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Ed Cesnalis

3 Steps to determine Vx Best angle of climb.

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STEP 1 THE DRAG CURVE 

The Drag Curve (Thrust Required)

An aircraft has two types of drag, right? Induced drag, which is created by lift, and parasite drag, which is caused by air friction and some other things. 

Check out this diagram below - it shows induced drag, parasite drag and total drag (the sum of the two):

drag-curve.thumb.jpg.3b48c7cc7c8f7b5b81f2c6f440caaa38.jpg

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STEP 2 (TA) THRUST AVAILABLE

Your propellor can generate the most thrust at a slow speed. Why? Because it's at a high angle of attack. As your airspeed increases, the propellor's angle of attack decreases, and it can generate less thrust.

Look at the diagram below - it shows your total drag (the thrust required for level flight), and it shows your maximum thrust available

thrust-available-chart.thumb.jpg.b6529c4653aa9fd8d2bc9ea1cffd1723.jpg

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STEP 3 FIND Vx ON YOUR CHART

The amount of power you have available to climb is the difference between the thrust line (blue) and the drag curve (black). But, if you measure it, this doesn't happen at the lowest point of the curve!

The largest space between thrust available and thrust required happens a little to the left of that point. Why? Because of the slope of the thrust available line. That's your Vx speed. Check out the animated GIF below:

thrust-available-animation.gif.dbf6ad214c0cfb068daa431ad1ee26be.gif

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1 hour ago, Ed Cesnalis said:

STEP 3 FIND Vx ON YOUR CHART

The amount of power you have available to climb is the difference between the thrust line (blue) and the drag curve (black). But, if you measure it, this doesn't happen at the lowest point of the curve!

The largest space between thrust available and thrust required happens a little to the left of that point. Why? Because of the slope of the thrust available line. That's your Vx speed. Check out the animated GIF below:

thrust-available-animation.gif.dbf6ad214c0cfb068daa431ad1ee26be.gif

In step 3 it is the amount of thrust not power that you have available. Climb at Vx has to do with reserve thrust. Climb at best rate has to do with reserve power.

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14 minutes ago, Tom Baker said:

In step 3 it is the amount of thrust not power that you have available. Climb at Vx has to do with reserve thrust. Climb at best rate has to do with reserve power.

Yes exactly. Notice the chart is showing lbs not hp. Good correction

 

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Did you notice that what you posted contradicts some of your earlier statements? Like, "Steep climb performance means a high rate of velocity that includes a large vertical component."  when in fact you chart show that best angle of climb comes at a lower velocity.

Why don't you go afead and post the Vy information from where you got the Vx info?

http://www.boldmethod.com/learn-to-fly/performance/vx-vy/

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I'll stick with that definition of steep climb performance.

Steep climb performance only comes at a low forward velocity but when you add in the large vertical component its a high rate.  

Posting the Vy stuff doesn't lead to a conclusion about what flap settings on our CTs have the steepest climb.  I think the closest we are going to get is going to be based on the chart above.

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1 hour ago, Ed Cesnalis said:

  I think the closest we are going to get is going to be based on the chart above.

 
I don't know. I think that Flight Design engineers using sophisticated and calibrated equipment might get a little closer.

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11 minutes ago, Tom Baker said:
 
I don't know. I think that Flight Design engineers using sophisticated and calibrated equipment might get a little closer.

The ultimate point of this discussion is to challenge FD's numbers, at least some of them. Calibrated equipment measures results where we are discussing theory.

This is from my POH and its wrong, they advise the wrong flaps that could put me short of a runway I otherwise have made.

Capture.thumb.PNG.56d4459ce9780af61a1246f1311d8954.PNG

 

I learned this was wrong from someone else that posts here or posted here, I forget who it is now I just know I went out and tested it and it was no contest.  

Here's the challenge and I took it and became a believer, I'm going to confirm it this weekend it has been a while.

Challenge:   

  • Set up a glide to at -6 - use what feels like best glide
  • Follow instructions above and got to best glide at 63kts 15* flaps
  • Maintain the target that you had made at -6

 

Testing this I was unable to come anywhere close at zero at 15.

I'll test again this weekend, its a persuasive test.  If the books wrong about flap setting for best glide it could be wrong about flap setting for best climb.    

