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Cruise Climb


Hotel26
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Introduction

Before taking to the air, any pilot will desire to know the following about the equipment to be used:
  • how far it can fly
  • how high
  • how fast
  • how much fuel it will require
  • and what is the most efficient speed & altitude to fly
 
Equipped with this information, the pilot may then choose to fly:
  • in the shortest time (speed)
  • via the most scenic route (altitude and route)
  • maximizing the range (efficiency)
  • with a reduced fuel load
 
Before any others, the first datum to determine must be maximum altitude.  This is because one must explore the whole altitude range to determine the most efficient cruise altitude.
 
Our  primary metric for efficiency, η, will be η = speed / fuel-consumption.  [This is effectively/approximately, "the maximum distance that can be covered given the fuel available.]
 
(The above introduction intentionally omits the complications of evolutionary cruise trajectories due to decreasing fuel load and those introduced by multi-mode engines which may introduce a choice of two power profiles that may be chosen or blended.)

Conclusion

The question now posed is, "how to fly an unknown machine to its maximum altitude?"  Seems simple, doesn't it?
 
In truth, the means to achieving the highest possible altitude can often be quite surprising.  In my next post, I will give an outline of one such technique.
 
I do know of others, however, so if you'd like to share your own favorite method, by all means, please post here.
 
Blue skies!
 
Edited by Hotel26
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Posted (edited)

Climb to Maximum Altitude

In the technique I employ, that I will now describe, I use KER and Atmospheric Autopilot.  Please note the following instrumentation:

G6KLBKA.jpg
 

Instruments we are observing:

  1. Altitude
  2. Vertical Speed
  3. Pitch (angle of body incidence to the horizon)
  4. Fuel Flow
  5. Airspeed
  6. and commanding Vertical Speed (Rate of Climb, m/s), via Atmospheric Autopilot.

Flight Phases from take-off:

  1. good rate of climb, whilst keeping airspeed increasing, is primary, as we know we have surplus energy -- at any altitude much lower than the theoretical maximum
  2. increasing airspeed, whilst maintaining a positive rate of climb, is primary, so as not to get trapped on the back side of the power curve
  3. approaching the level-off, Pitch is primary and should desirably approach zero asymptotically -- but the final value will depend on the design setting of wing Angle of Incidence

Stratagem:

In order to minimize parasitic body drag, maintain small/zero angle of Pitch incidence between the longitudinal axis of the body and the prograde direction.  You could think of this as the Angle of Attack of the body rather than the all-important AoA of the wing, usually spoken of.  Now although the attitudinal Pitch is relative to the horizon rather than to prograde, we note that at or near maximum altitude, the climb rate is very minimal when compared to horizontal airspeed: prograde is very nearly identical with the horizon.  Therefore we can substitute Pitch for Body Incidence.

Scenario:

In the screen shot above, this little 'plane (Drosophila, the "common fruit fly"; having a maximum altitude of at least 9800 meters), is still climbing at a healthy 4 m/s, commanded (4.01 actual).  Pitch shows 0.01010 degrees, which is very close to the "null point".

As the climb continues, the airplane performance will ordinarily decrease, most naturally.  Atmospheric Autopilot will respond by raising the nose in order to maintain the commanded climb rate.  The increased resulting drag may soon have an effect upon speed, which we watch constantly with a view to never permitting it to decrease.

Tactics:

So when the Pitch is positive and increasing, we should consider lessening the climb rate as necessary to send the Pitch rate back downward toward zero.  We may not allow this situation to worsen.  The closer we get to maximum altitude, the less (excess) performance we can expect and the more sensitively we must respond.

When the Pitch is negative, it indicates that the airplane is likely enjoying a surplus of energy and we can consider commanding an increase in the climb rate. 

As long as the Pitch is positive and decreasing toward zero, or negative and increasing toward zero, we are in good shape.

