain.
Thrust reduces as you go faster This is the first thing that we need to remember - the first rule of thrust. As you get faster, the thrust produced goes down. Even with the same power applied. By 35mph the average 900mm fan has reduced it's thrust by a huge 40%! You can check this out using the craft
performance calculator.
Why, I hear you say? Think of it like this. When you are static, you bring air in at nought mph and chuck it out the back at say 80mph. Theres a huge difference between the in and out speed and that's what creates the thrust. However, when you're doing 40 mph you're bringing in air that (as far as the craft/fan is concerned) is already doing 40mph, and sadly you're still chucking it out the back at the same 80mph. Theres less difference and therefore less thrust. If you were lucky enough to be doing 80mph then you'd be getting air in at 80 and sending it out at 80. Therefore no thrust at all ! This gives an absolute on craft speed and (massive understatement!) limit is really important if you're designing a speed record craft.
Bigger fans give more static thrust.
This is the second rule of thrust. For a given power, a bigger fan will give more static thrust. It's a big deal, if you double the fan area you can get 40% more thrust from the same hp. That's why fans are getting bigger all the time. So why does this happen?
This is because of a weird relationship between the power consumed and thrust produced. Turns out that the power needed to throw air out the back depends on the air flow rate times air exit speed squared. But the thrust depends on the air flow rate times exit speed straight. 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! Now, that's why ducts are getting bigger.
Props give less thrust than fans but they go faster. This is the third rule that you need. Once you have these three, then you have everything you need to understand why Guy failed in his attempt.
It turns out that an open propeller behaves like a duct of half the area. The reasons for this are complex, but it turns out the air carries on accelerating even after it had left the props. It's all due to static pressure, and we don't need to worry about the whys of this but, just know that it happens. Ducts were originally designed to stop this happening - which they do, if they are long enough. Back to this point later.
What is a fan anyway?Sounds like an obvious thing. We all know what a fan looks like - it's got a duct, a prop does not, and that's the difference. Well, yes that is true but we do need to know a bit more than that to understand things.
The key is what the duct does. This is going to sound barmy, but what the duct does is slow down the exit air, and thats why it creates more thrust. !!! Yep, hp for hp, slow air gives more thrust - this is exactly the same as the second rule of thrust, the duct makes the fan behave like a bigger prop. For this to work, the duct has to be long enough - about the same length as the diameter.
So .... what's a prop? A prop is exactly the same as a fan except no duct. Obvious statement really - but no, there's no fundamental difference. One little difference should be mentioned, whilst a fan is 'active' right to the tip, a prop has the tips washed out so that they're not doing anything, to reduce eddys that otherwise would form at the tip sucking away power without creating thrust. The fan gets away without this, as it has the duct to do the same job.
Key to understanding a prop is the air exit velocity. It's exactly twice that of the fan! Remember what it said about slow air makes more thrust, hp for hp? So the prop will make less STATIC thrust. Note the naughty little word 'static' ,we'll come back to this later. But here's a hint - if you want to go fast in a straight line, you want a prop.
Incidentally, if you've ever tried to use a multi wing fan without a duct, and found the result to be very poor thrust, it's the tip eddys that got you, consuming hp without making thrust.
What about shrouded props? If a prop must have the tips washed out, and a duct must be as long as it is wide, what is it if you get a fan but put it in a short duct? That's a shrouded prop! A shrouded prop has the advantage that no washout is needed, and the shroud (duct) only had to be very short.
A shrouded prop has a higher exit velocity than a ducted fan, nearer to a prop in fact, and does not need the washout tips. So it sits in the middle, and can combine the advantages of both to some extent. Of course , it does not have the full range of advantages either!
And yes, before you ask, the Otter does have a shrouded prop. I know it's a multi wing fan blade in there, but technically it's a shrouded prop.
We all know fans are faster, just look at the race results - right?
Wrong. That depends on how you define fast. Races are typically on tight little courses with lots of bends, and cornering is all about static thrust. They have few long straights and so race winning craft have lots of static thrust. They don't need moving thrust to win. So race winning craft are not speed record craft, as Guy Martin found out. We define fast as in straight line.
