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More climb, same cruise

Heretic

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Is there a table or method to improve climb rate without touching the drag and thrust?

I've got a Learjet 35A that I've made to fly on the numbers in cruise, with adjusted drag, lift and thrust figures, but climb performance fizzles out way too quickly compared to the POH tables, even falling out of the ball park at higher altitude.

Since table 404 governs lift by AoA, I'm considering dividing it into a "cruise" portion, using the established slope and a "climb" portion with a steeper slope for more lift, but I'm not sure if that's the right way to go.

Any tips?
 

Roy Holmes

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Is there a table or method to improve climb rate without touching the drag and thrust?
There is no table.
Do you have a chart of thrust versus altitude? For both the real airplane and your model.
Usually when performance fizzles out at altitude it is because thrust is too low.
I seem to remember quite a few posts on that topic.
Basically to climb you need excess energy and you will not get that by artificially increasing lift, because that will use up energy. Why? because lift creates induced drag.
It is also a fudge and you absolutely do not have to fudge anything in the sim except supersonic thrust.
If you want to PM your air file and aircraft.cfg I can have a good look for you and come up with suggestions

Roy
 

Heretic

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For the .air file tables, I made assumptions for mach 0 at sea level and then pulled an operating point from a Youtube video (very scientific and accurate, I know). That video was used for engine instrument indications and setting up drag and thrust. So at mach 0.75 and 43000 ft, I have a static set point now.


There's a thrust chart for a TFE731-2, but it doesn't reflect the 3500 lbs static thrust.
http://onlinelibrary.wiley.com/doi/10.1002/9780470117859.app4/pdf
And there's this table:
https://books.google.de/books?id=bSq-cEf0EWsC&pg=PA134&lpg=PA134&dq=tfe731-2+thrust+vs+altitude

Not quite sure if that's usable.
 

Roy Holmes

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It is one thing to have real world data and another to know that you are getting the same results in the sim.
Dead easy to find out with a performance test program.
Without test data you are not going to find any answers.
Your google book thrust data at sea level is static gross and at 40K is net. Not a straight comparison
Roy
 
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Heretic

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You're right, the chart is nearly unusuable. At least it confirms that my idle thrust is realistic.


Anyway, I added some intermediate steps for some mach numbers to the thrust table and shifted each upward progressively (more shift at lower mach) to de-linearize the falloff with a change in delta2. Intended to make things less linear.


More importantly though, I started to read my "Climb to altitude" chart differently. The columns list gross weights while the rows contain altitudes. In the fields, one can read time, fuel (not important since FSX has this way off by default anyway) and distance. If I interpret the "weight" column as "weight at brake release" and then fly a maximum climb profile, I can attain the times listed in the charts. I used 270 KIAS as a ballpark figure below mach transition (29000 ft) and then stuck to around mach 0.65 for the remainder to cruise altitude while keeping things around the maximum N1 chart for a given ram/total temperature and altitude.

So I guess the whole issue was just a case of "dumb pilot".
 

jx_

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Bjoern,

There is one thing that the guys at FS left out that will cause this: Mach effects on angle of attack. At 30,000 feet, the CL vs Angle of Attack table compresses and so the airplane flies at much lower angles of attack than it does at the same weight and airspeed at sea level; while still stalling at the same point on the compressed scale. The stall angle of attack reduces by roughly half, thus reducing drag for any given AoA. In the sim, an airplane at 30,000 feet will still fly at 3 degrees AoA instead of 2 degrees in cruise, or 10 degrees AoA during climb, even though that should be around 6 degrees by that point. So FS Developers like us end up seeing too much drag in our cruise numbers and try to compensate to over come this. Since induced drag is not linear (it becomes a factor), the climb will require more power than level cruise to get accurate compensated performance. I was able to overcome the issue by using the AoA vs Mach, CD0 vs Mach, and Mach Drag tables; but the airplane will not stall at the correct speeds in this configuration (too much available lift).
 

Roy Holmes

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At 30,000 feet, the CL vs Angle of Attack table compresses and so the airplane flies at much lower angles of attack than it does at the same weight and airspeed at sea level; while still stalling at the same point on the compressed scale. The stall angle of attack reduces by roughly half, thus reducing drag for any given AoA.