The closer we approach maximum altitude, the more critical body drag becomes and so our feedback against it.  At lower altitudes we can afford to be more aggressive.

Keys:

  1. "In general", I strongly resist allowing speed to decrease or the climb rate to go negative.
  2. some airplanes (those with "strong personalities"!) may give the appearance of "topping out" but -- while speed or climb rate (or both) are only vanishingly increasing-- catch a "second wind" and burst out of the gate once passing some hidden threshold.  (The feeling is like squeezing through a choke point.)  This can especially happen in a second phase of Cruise when fuel load depletion unleashes a new maximum.  (My Hexapen Deluxe is one of those, as I recall.)
  3. There is a sense, I am sure, in which this technique is "nothing new".  It may be quite like what an experienced pilot might do, "seat of the pants", purely from a few instruments and, particularly, the Artificial Horizon.  But I find the trend in the Pitch indicator is much more sensitive and that this whole approach is more amenable to explanation and comprehension.

Take-Away:

If, after finding an aircraft's maximum altitude, the final Pitch is positive, it may indicate that it is profitable to return to the SPH and adjust wing incidence to be commensurately greater.  A negative Pitch may indicate the opportunity to decrease the wing incidence, which will offer new dividends.  Either of these incur the need, of course, to once more fly the modified airplane and (re-) determine its maximum altitude/performance.

Disclaimers:

I have knowingly simplified aspects of this Introduction in order to make an excitingly-complex subject more approachable.  Experts in the field will understand already the simplifications (omissions) made.

I do believe though that this technique gives an understandable and metric approach to "climbing Everest" that will equip the newly-minted with a basis to develop their own intuitive comprehension of the rigors of this particular sport.

In particular, the trans-sonic regime and the usage of multi-mode engines (particularly the engine I have painfully grown to dearly love over time, the Panther), often demand some skillful optimization of technique.

Finally, I am aware that there are alternative techniques available and I am certainly most happy about opening a forum here for pilots, both experienced and more novice, to "compare notes".

Blue Skies!

Spoiler

[This one is for you, Spacejet!   You are missed.]

 

Edited by Hotel26
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On 7/23/2022 at 1:10 PM, Hotel26 said:

some airplanes (those with "strong personalities"!) may give the appearance of "topping out" but -- while speed or climb rate (or both) are only vanishingly increasing-- catch a "second wind" and burst out of the gate once passing some hidden threshold.  (The feeling is like squeezing through a choke point.)  This can especially happen in a second phase of Cruise when fuel load depletion unleashes a new maximum.

I have been dealing with a rather severe case of this ailment this weekend.

Ever since the Juno was added to the game, back in 1.0.5 if I remember correctly, I have wanted to build myself a griffinfly (long extinct giant dragonfly). My game saves are full of half-done examples, but somehow I never got one to a point where I was happy with it. Yesterday, the bug bit me again, so to speak: a bit of random fooling around in the SPH started to look suspiciously insect-like. For the first time in all those attempts I got one that combined looks and performance (or that doesn't auto-destruct on loading... story for another day/thread). Except... it's stuck at the very end of the transsonic region. It keeps oozing up to that edge, but burns through its fuel load before punching entirely through.

There's a few very obvious things I could do to resolve it - drag reduction, involving removal of radially attached parts etc. But that would kill the design. I could add additional engines, but one of the particular satisfying details was the exact nr of engines it has now. So here I've been, self-restricted in what I can change, spending the entirety of yesterday and today's playtime launching and tweaking and relaunching to try and get it to break free.

Anyway, long story short: a silly anecdote that doesn't really contribute much to this very interesting dissertation, but that particular phrase really touched a nerve today. :D Keep up the good work, and I hope to revisit this thread soon.

