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NC Bill

Glide Ratio

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BTW,

 

How does an increase in weight affect the distance you can glide?

 

Discuss...

 

The gross weight does not affect the distance you can glide. This remains the same given any weight.

What does change is the speed at which you achieve best l/d. The heavier the plane (glider) the faster the best l/d speed is.

 

This is the reason why sailplanes put water balast in their wings to go faster around a particular course and dump that at the end of the day when thermal conditions become weaker and they need to lighten up to still use the (now) weaker thermals so they can still climb in these , whereas they might not be able to if they are too heavy.

 

Hope this helps.

 

Regards,

 

Jos

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The gross weight does not affect the distance you can glide. This remains the same given any weight.

 

Jos,

 

Bingo!

 

It's hard to conceptualize, but given a calm day, from 5,000', a CT made of balsa wood weighing 200 lbs and one made of lead weighing 20 tons would glide the same distance.

 

I imagine the balsa wood plane gently wafting down at just a few knots, and the lead plane screaming down at hundreds of knots, but in theory they would hit the ground at the same point.

 

Just one of those counterintuitive things that keeps flying interesting!

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So...

Is 1.5NM/1000' AGL, 63 KIAS (at avg weight), zero flaps a decent no-wind max range ballpark figure?

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2 minutes ago, Hambone said:

So...

Is 1.5NM/1000' AGL, 63 KIAS (at avg weight), zero flaps a decent max range no-wind ballpark figure?

make sure you want zero flaps, when I go to zero from negative six my glide reduces.  only takes a minute to test, start a glide at neg six, observe your glide and what point you will make now with zero flaps can you still get there?  is it even close?

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Ed,

Are you using 63 KIAS for both flap settings for your test? Or do you add a few knots for the flaps -6° setting?

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1 hour ago, WmInce said:

Ed,

Are you using 63 KIAS for both flap settings for your test? Or do you add a few knots for the flaps -6° setting?

No need to add a few knots.  Just add 6degrees and kill your glide.  A few knots might show a bigger differential but then its no longer a simple no-brainer type test.

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1 hour ago, Ed Cesnalis said:

No need to add a few knots.  Just add 6degrees and kill your glide.  A few knots might show a bigger differential but then its no longer a simple no-brainer type test.

Let me ask in another way.

If you added, say 3 knots, (66 KIAS) to your -6° setting, would that give you a longer glide than flying at 63 KIAS at flaps 6°?

I invite others to weigh in on this.

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1 hour ago, WmInce said:

Let me ask in another way.

If you added, say 3 knots, (66 KIAS) to your -6° setting, would that give you a longer glide than flying at 63 KIAS at flaps 6°?

I invite others to weigh in on this.

I have no idea.

If you make assumptions like, 63 is in fact the right number for the zero setting than it stands to reason that the neg six number is a few knots faster.

I bet the ball park is full nose up trim and if you go thru min sink on the way there you're too slow.

Best flap setting is a far more important.

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1 hour ago, Ed Cesnalis said:

If you make assumptions like, 63 is in fact the right number for the zero setting than it stands to reason that the neg six number is a few knots faster.

Not an assumption Ed, that's published in the (CTSW) Aircraft Operating Instructions.

I agree, stands to reason,  for -6°, a bit faster airspeed would give better glide.

1 hour ago, Ed Cesnalis said:

Best flap setting is a far more important.

Best flap setting, flown at the best airspeed for that flap setting, will result in the best performance.

Concur?

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2 hours ago, WmInce said:

Not an assumption Ed, that's published in the (CTSW) Aircraft Operating Instructions.

I don't think it is in mine.  The POH is also telling you that its your best glide yet its not close to the glide I get with reflex.  If FD is wrong about best glide configuration I don't trust them on the number.

You could be all set up to make your field and en route read  your POH and drop to zero degrees and not make it.

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2 hours ago, WmInce said:

Best flap setting, flown at the best airspeed for that flap setting, will result in the best performance.

Concur

No not at all.  First you have to have the right flap setting or performance will be way off even if your speed is perfect.

I would concur if you added 'will result in the best performance at that flap setting'

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6 hours ago, Ed Cesnalis said:

No not at all.  First you have to have the right flap setting or performance will be way off even if your speed is perfect.

I would concur if you added 'will result in the best performance at that flap setting'

 

6 hours ago, Ed Cesnalis said:
  8 hours ago, WmInce said:

Best flap setting, flown at the best airspeed for that flap setting, will result in the best performance.

Concur

Ed, I think you need to take a closer look at what he said.

I concur with Bill. If you have the best flap setting, which you say is -6°, and you fly at the best airspeed for that setting you will have the best glide performance. Any speed above or below that speed will have less performance.

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I have found the 14:1 glide ratio published by FD to be optimistic.  12:1 might be closer, but I always assume 10:1.  It's easy to calculate distances that way, and it's conservative; you might reach a place you were not sure of, but you can definitely reach anyplace in a 10:1 glide distance if you do your part and the winds are not howling against you.

After some testing, I like the -6° flap setting at 78-80 knots for best glide.  I tried using the factory 15° @ 63kt setting, and the glide distance was much lower.  Using my settings you just have to make sure you go to a landing flap setting at some point.  I usually wait until I know I will make the field and then add in flaps to 15°.  Once I'm on short final (or if I look high) I go to 30° flaps, and maybe 40° depending on what the landing zone looks like. 

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10 hours ago, Tom Baker said:

Ed, I think you need to take a closer look at what he said.

 

10 hours ago, FlyingMonkey said:

I tried using the factory 15° @ 63kt setting, and the glide distance was much lower. 