Size is everything. Whether it's a prop, a fan or a shrouded prop one thing holds true, hp for hp, it'll create more static thrust if it's bigger. The ducted fan makes the most, then the shrouded prop, then the prop. But don't forget about the duct length rule, it has to be the same as the diameter. So if you want a 1.5m ducted fan, it must be 1.5m long. Not very practical!
Actually we haven't used true ducted fans since the seventies, they are somewhat in between ducted fans and shrouded props really. So a current duct of maybe 1.1m diameter but maybe 0.6m long actually behaves somewhere nearer to a prop than a duct, and doesn't get the hoped for thrust benefit.
Now we have to talk about moving thrust ! As we said, slow exit air gives the best thrust hp for hp. But what if the exit air leaves the fan at the same speed as the crafts forward speed, surely that can't be making any thrust can it??? Essentially you just took the air aboard the craft and then sort of left it behind rather than blasted it out behind you.
That's absolutely true. If your fan sends air out the back at, say, 80mph then you can't go any faster than that - even if you have no drag at all. That's the problem Guy Martin had. His fan chucked air out not much faster than 85mph. So no matter what he did, that was it. And, of course this doesn't happen all of a sudden, as the craft speeds up the thrust reduces slowly from the static thrust value until eventually there's no thrust left. Of course the maximum speed happens a bit before that when the increasing drag equals the reducing thrust.
So - the ducted fan has a greater static thrust but the lowest exit air speed. That means its thrust reduces fastest. The prop, on the other hand, has the least static thrust but the highest exit air speed so it's thrust reduces least. A prop powered craft can go faster than a fan powered craft. And as usual the shrouded prop sits somewhere in the middle.
What this means is that if you want to design a fast craft you have to design for fast prop/fan exit air speed, and that'll mean smaller fans or preferably an open prop with its built in exit airspeed advantage.
Anything else to be considered?As we have discovered, a big ducted fan gives the most thrust per hp, so surely it's the best choice for a hovercraft? Perhaps, but we do need to consider other practicalities before we decide what's best. The size and weight of a big duct can have a big impact on craft performance, especially in a side or tail wind. We talked about yaw stability in the first article, recall that if we have a large side profile at the stern it must be balanced by a large side profile at the bows otherwise the craft will shuttlecock in a strong tailwind. Put another way, there's a practical maximum size of duct for our size or craft , once you get above about 1.2m or 1.3m it's getting difficult to balance the yaw stability up. This is why shrouded props become important, gaining some of the advantages of ducted fans but without the bulk.
Once you are looking at fan or prop sizes greater than about 1.5m, even shrouded props cease to be practical and you'll finish up with an open prop due to the size and weight of the shroud.
So what about that speed record ? From all this discussion you should be getting the idea that a speed record craft should have a specially designed prop. A ducted fan similar to those used in racing will lose thrust too quickly at high speed and that'll limit things. On the other hand, a prop with its highest air exit speed may not produce as much static thrust but it hangs onto what it has longer as the speed increases. On my I old Surveyor I once experimented with a 1.2m open prop in place of the 1.5m shrouded prop you are all familiar with. It had less thrust but went MUCH faster in terms of top speed, using the same engine. It was hugely noticeable when we tested it with the fan and swapped it out to the prop, the craft didn't accelerate or get over hump as well but once the speed was up it just kept getting faster.
So if Guy Martin fancies another go he needs to forget the racing duct - it's too slow! - and build a craft with either a smaller ducted fan or better still an open prop. Of course, since this small ducted fan or prop will produce less static thrust, he'll need lots of power installed in order to get over hump, but once over hump he'll hang onto much more thrust to a much higher speed. He could and should get well over the ton with that set up, and if money (serious money) were no object then 200 mph should be achievable. Yep, that speed record could be smashed into little pieces by a suitably determined team of Engineers.
And of course noise ...No discussion on fans and props is complete without talking about noise. This again is pretty simple when you boil it down. High tip speed is bad. More blades is bad. But you do need to absorb the power, and you do need to create thrust. So you have to go big. It's no coincidence that the quieter craft have the bigger fans and the quietest craft have the biggest open or shrouded props. And that is why the Otter has a large shrouded prop.
Of course Guy Martin's new craft would be really really noisy! But very fast too.