 

Spoiler

cRshQEK.png

ai0A0e4.png

 

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Posted (edited)

It's not often (never) that I quote so-called "real life" in regard to KSP, but in case it soothes:

  1. heavy, long-distance wide-bodies very typically climb to an initial cruising altitude and then request higher once some of the fuel load has been expended.  (Nothing at all like a two-stage rocket but that's my metaphor for today.)
  2. in the very early days (before the prevalence of ATC), cargo pilots cruised in a continuous but slight climb instead of using the modern "step-ladder".  I imagine their primary control reference was to never decrease airspeed while maintaining some small, positive climb rate.

re: your Hidden Contents: exquisite entomology!

Spoiler

Pardon my humor, but I see a use for this immediately, involving  an insecticide called Chainsaw[tm]: it looks too real!  :)

 

Edited by Hotel26
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On 7/20/2022 at 9:34 PM, Hotel26 said:

I do know of others, however, so if you'd like to share your own favorite method, by all means, please post here.

My method is much simpler, assuming the plane is my own design. Just fly prograde and see at what speed altitude it stabilizes.  If the cruising altitude is too low I add more wing area / wing incidence.

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Posted (edited)
2 hours ago, TheFlyingKerman said:

Just fly prograde and see at what speed altitude it stabilizes

Yes!!  I've heard of this method.

Let me riddle you this in two parts:

  1. in which direction do you conduct the test?
  2. can you explain why it works?
  3. Bonus question: are there any special things you have to do in design to make your planes "obey" this trajectory?  (As I know that many of mine do not...!)

Thanks for chiming in!

P.S. I read this a bit hastily but notice now you don't mention what mode you have SAS in.  Are you using SAS or hand-flying this?  If SAS, which mode?

Edited by Hotel26
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That's a lot of deep dive questions for a game in which you can make something flying out of 5 parts and one rule of thumb (CoM/CoL arrangement). As the saying goes, if it flies, it flies.

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Posted (edited)
2 hours ago, The Aziz said:

one rule of thumb

Well, this has to be acknowledged.  There are a lot of simpler ways to get high!  (Just about anything is simpler.)

So let's state the baseline: "take-off and climb, and keep climbing until she won't climb any more, and when you get to there, that's your maximum altitude."  That's the simplest way I know of.  :)

So I'll spill the beans on another technique I've heard of.  "Fly East on SAS Orbital."  This may not work initially for some planes (I'm not an expert on why not), so you may have to do the initial climb yourself and then switch to this.  (I kinda think @TheFlyingKermanmight have been onto something like this...?)

It works, I think, because of the following:

We5lwE0.png

If the airplane at the circle (Origin) on the left is flying one of the blue lines (easterly) in the atmosphere, this corresponds to the SAS Sfc direction.  The difference between Sfc (atmospheric reference) and Orbital, is that Orbital adds the easterly spin of Kerbin (175 m/s?) horizontally to your velocity vector.    If you allow SAS to fly your airplane aiming for the SAS ORB prograde, you'll notice that:

  1. in the case that you are climbing, ORB will tend to aim lower, and
  2. in the case that you are descending, ORB will tend to pull you higher.

As a result, SAS ORB in control has a moderating influence providing a neg feedback loop to permit the airplane to balance its surplus energy to its climb rate.  This will asymptotically climb to the point where there is no longer any surplus energy, and therefore the aircraft will no longer (sustainably) climb higher.

Once it takes over (properly), this method is automatic and hands-off.  While it only works in a precisely easterly direction, it does yield your theoretical maximum, which is what you are trying to improve in the design/testing process.  (It being automatic (over and over again) is a boon to getting though numerous cycles without tedium.)

Once you finish designing your aircraft, you will now have to fly the climb manually when on any other course.

Edited by Hotel26
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38 minutes ago, Hotel26 said:

"Fly East on SAS Orbital."

That neatly summarizes my BASIC rapid-iteration flight-test technique:

  1. Launch to runway.
  2. Set SAS ORB PRG and fine control.
  3. Punch the throttle and stage to ignite engine(s).
  4. Observe the plane take off and climb to its current natural cruise altitude and speed. (*)
  5. IF observed performance = satisfactory END.
  6. ELSE revert and make adjustments as needed according to observations.
  7. GOTO 1.