 

I stand by what I said.  The published 15° @ 63kt setting will leave you short of the field you would have made at a variety of speeds at neg six.

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I think each and every flap setting would have an airspeed that gives you the best glide distance FOR THAT FLAP SETTING.  Then the question becomes, which flap setting gives you the best glide distance when using the correct airspeed for that flap setting?  Of course, wind conditions might cause you to use less flaps and a higher airspeed.  Flying at 63 knots into a 63 knot headwind might not get you there.

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ED, in Bill's post he said,

On ‎3‎/‎29‎/‎2018 at 12:27 AM, WmInce said:

I agree, stands to reason,  for -6°, a bit faster airspeed would give better glide.

and,

 

On ‎3‎/‎29‎/‎2018 at 12:27 AM, WmInce said:

Best flap setting, flown at the best airspeed for that flap setting, will result in the best performance.

Concur?

To which you replied,

 

On ‎3‎/‎29‎/‎2018 at 2:57 AM, Ed Cesnalis said:

No not at all.

If the best airspeed for -6° flaps doesn't provide the best glide, what speed does?

I think any speed above or below best glide speed for the flap setting will provide less performance.

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Best glide will be whatever is lowest drag. For conventional aircraft, it's probably going to be in whatever configuration generates just the right amount of lift for goals such as the following:

  • Keep the angle of attack low as possible. This reduces the presented aircraft cross section, reducing form drag. Flaps also increase that presented cross section.
  • Reducing lift-induced drag. More lift means more lift induced drag at slower speeds. Flaps are VERY guilty of raising this very quickly!

On some aircraft, flaps can go so high, as to act more like spoilers (but are not technically true spoilers), such as with gliders. These are called reflexing flaps. What they do is reduce the lifting ability in certain sections of the wing, which reduces coefficient of lift, which also reduces some forms of drag. HOWEVER: as emphasized, SOME. There's an airspeed where below it, reflexive flaps will do more harm than good (probably an upper airspeed too but much higher than we'll ever fly at). Reflexing flaps are typically designed for cruise, where airspeed is plentiful and the wings are generating so much lift, that we can afford to sacrifice some of it for a lower coefficient of lift, which means a lower drag. Not all airfoil designs can use reflexing flaps (usually, wings designed for high lift benefit from reflexing). FDCTs flaps ARE considered reflexing flaps.

So I suppose the question is: is the airspeed where reflexing the flaps are beneficial higher or lower than best glide?

If you want a good bit of reading: https://en.wikipedia.org/wiki/Parasitic_drag and https://en.wikipedia.org/wiki/Lift-induced_drag

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50 minutes ago, Anticept said:

So I suppose, the question is, is that airspeed where reflexing the flaps is beneficial higher or lower than best glide?

My experience says its beneficial at most airspeed, the benefit can be fine tuned but its there even at 63kts.

To use your way of saying it 'I always reflex my flaps for performance much better than best glide' .    hmmmmm,   just don't sound right  ....   

 

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So then it's possible that CT wings are designed for very high lift, and as such, the airspeed which reflexive flaps are useful isn't much higher than stall? I could see that being the case.

Out of curiosity, I looked up the wingspan of pipistrel's virus, it's ~41 feet vs flight design's ~28 feet. I know the virus outperforms a CT in speed, and I think I'm seeing why! Their wings are razor thin but extremely long (this won't fit in some t-hangars)! Flight Design might have needed to approach the short wing with a higher lift to get the lower stall speeds.

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My POH for the CTLS says typical glide ratio is 8.5 to 1 and max distance is achieved at -6 flaps and 78 knots (which is the same as their -6 climb Vy). They also show an example at 15 flaps and around 63 knots (100 km) which is about 1 mile less. I'm guessing 0 flaps is in between at around 73 knots (again 0 flaps climb Vy). My guess is that more flaps allows more time in the air, but less glide distance.

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I know nothing about negative flaps, but it's not hard to see how some amount of negative flaps might produce less drag than zero flaps and therefore result in increased glide distance.  The question then is, what airspeed results in the longest glide distance.  Once you know that, any airspeed lower or higher than that will result in less glide distance in zero wind condition.  Add a head wind, and you need to increase your airspeed for best glide distance.

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Indeed, the only reason negative flaps comes up here is that Flight Design created its wing with that in mind. They consider zero flaps to be 'some' flaps, including a Vfe speed limit of 100 knots for zero flaps.

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On 3/31/2018 at 10:15 AM, SportFlyer1 said:

My POH for the CTLS says typical glide ratio is 8.5 to 1 and max distance is achieved at -6 flaps and 78 knots (which is the same as their -6 climb Vy). They also show an example at 15 flaps and around 63 knots (100 km) which is about 1 mile less. I'm guessing 0 flaps is in between at around 73 knots (again 0 flaps climb Vy). My guess is that more flaps allows more time in the air, but less glide distance.

 

Mine says this also but shows best glide speed varies by weight. 78kts is with max weight. At 1100lbs 69kts is best. 

Anyone with the angle of attack indicator used this to find best glide speed?

 

 

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You are lucky,

wednesday we compared the climb performance and the glide ratio of our CTSWs. I had a logger running while my companion did not. Here are the figures:

CTSW 2007, takeoff weight 1027 lb, glide at 65 kn, -12° flaps, engine idle (fixed pitch prop), winds calm (see metar). The result is a glide ratio of 13.2.

 

GlideRatio-DMASV.png

GlideRatio-DMASV-metar.pdf

Glidepath-GoogleEarth.kml

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