Good maths Thanks Steve Ive been reading about some Oil Tests conducted by Johnson etal., perhaps that is worth a try > Andrew, > I have played around with polar measuring in the past (It can be > interesting and fun) However, at sensible glider performance levels > (30:1 and better) it is VERY hard to get an accurate measurement. > > Lets pick some easy numbers. > If we flew at, say, 60kts for 3 minutes we would cover 3nm (and be 1/2 > way to Roseworthy!) If our glider had a 30:1 ratio at this speed we > would lose 3x6050/30 feet=605. If we had a ratio of 31:1 we would lose > 3x6050/31=585 feet. So we need to be able to measure height with a > resolution of better than 20 feet after flying for 3 minutes. Note > that as the glide angle gets better (more Cirrus like?) the difference > reduces. The same examples with 37 or 38:1 gives a height difference > of 12 feet after 3 minutes. > > To get any meaningful results you would need to have (using our easier > example) about 15 minutes worth of glide to get a 100 foot difference. > Now if we did that in one go we would need 5x600=3000 feet of height > to burn off and we get into the area where we should be thinking about > air density and pressure too (because of the height difference through > the flight). If we did a lot of shorter flights (which we could) to > get a statistically meaningful result it would cost us an arm and a > leg and we still only have one point on our polar! > > There are other practical aspects, none of which are insurmountable > but all need addressing. We assume perfectly smooth air of course. 20 > feet in 3 minutes is 7 feet/min. Any variations in vertical air > movement of this magnitude will also stuff your results. How > accurately can you hold speed? Exactly 60 knots for 15 minutes (I > don't doubt you could do it, but I bet your attention would wander > after 5 minutes, I know mine would!). And finally after 15 minutes you > are now 15nm away from the start point of your glide, hope you didn't > start directly over Gawler :-) > > Talk to me more about it when I see you at Gawler next. > > Regards > SWK > > > -----Original Message----- > > From: ANDREW WRIGHT [mailto:[EMAIL PROTECTED] > > > Does anyone have any advice on how to accurately measure the real > > flight performance of a glider? Ideally I would like to compare the > > polar curve of my glider, a Standard Cirrus 75, with some measure of > > what it should be. My reason for wanting to do this is that I would > > like to determine if I really do need to reprofile a problem section > > of my wing. I suspect a performance loss due to this problem > > section of my wing and would like to measure its effect. Is there a > > reliable source of polar information on gliders (I.e. actual flight > > measured polars.) On or off line comments and advice welcomed. > > > > ANDREW WRIGHT > > > > > -- > * You are subscribed to the aus-soaring mailing list. > * To Unsubscribe: send email to [EMAIL PROTECTED] * > with "unsubscribe aus-soaring" in the body of the message * or with > "help" in the body of the message for more information. > >
ANDREW WRIGHT -- * You are subscribed to the aus-soaring mailing list. * To Unsubscribe: send email to [EMAIL PROTECTED] * with "unsubscribe aus-soaring" in the body of the message * or with "help" in the body of the message for more information.