*: this step is usually entirely hands-off and I can alt-tab to other things, with intermittent switching and observation at key points of the flight envelope. I'll sometimes have a second instance of KSP running along-side this, already implementing the adjustments I think may be needed, or moving ahead with other details of the design.

Flying on SAS ORB PRG may not have any further practical use when actually using planes for 'real' flight, and it may not even be an optimal climb strategy (although my testing suggests it's quite close to it - works quite well for spaceplane ascent to EQ orbit too), but it's the proverbial heaven-sent during the design process.

 

38 minutes ago, Hotel26 said:

As a result, SAS ORB in control has a moderating influence providing a neg feedback loop to permit the airplane to balance its surplus energy to its climb rate.

Pretty much. It's a dampening influence on the design's natural porpoising, letting it reach equilibrium -often a lot- faster. And it's entirely hands-off. I can recommend it.

 

EDIT and IMPORTANT NOTE: this only really works with planes that are built to produce lift while pointing at pure prograde. In KSP, due to the lack of aerofoil modeling, this requires the plane's wings to have at least a few degrees of positive angle of incidence compared to the fuselage. Plane designs that use wings parallel to the body -the way the game compels players to build- WILL NOSEDIVE off the runway if you let them take-off by themselves on SAS ORB PRG.

I tend to use 5 degree AoI because I like to build with best-ROI-for-least-effort techniques - 5 degree is exactly one fine snap rotation in the stock game. The real optimum is often somewhere around 3-4 degrees.

Edited by swjr-swis
IMPORTANT NOTE
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7 hours ago, swjr-swis said:

use 5 degree AoI because I like to build with best-ROI-for-least-effort techniques - 5 degree is exactly one fine snap rotation in the stock game. The real optimum is often somewhere around 3-4 degrees.

This is an example of a 90/10 technique.  90% of the result for 10% of the effort.  The lift to drag curve absolute peak when supersonic is remarkably shallow and broad.  l/d ratio absolute peak is at an AoA of 3.81 degrees at Mach 2.5

Using a value of 5 instead of 3.81 degrees nets you a loss of efficiency of... 3.46%

5 vs 4 degrees is a loss of 3.33% at Mach 2.5
5 vs 4 degrees is a loss of 1.95% at Mach 6

So yeah, if that 2-3% is super vital, one can feel free to faf about setting up 4 degrees, but most of the time, it just isn't worth it.

For what it's worth, I often opt for slightly more wing area at a lower AoI for the superior landing flare and pitch up handling  (5 degree AoI with 3 degree flare = 60% more lift, 3 degree AoI with 3 degree flare = 100% more lift)

Edited by Lt_Duckweed
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Posted (edited)
7 hours ago, camacju said:

I use either 0 or 5 degrees because I'm too lazy to install a precise editor

We can understand that some people are allergic to mods (the way I am allergic to, oh, Real Life, :)), I guess.

Precise Editor really beats the mouse for intricate work, though, doesn't it?

Spoiler

(My only beef with it is that I get the sense it is not quaternion-based inside(?) and, critically, it reasonably often has trouble applying a rotation in one Local axis when rotations have been applied in the others...)

I found @Lt_Duckweed's view expressed above both surprising and interesting!  Thank you.

                                                                       

In my next post here, I'll make some elementary comments about determining max range, certainly nothing new for the experts, but hopefully of value to those like me, who have been toiling now for some years, learning things the hard way (only to find out "everybody" already knows that!).  :)

I really do invite anyone with anything to share on the whole, fascinating topic, loosely "aerodynamic optimizations", to speak forth!

