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Thread: Noise -Effect of blade angle

  1. #1
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    Default Noise -Effect of blade angle

    When I published here late last year a draft formula for calculating fan noise on multiwing style fans I didn't include a term for the effect of blade angle or pitch. The logic was that most folk operate with blade angles in the 35-45 degree range and I didn't have any data on the blade angles used on the particular craft I'd sampled. Conrad has suggested that blade angle might be major component of why these fans are apparently noisier than ultralight aircraft props, which typically run at around 20 degrees.



    I've conducted some tests using my Kohler engined F25 running a 3 bladed fan at different angles to explore this. The results from the tests show there is about an 8db increase in noise from the same m/w fan at a given tip speed when blade angle is increased from 20 to 40 degrees and a further 3 db increase from 40 to 45 degrees. The tests were conducted over tip speeds from 60 to 110 m/sec, which is lower than the 120-150 m/sec used by most craft so its possible the difference is more marked at higher tip speeds.



    Of course the bhp absorbed by the fan at different angles but at a given tip speed varies significantly - the fan is doing more work at higher angles but perhaps thats a clue to the lower noise of the prop. Each foot length of the prop blade is doing less work, causing less noise but the overall longer length of the prop blade and often it's higher tip speed means it does as much or more work overall.



    The lower pitch at the tip of a prop compared to a fan may also be a major component. The pressure on one side of a airfoil blade is greater than on the other and the result is a spanwise flow of air around the tip from the high pressure to the low pressure zone. This turbulence causes noise. The duct wall on the fan should help to contain this flow, in much the same way as the winglets used on some aircraft, but maybe the high tip clearances needed for practical reasons are less than fully effective.



    On another tack I reported before that from the Blackditch sample some craft seemed to have unexplained high levels of fan harmonics whilst others were much lower. My craft seems to be mostly in the latter group with total fan harmonic noise 5-6db lower than the fan blade passing frequency (bpf) noise, even though the flow straightners are close to the fan. However when using the 45 degree angle at max rpm (just 3120rpm because of the loading) the fan harmonics rise dramatically so that the total harmonic noise is 1 db higher than the bpf. This is probably due to cavitation, which Conrad tells me may go away if the craft was allowed to move forward.



    Other byproducts of these tests are that its confirmed that my Kohler F25 even with a poor exhaust is significantly quieter than the fan. The fan guard does not appear to cause noise. I can see no increase in either overall dba or broadband (indicating more turbulence) noise with or without the guard.



    A much larger and wider ranging series of tests is needed to see if there is a sweet spot offering minimum noise with the same or better propulsion efficiency.



    More to come in my AGM lecture. Comments welcomed.

  2. #2
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    Default Re: Noise -Effect of blade angle

    Excellent work Keith.



    <table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote:</td></tr><tr><td class="quote">
    .. but perhaps thats a clue to the lower noise of the prop. Each foot length of the prop blade is doing less work, causing less noise but the overall longer length of the prop blade and often it's higher tip speed means it does as much or more work overall.
    </td></tr></table>



    My basic experiments with the Scout prop and fan (both the same diameter) didn't show this. The prop did more work (and generated less noise) than the fan at a slightly higher tip speed both with the same power input. The fan pitch and tip speed were optimized for maxumum thrust as was the prop.



    <table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote:</td></tr><tr><td class="quote">
    The lower pitch at the tip of a prop compared to a fan may also be a major component.
    </td></tr></table>



    A 5Z fan blade has a very similar tip angle to a prop (about 3-10degrees) at the most common pitch settings.



    The only really obvious difference I can see between a fan blade and a prop blade is the width (I think chord is the 'proper' name). They both have similar aerofoil sections, blade twist, blade angle (prop blade angle is usually measured at about 70% diameter whereas fan blades are set at the root), etc. As fans are designed specifically to shift air efficiently (and quietly) at slow speed (much slower than hovercraft) it could be that the wider blade just doesn't work properly at higher speeds and the noise is an indication of this.



