Martin Harris - Moderator Posted January 14, 2021 Share Posted January 14, 2021 How complex do you need to go? In the real world, winds are rarely steady and you rely on forecasts, observations and remote ground level measurements during flight planning. Pressure systems move in such a complex manner that the Met Office have to run the latest super computers to attempt to model them. The answer to the question is only illustrative - part of the reason why there is always a minimum quantity of unused fuel factored into pre-flight planning! Edited By Martin Harris - Moderator on 14/01/2021 12:59:38 Quote Link to comment Share on other sites More sharing options...
David Holland 2 Posted January 14, 2021 Share Posted January 14, 2021 Chris, as fuel burns off and its weight reduces the aircraft will have an excess of lift which, if power is not adjusted, will result in the aircraft climbing or, if trimmed more nose down to compensate, an increase in airspeed. As Martin says this is a very artificial scenario, in reality the environment in which the aircraft is operating is so variable and changeable that the question is meaningless. Interestingly, weather forecasts , certainly as far as GA is concerned, are only valid for 6 hours from the time of issue. David Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted January 14, 2021 Share Posted January 14, 2021 Posted by Chris Walby on 14/01/2021 10:48:13: but what happens if the aircraft is less efficient at the lower mass without retrimming? will it have less or more drag? IIRC gliders sometimes carry water as ballast to suit certain weather conditions and beat others without ballast over a measured course/time? How complex can we go without formula Edited By Chris Walby on 14/01/2021 11:17:17 A water ballasted glider (with a high wing loading), can achieve the same glide ratio as an unbalasted light one - but the heavy one will do so at a higher speed. The water-ballasted one can therefore cover the same distance as the unballasted one but in less time. However the ballasted one will have a higher rate of decent, (I believe). As a general observation, heavy aeroplanes do not fly as well as light ones but if you are flying a high performance sailplane on a cross country race and the lift is plentiful, then fill up the water ballast. You will cover the same course but quicker. If the lift is plentiful you are not concerned with gliding efficiency. If the lift is patchy it will take you longer to climb at a higher weight and it will also be more difficult to stay in the lift with your higher airspeed. In the question in the OP, varying the airspeed wasn't considered as the aircraft was stated to cruise at 100mph. Edited By Piers Bowlan on 14/01/2021 15:02:50 Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted January 14, 2021 Share Posted January 14, 2021 Stalling speed and radius of thermalling turns both increase leading to a loss of climb performance but on a strong lift day, the increased speed between thermals is beneficial to a fast average speed - plus it's a lot of fun doing a competition finish (A.K.A. beat-up) trailing water plumes! Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted January 14, 2021 Share Posted January 14, 2021 I agree Martin, certainly radius of turns/steeper bank angles will reduce rate of climb but also a heavier aircraft will always have a reduced rate of climb compared with a lighter one for a given amount of lift in a thermal. The rate of climb will be dependent on the excess lift over aircraft weight and that is true for any aircraft not just a glider. So a ballasted glider will not climb so rapidly in a thermal compared with a non blasted one. However it will able to fly from thermal to thermal far quicker. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted January 14, 2021 Share Posted January 14, 2021 Plus the fact that the strongest lift tends to be in the central core of a thermal and a lightly loaded slower flying glider can outclimb "better" gliders even at theoretically less efficient high angles of bank. I assume you were referring to wing loading rather than total mass... Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted January 14, 2021 Share Posted January 14, 2021 I was really comparing the same make/model glider flying ballasted as opposed to when it is flying un-ballasted. When ballasted it will indeed have a higher wing loading and it will climb less well in a thermal. When thermals are strong and plentiful it is not an issue for a high performance glider. I wasn't really comparing different types of glider. Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted January 14, 2021 Share Posted January 14, 2021 I think we're singing from the same songsheet Piers... Quote Link to comment Share on other sites More sharing options...
Piers Bowlan Posted January 14, 2021 Share Posted January 14, 2021 Quote Link to comment Share on other sites More sharing options...
kevin b Posted January 14, 2021 Share Posted January 14, 2021 This thread is very interesting, but unfortunately it is way over my head. Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted January 15, 2021 Share Posted January 15, 2021 Posted by kevin b on 14/01/2021 23:18:56: This thread is very interesting, but unfortunately it is way over my head. kevin b, that's the thing with flight, it usually is. If its not, there's usually a Buccaneer involved. Quote Link to comment Share on other sites More sharing options...
J D 8 Posted January 15, 2021 Share Posted January 15, 2021 It's that old question Robin. Why does a Buccaneer retract its wheels ? So it can fly lower ! Quote Link to comment Share on other sites More sharing options...
Robin Colbourne Posted January 15, 2021 Share Posted January 15, 2021 Posted by J D 8 on 15/01/2021 15:23:18: It's that old question Robin. Why does a Buccaneer retract its wheels ? So it can fly lower ! Love it!!! Quote Link to comment Share on other sites More sharing options...
Gary Manuel Posted January 15, 2021 Share Posted January 15, 2021 I'm slowly making my way through "Flight without formulae" by A.C.Kermode, which inspired this thread. I'm thoroughly enjoying the book and learning a lot from it although some of the language and culture appears a little dated. How about this for a quote from the book when the author was discussing that early pilots were reluctant to accept aeroplanes that were inherently stable. He compared it with the purist car driver being reluctant to accept cars with automatic gear boxes. "Why was the opinion of pilots against the stable aeroplane? Probably because if one can do a thing, one likes it to be difficult. With self-changing gears even women can learn to drive without ----- Well, let us leave it at that!" It made me chuckle - and cringe a little. Quote Link to comment Share on other sites More sharing options...
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