 

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Andy   

This discussion has my interest and I hear what Ed's saying and think there's some merit in it, but I have a lot of questions since the relationships are complex.  I do intend to dig into this but it'll take me a while; I've got a lot of things on my plate and there simply isn't much in the literature about the use of reflex flaps and we never talked about them in any of my engineering courses.  I agree there's a better config than 15 flaps at 63 for engine out; but it's also possible they assumed you were performing a 15 degree flap takeoff (normal for our CT's) and were talking about that case in the quote above (in which case transiting back to minus 6 would not be the thing to do).  Someone involved with it would have to say. While the overalll thrust of the information in what Ed posted is correct, like a lot of things on the Internet concerning performance and aerodynamics, the devil is in the details. While power and thrust required are driven by the drag curves, it is not actually correct to superimpose thrust or power available curves over them and make an analysis; the correct analysis is a comparison of power available vs power required or thrust available and thrust required.  They are both dependent on drag but other terms are also involved, so the shapes of the curves are similar but not the same. 

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8 minutes ago, Andy said:

While power and thrust required are driven by the drag curves, it is not actually correct to superimpose thrust or power available curves over them and make an analysis; the correct analysis is a comparison of power available vs power required or thrust available and thrust required.  They are both dependent on drag but other terms are also involved, so the shapes of the curves are similar but not the same. 

Andy,

Thanks a ton for contributing.

I'm sure what you say above is true but it wasn't until I thought about the animation above that I advanced my understanding and see I was jumping to conclusions that -6 has a steeper climb simply because of a TA TR comparison, comparison.

The animation allows me to visualize a comparison and I see that either result is possible even if done correctly.

I also now see now that I am bias towards the -6 steep climb because its faster Vx better matches the higher speeds I use in the high sheer environment I fly in.

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@Tom Baker

My thinking was on the right track but too simplistic.  I see now that either result is possible.  Quote below is what I should have been saying.  

Quote

Clean configuration does give best speed, obviously, due to reduced drag, and should also give best rate for the same reason... but not necessarily best angle, because the extra lift the flaps provide may add more vertical speed than you lose from the additional drag, especially at lower speed (drag increases with the square of airspeed, remember)

And then we have to factor in that the power of the aircraft will change whether you want to climb at a higher angle at a lower airspeed, or a shallow angle at a higher airspeed.

 

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WmInce   
56 minutes ago, Ed Cesnalis said:

. . . “I also now see now that I am bias towards the -6 steep climb because its faster Vx better matches the higher speeds I use in the high sheer environment I fly in.“. . .

Ed,

What did you mean by “faster Vx?”

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3 minutes ago, WmInce said:

Ed,

What did you mean by “faster Vx?”

Vx at zero for my SW is 66kts. Vx for -6 is faster guessing 70kts.  If I have to get below 75kts I exit with energy in reserve so the -6 Vx speed is the closest thing I have to work with, the lower flap settings at Vx are well below my minimum speed for wind sheer.

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My POH for the CTLS says Vx is 61 CAS at 15 degrees, and 66 at 0 degrees. They don't quote one for -6 since they must think you would be mad to try it, but it would be higher. The same is true for Vy which is quoted as 67 CAS (15), 73 (0), and 78 (-6). That's all I know....

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On 9/29/2017 at 9:36 PM, Ed Cesnalis said:

The ultimate point of this discussion is to challenge FD's numbers, at least some of them. Calibrated equipment measures results where we are discussing theory.

This is from my POH and its wrong, they advise the wrong flaps that could put me short of a runway I otherwise have made.

Capture.thumb.PNG.56d4459ce9780af61a1246f1311d8954.PNG

 

I learned this was wrong from someone else that posts here or posted here, I forget who it is now I just know I went out and tested it and it was no contest.  

Here's the challenge and I took it and became a believer, I'm going to confirm it this weekend it has been a while.

Challenge:   

  • Set up a glide to at -6 - use what feels like best glide
  • Follow instructions above and got to best glide at 63kts 15* flaps
  • Maintain the target that you had made at -6

 

Testing this I was unable to come anywhere close at zero at 15.

I'll test again this weekend, its a persuasive test.  If the books wrong about flap setting for best glide it could be wrong about flap setting for best climb.    