    <table border="0" align="center" width="90%" cellpadding="3" cellspacing="1"><tr><td class="SmallText">Quote:</td></tr><tr><td class="quote">
    The duct wall on the fan should help to contain this flow, in much the same way as the winglets used on some aircraft, but maybe the high tip clearances needed for practical reasons are less than fully effective.
    </td></tr></table>



    I suspect that you probably need very close clearances to reduce tip turbulance. A duct is, after all, just an aerofoil section wing folded around in a circle. As the angle of attck of the 'wing' duct is fixed, there have to be a limited range of airflow conditions under which it isn't in either the stalled or high drag state.



    Any chance of a copy of your presentation for us people who won't be at the AGM (after you've presented it of course!)?


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    Default Re: Noise -Effect of blade angle

    Hi



    Did you measure the thrust? If not, I could estimate it given a few details. It would be interesting to plot noise against thrust & power for a range of blade angles. Let me know if you want me to work the estimates.



    Ian
    Attached Files Attached Files

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    Default Re: Noise -Effect of blade angle

    Ian

    Thanks - no the craft was on a trailer and with just 3 blades instead of the normal 6 and the splitter plate removed to eliminate it as a source of noise it was a bit tricky to measure thrust! I'm keen to see some kind of power output v noise plots. I'll send you the data via email.

    Clearly from Johns comments I should stick to reporting data not speculating on aerodynamics conclusions!


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    Default Re: Noise -Effect of blade angle

    No you keep at it Keith.



    I would not be surprised at all to find that blade angle of attack is a key parameter. But it has to be the actual angle of attack with the airflow, which is a bit more difficult to work out, but is a measure of "how hard the blade is working".



    Nick


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    Default Re: Noise -Effect of blade angle

    Nick is correct on this one Keith. Most ducted fan systems (industrial) are designed to move air at the lowest speed possible conducive to providing the required volume of air. The main systems which deviate from this, are extraction systems where a high design velocity is chosen to ensure entrainment of the contaminant within the airflow (smoke, dust, wood-chip etc). The MW type fans are designed for use in the former type of system (they will not handle anything heavier than smoke very well). As such, the twist in the blade and the cord is designed about an "average" system, having a slower fan, than we use. The result is an overloaded tip and poor efficiency over a relatively large part of the blade length. As the efficiency drops, the noise level climbs with increasing power absorption.



    With a correctly designed fan blade for hovercraft travelling at a maximum speeds of perhaps 40mph, It is likely that the efficiency could match that of proprietry propellers which are mainly designed for cruising speeds in excess of 80mph. Add to that the shielding effect of the duct and the lower fan speeds that could be used with a high efficiency blade, I suspect that the noise levels could fall below that of current prop craft.



    The tip clearance problems can be solved - look at the Australian craft!! it just requires better engineering of the duct assy. than most craft use in the UK. The problem regarding the angle of attack with respect to the incoming air will of course always remain, because of the typical sideways attitude of a hovercraft and the limited length of affective duct which is practical.



    Please don't stop offering opinions, speculative or otherwise, they drive the ideas and suggestions from others.


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    Default Re: Noise -Effect of blade angle

    Hi all

    Interesting reading, If someone develops a purpose built

    hovercraft fan (quiet + efficient) I will be looking to replace

    my MW setup as I find it unsatisfactory



    Would it not be in the clubs interest to fund the development

    of such a blade?

    If the club was sole supplyer of the blades i am certain

    development costs would be more than covered.



    does FUD forum stand for

    Fan

    Under

    Development

    forum

    TrevB


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    Default Re: Noise -Effect of blade angle

    Thanks for the support guys. I think the question of funding is cart before horse. Setting up moulds is expensive (5-10k?) but in the scheme of things affordable either by the club or other bodies but the real issue is to identify a design with some certainty before anyone invests real money. Thats were we need to get a better understanding and preferably reliably computer model a design first. And sorry thats beyond me!

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