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David Holland 214/01/2021 09:19:09
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Doc, the airspeed is exactly the same in both cases. The aeroplane is completely unaware of its groundspeed, it is flying at 100 mph airspeed within an air mass that is moving at 20 mph. Therefore it’s groundspeed is 80 mph into wind and 120 mph downwind but it’s airspeed remains 100 mph and drag remains constant.

David

Piers Bowlan14/01/2021 09:37:36
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ZFlyer, this is a point of no return (PNR) question.

The formula is:- Endurance X Ground Speed out X Ground Speed back / 2 X True Airspeed

so...

4 X 80 X 120 divide by 2 X 100

ANS:- 192, point of no return or radius of action.

As David said, the airspeed is constant so no increase of drag - a red herring!

BUT, to put the cat among the pigeons. coolThe aircraft will be heavier at the start of its flight, being full of fuel. Therefore more lift will need to be generated at the start of the flight than the end, which mean that more induced drag will be produced too (which is not dependent on airspeed). The consequence is that less power will be required to maintain 100mph at the end of the flight so less fuel consumption. If the fuel flow is not constant this must move the PNR further from the origin? Discuss! wink 2

'Flight Without Formula' A brilliant book for anyone interested in aircraft flight yes

 

Edited By Piers Bowlan on 14/01/2021 10:06:34

Dickw14/01/2021 09:58:37
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Posted by Peter M on 13/01/2021 23:40:21:

Well done Dick, Mr. Lane was a good teacher!

Wow! That jerked some memories!

Must be nearly 60 years ago - but you have an advantage over me as I didn't keep in touch with anyone or keep any records from that period. I assume we knew each other back then?

Dick

ps ??? Did you ever own a Morgan?

Edited By Dickw on 14/01/2021 10:19:51

Chris Walby14/01/2021 10:48:13
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386 photos

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 laugh

Edited By Chris Walby on 14/01/2021 11:17:17

Martin Harris - Moderator14/01/2021 12:57:55
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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

David Holland 214/01/2021 13:29:37
253 forum posts
28 photos

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

Piers Bowlan14/01/2021 14:59:45
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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 laugh

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

Martin Harris - Moderator14/01/2021 15:53:33
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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!

Piers Bowlan14/01/2021 19:37:35
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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.

Martin Harris - Moderator14/01/2021 20:24:59
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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...

Piers Bowlan14/01/2021 23:01:12
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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.

Martin Harris - Moderator14/01/2021 23:10:01
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I think we're singing from the same songsheet Piers...

Piers Bowlan14/01/2021 23:18:52
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yes

kevin b14/01/2021 23:18:56
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2046 forum posts
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This thread is very interesting, but unfortunately it is way over my head.

wink 2

Robin Colbourne15/01/2021 15:02:36
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778 forum posts
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Posted by kevin b on 14/01/2021 23:18:56:

This thread is very interesting, but unfortunately it is way over my head.

wink 2

kevin b, that's the thing with flight, it usually is. If its not, there's usually a Buccaneer involved. laugh

Buccaneer

J D 815/01/2021 15:23:18
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1750 forum posts
88 photos

It's that old question Robin. Why does a Buccaneer retract its wheels ? So it can fly lower ! yes

Robin Colbourne15/01/2021 17:00:42
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778 forum posts
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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 ! yes

laughlaughlaugh Love it!!!

Gary Manuel15/01/2021 19:52:17
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2461 forum posts
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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.

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