I've been busy on other things, but those statements kept nagging at me.
As everyone knows I'm a duffer when it comes to big airplanes, but for the types I deal with the statements are not borne out by the performance data.

I looked at two types the F-4 and the Mirage F-1.
The F-4 at 1g and 37500 lBS AUW has the following Calibrated Airspeed (CAS) when climbed at 7° AOA.
Sea level and 250 CAS/0.378M
20K 290CAS/0.631M. If at 250CAS AOA should be 8 at 1g
30K 285CAS/0.754M. If at 250CAS AOA should be 8.7 at 1g
40K 280CAS/0.909M. If at 250CAS AOA should be 8 at 1g
These numbers imply a slight increase in AOA when flying at the same CAS as at SL but at altitude. The statement said the opposite
Mach effects on AOA begin at around 0.75M but are relatively flat until well supersonic.
Stall AOA 20° and 1g is at:
Sea Level and 150CAS
20K and 150CAS
30K and 155CAS
40K and 165CAS
These numbers imply little change in stall speed or AOA. The statement said AOA would be halved

The Mirage F-1 at 1g and 10000 Kg AUW has the following characteristics when climbed at 5° AOA.
Sea level and 271 CAS/0.41M
20K 290CAS/0.63M
36K 288CAS/0.86M
40K 280CAS/0.91M
Mach effects on AOA begin at around 0.85M but are relatively flat until well supersonic.
Stall AOA 16° and 1g is at:
Sea Level and 145CAS
20K and 154CAS
36K and 167CAS
40K and 164CAS
In general the same value trends as the F-4 and opposite to the statements.

The amount of g that can be pulled at limiting AOA decreases with altitude because of reduced density.
The F-4 can pull 8.5g at 440CAS and SL
The F-4 can pull 7.0g at 440CAS and 20K
The F-4 can pull 6.5g at 440CAS and 30K which is the structural limit
The F-4 can pull 5.4g at 440CAS and 40K
In all cases these occur with 20°AOA.which is a constant stall AOA irrespective of altitude.

Given the fact that airliners generally have relatively high Mach wing sections similar the the F-4 and F-1 I can not see how the statements would be valid. But then I am a duffer at big airplanes
Roy
 

jx_

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hey Roy.

Remember you are flying a highspeed supersonic wing with enhancements and vortex manipulators, not a supercritical subsonic wing!

You wouldn't see it in your planes until you approach high speed stall. FS does not model high speed stall, and that characteristic is what reduces stall AoA up high. It's not actually related to altitude, but airliners can't reach the required mach number down low.....

This is why airliners and other long slender subsonic wing designs suffer from coffins corner and mach tuck. As high speed stall gets closer, stall AoA comes down to you at the same CAS/higher TAS. Because FS doesn't model this, we can't set the correct coefficients and get the correct results. Due to compression effects the real CL alpha table is squeezed horizontally so the stall point move left as the center of pressure moves and the tips stall.

The other factor is the supercrit wing design is most efficient at it's cruise speed, which means a lower AoA at the same CAS/higher TAS. (reason for lift vs mach table)

Add all that up and you get one of the very few things in FS that can't be done correctly. If you increase Mach Lift factor, FS does NOT apply CDi to that lift (this is the heart of the issue).

If we use the real low speed CL table, the airplane will have too much up pitch and too much CDi at cruise. If I add more lift in FS tables at high mach (using lift v mach) to get the pitch as indicated in performance charts, the pitch attitude is correct but it will have too little drag. If I set the drag correctly my pitch is off by a large amount. If I set both, I have too much drag in the climb because I boosted the low AoA drag setting to compensate (probably using drag v mach).

The additional lift from the lift factor v mach is not adding CDi.

you may be able to use ram drag as a fudge since power setting is usually the only thing that changes at high altitude from climb to cruise. Otherwise you have to do what I do, come up with a reasonable negligible fit of the drag polar and decide if you care about high speed stall.


Does the F4/Mirage suffer from Coffin's corner at some mach?
 
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