Edited by Hotel26
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Posted (edited)
16 hours ago, Lt_Duckweed said:

Using a value of 5 instead of 3.81 degrees nets you a loss of efficiency of... 3.46%

tl;dr I think the AoA at max altitude and speed, whatever it is, is the "best" you can do for that machine, for that altitude & speed.  So if you change the wing incidence, you will still need the same AoA to fly at the same latitude and speed.  But what you will have done is adjusted the nose (at the other end of the wing incidence angle!) such that you make it match prograde.  This could have a big effect on parasitic body drag (rather than wing efficiency) and put you into a new ball game: less drag at that altitude/speed means -- more speed; more altitude.

One will also note that fuel flow of certain engines changes remarkably at certain high altitudes with only slight differences in altitude.  So climbing 200 meters higher can make a whopping difference to ultimate range.

Spoiler

I'm going to be thinking about everything you said above (not just this quote) for a while, but your remarks are addressing the efficiency of the wing and very interesting.

The slant of my thinking about angle of incidence is focused on the effect on body angle and parasitic drag.  I am told that the Mk2 format is particularly sensitive to angle to prograde.  Is that true and is it significant?

So, I would think that, if we fly a particular airplane at a certain altitude and speed, we are going to find that we need a certain angle of attack to generate lift equivalent and opposite to gravity.  Is that true?

If that's true(?), then let's say that AoA (of the wing) is 5.5 degrees.  Then it follows that where the nose points is the angle of the wing incidence lower.  Let's say the AoI is the standard 5 degrees.  That puts the nose (down from 5.5 degrees (wing)) to +0.5 degrees to Prograde.  And in level flight, this is also the horizon, giving us a pitch of +0.5 degrees.  This angle now has no effect on wing efficiency or the flatness of the curve at this speed and altitude. 

But it may (I am told) have a remarkable effect on increasing parasitic body drag.

So consider the following: if I increase the angle of (wing) incidence) to 5.5 degrees -- and if I fly the same 5.5 degrees AoA (wing!) -- then the only thing I've done is...  lowered the pitch of the nose by 0.5 degrees: down to the horizon, minimizing body drag.  We don't have curves published for this??

So experimental evidence would be key to determining whether it is or isn't worth doing (varying from designer to designer).

I will make one further claim.  It's possible and necessary to make value judgements about effort & utility.  (See above!)  In practice, no one (including myself) might want to use my technique for determining max altitude as the method to routinely fly to max altitude.  (Knowing where it is, there are ways to exploit to get there more quickly with less effort.)  I think though that understanding what's involved in the theoretical approach might open a greater appreciation enriching ordinary flight?

 

Edited by Hotel26
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On 7/27/2022 at 2:24 PM, Hotel26 said:

Yes!!  I've heard of this method.

Let me riddle you this in two parts:

  1. in which direction do you conduct the test?
  2. can you explain why it works?
  3. Bonus question: are there any special things you have to do in design to make your planes "obey" this trajectory?  (As I know that many of mine do not...!)

Thanks for chiming in!

P.S. I read this a bit hastily but notice now you don't mention what mode you have SAS in.  Are you using SAS or hand-flying this?  If SAS, which mode?

1. I use SAS surface prograde, so it works for any direction that I set up by hand.

2. It works because at the right altitude, the lift equals gravity and the plane flies level, exactly where the nose is pointing. Below that altitude the air is denser given the same speed there is excessive lift. This makes the prograde vector pointing above the nose, and the SAS prograde mode tracks the change and starts climbing. Above cruising altitude the SAS does the opposite.

3. I only build 'normal' planes with everything straight.

19 hours ago, Hotel26 said:

I will make one further claim.  It's possible and necessary to make value judgements about effort & utility.  (See above!)  In practice, no one (including myself) might want to use my technique for determining max altitude as the method to routinely fly to max altitude.  (Knowing where it is, there are ways to exploit to get there more quickly with less effort.) 

I do. I have at least a few long science flights for each science / career save, also SSTO space planes sending stuff into orbit. Fortunately my designs are multi purpose so I only need a few models to do everything.

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