 

 

My testing shows a significant glide range advantage to using -6° flaps at 78kt, over 15° at 63kt.  It does feel fast, but you cover ground quickly, the drag profile is lower, and you can select flaps appropriate to the landing site once it's made.  I think if I were engine out at 15° on takeoff I'd just leave it alone and do what needed to be done, giving myself plenty of margin to make a parachute deployment decision if the landing was not working out.  But for me that statement applies to any off-airport landing; if the landing is in doubt, I'm riding the silk elevator for a better chance of survival. 

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JLang   
11 minutes ago, FlyingMonkey said:

 

My testing shows a significant glide range advantage to using -6° flaps at 78kt, over 15° at 63kt.  It does feel fast, but you cover ground quickly, the drag profile is lower, and you can select flaps appropriate to the landing site once it's made.  I think if I were engine out at 15° on takeoff I'd just leave it alone and do what needed to be done, giving myself plenty of margin to make a parachute deployment decision if the landing was not working out.  But for me that statement applies to any off-airport landing; if the landing is in doubt, I'm riding the silk elevator for a better chance of survival. 

Andy, I believe we touched on this on another topic, but it's my belief/guesstimate, based on interpolating info from both the CTSW and CTLS documentation (what little there is) that CTSW Vy (-6) is about 83kt and Vy (15) is 63kt, at gross weight.  If I reason correctly, Vy should be close to if not exactly best glide, and 78kt should be right at Vy for your typical, less-than-gross weight, so that makes sense.  Out of curiosity, did you try 15 flaps at something less than 63kts, assuming you were less than gross weight?  I would expect the same result, though maybe less dramatic?

Related to this, I'm thinking that if the goal was not max glide distance, but to maximize time aloft -- say, if the field was made, but to gain extra time to restart, make radio calls, etc. -- then flaps at 15 and ~63kt would be more suitable, right?

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34 minutes ago, JLang said:

Andy, I believe we touched on this on another topic, but it's my belief/guesstimate, based on interpolating info from both the CTSW and CTLS documentation (what little there is) that CTSW Vy (-6) is about 83kt and Vy (15) is 63kt, at gross weight.  If I reason correctly, Vy should be close to if not exactly best glide, and 78kt should be right at Vy for your typical, less-than-gross weight, so that makes sense.  Out of curiosity, did you try 15 flaps at something less than 63kts, assuming you were less than gross weight?  I would expect the same result, though maybe less dramatic?

Related to this, I'm thinking that if the goal was not max glide distance, but to maximize time aloft -- say, if the field was made, but to gain extra time to restart, make radio calls, etc. -- then flaps at 15 and ~63kt would be more suitable, right?

Yes, I think you are right about all of that.  I didn't test with 15 at less than 63kt, but I think that would give less glide distance than 63kt would at that setting.  Possibly more time aloft though, I'd have to test that to know.

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45 minutes ago, JLang said:

Andy, I believe we touched on this on another topic, but it's my belief/guesstimate, based on interpolating info from both the CTSW and CTLS documentation (what little there is) that CTSW Vy (-6) is about 83kt and Vy (15) is 63kt, at gross weight.  If I reason correctly, Vy should be close to if not exactly best glide, and 78kt should be right at Vy for your typical, less-than-gross weight, so that makes sense.  Out of curiosity, did you try 15 flaps at something less than 63kts, assuming you were less than gross weight?  I would expect the same result, though maybe less dramatic?

Related to this, I'm thinking that if the goal was not max glide distance, but to maximize time aloft -- say, if the field was made, but to gain extra time to restart, make radio calls, etc. -- then flaps at 15 and ~63kt would be more suitable, right?

your belief numbers look very good to me.  I relate Vy and best glide speeds and Vx and minimum sink speeds.  

I'm guessing min sink is determined like Vx and is possible that any given flap setting wins but not a given.

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Without testing 15° flaps may give minimum sink instead of best glide. In some cases more time to set up for a landing is what one needs instead of having to travel further to get to a landing site. You have to remember that something may have got lost in translation, as many things did back in the early days of the CT's.

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JLang   
52 minutes ago, Ed Cesnalis said:

I relate Vy and best glide speeds and Vx and minimum sink speeds.  

 

Right -- any reason why Vy for best rate should not always be speed for best glide, and Vx for best angle should always be speed for minimum sink, for any airplane?

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