Best Westerly Heading Performance Altitude on Long Hauls - Theory

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USAF-LT-G

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USAF-LT-G
So I've been trying to figure out the math here in the USA. It's pretty obvious that on an easterly heading, the winds aloft following the jet stream favor a higher altitude for quicker speeds to your destination. Example - IAS = 120 while TAS = 170 @ 11,500.

However, I'm unconvinced that the same holds true on a Westerly direction. I read some theory that you gain 2% of TAS for every 1,000 ft of climb.... which, ok, I buy that for an Easterly direction.... but I'm not convinced on a Westerly direction.

I would think that given the AOA is properly calibrated.... that an unpressurized aircraft flying below oxygen requirements, would actually fly "faster" to it's destination (fuel leaning aside) at a "lower" altitude, in a westerly direction.

Am I off my rocker here? :confused:
 
Yes, you gain about 2% TAS for a given CAS, per 1000 feet altitude, in the troposphere.

Direction doesn't matter for TAS. You can have a 200 knot jet stream in your face or calm winds. TAS is the same.

Jet stream is not always westerly and not present at every latitude, and you can climb above it with enough performance (it peters out at the local tropopause). Whether you gain or lose by climbing depends on winds aloft on that day, as well as the cost of the climb for your airplane. It's different for a jet than it is for a light piston.
 
Understood about winds aloft. Agreed "winds do change" but, as they stand right now, anything below 14,000 has pretty much consistently been going in some easterly fashion. What I'm really trying to get at..... for light aircraft (A DA-40 for example), heading west below 14,000 on a long haul (in excess of 4.5 hours)..... it "seems" as though altitude is not your friend purely from a "speed to destination" perspective. Am I wrong there?
 
You may be off your rocker but we'd need pictures or it didn't happen. Rockers are generally low speed so seatbelts shouldn't be required. If drinking heavily all bets are off.

Back to your story. Yes TAS increases with altitude for a given indicated IAS. Direction of flight is not relevant. I think your story was intended to address ground speed, GS. For some aircraft remaining low while traveling west might be better. What I have found is that it's usually a wash, as TAS increases with altitude so does the headwind. Since the ride is usually better at higher altitudes and there aren't many obstacles I'll go ahead and climb. I do have a turbocharger so I don't lose much performance with altitude. Normally aspirated aircraft will be a different story.

Moral of the story? Dunno. Find out where the aircraft you are flying is most efficient and operate there if you can. For me the book sez 8,000 ft is most efficient but not a lot of difference in the range of 6,000 to 10,000 ft.
 
Why would you think the atmosphere acts differently when you fly west vs. east? Does it know which direction you are flying?

Please tell me you're not a USAF LT who is a pilot and asking this question . . .

You're still gaining TAS as you climb - but if you are not turbocharged, then your IAS/CAS is going down as well since you are producing less power - so your TAS is not as high as it could be. . . .

Now - flying into the wind you will have a lower GS - but finding the right altitude is a matter of simply knowing your airplane, knowing how fast your IAS/CAS drops off with altitude - and what the TAS actually is - and then subtracting the wind . . .

Thats called pilot in command skills.
 
Clark1961 said:
You may be off your rocker but we'd need pictures or it didn't happen. Rockers are generally low speed so seatbelts shouldn't be required. If drinking heavily all bets are off.

Back to your story. Yes TAS increases with altitude for a given indicated IAS. Direction of flight is not relevant. I think your story was intended to address ground speed, GS. For some aircraft remaining low while traveling west might be better. What I have found is that it's usually a wash, as TAS increases with altitude so does the headwind. Since the ride is usually better at higher altitudes and there aren't many obstacles I'll go ahead and climb. I do have a turbocharger so I don't lose much performance with altitude. Normally aspirated aircraft will be a different story.

Moral of the story? Dunno. Find out where the aircraft you are flying is most efficient and operate there if you can. For me the book sez 8,000 ft is most efficient but not a lot of difference in the range of 6,000 to 10,000 ft.
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Lol - Ok.... I'm an idiot, and wow, clearly tired this morning. OMG I meant GS, not TAS. Sorry.... I did not find an example in the 2005 DA-40 book that gave a suggested altitude.

comanchepilot said:
Why would you think the atmosphere acts differently when you fly west vs. east? Does it know which direction you are flying?

Please tell me you're not a USAF LT who is a pilot and asking this question . . .

You're still gaining TAS as you climb - but if you are not turbocharged, then your IAS/CAS is going down as well since you are producing less power - so your TAS is not as high as it could be. . . .

Now - flying into the wind you will have a lower GS - but finding the right altitude is a matter of simply knowing your airplane, knowing how fast your IAS/CAS drops off with altitude - and what the TAS actually is - and then subtracting the wind . . .

Thats called pilot in command skills.
Click to expand...

Calm down.... I'm asking the question because the aircraft is not mine. I've flown it plenty, but don't feel i've honed in on the best possible GS over a long haul. I probably should have specified was looking for real life figures on the DA-40.
 
USAF-LT-G said:
jet stream favor a higher altitude for quicker speeds to your destination. Example - IAS = 120 while TAS = 170 @ 11,500.
I read some theory that you gain 2% of TAS for every 1,000 ft of climb.... which, ok, I buy that for an Easterly direction.... but I'm not convinced on a Westerly direction.
Click to expand...

The polar jet stream is between 30 and 39k MSL, the higher altitude tropical jet is 33k to 52k MSL. Winds at lower altitudes are about pressure gradients not the jet stream.
TAS increases as altitude increases as you said, but it's ground speed that dictates your travel time. Ground speed is TAS +- Wind.
 
luvflyin said:
Do you care about endurance? Or just the fastest ground speed you can get?
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In this scenario.... more about the fastest possible ground speed. There's a stop for refuel involved regardless, but I suppose, a combo (best endurance / speed) is interesting. I know that lately at altitudes between 10,000 - 13,500 gph is typically 8.5 / 8.9 leaned on a 21.3 man / 2200 rpm.
 
USAF-LT-G said:
In this scenario.... more about the fastest possible ground speed. There's a stop for refuel involved regardless, but I suppose, a combo (best endurance / speed) is interesting. I know that lately at altitudes between 10,000 - 13,500 gph is typically 8.5 / 8.9 leaned on a 21.3 man / 2200 rpm.
Click to expand...

Foreglight and Garmin Pilot both give winds aloft in graphical form. The wind varies hour by hour and from the ground to FL180, if VFR. And these reported winds most often vary in realtime as you fly. If you don't have a wind meter in the plane you could vary your altitude as you fly using one of the EFBs as a guide. The higher you fly the better fuel efficiency but picking the least wind resistance will be a moving target regardless of direction of flight.
 
It looks like figure 5.3.9 shows most efficient cruise altitude. At least that is the figure in the POH on Diamond's website.
 
While west to east air movement is the norm, on any given day the local highs and lows will determine the wind in which a GA aircraft flies. You want to get on the side of the pressure system that works for you. The clockwise high should be to your north and the counter-clockwise low should be to your south for a westerly flight. Of course, lows carry fronts and you want to avoid them.
 
cgrab said:
While west to east air movement is the norm, on any given day the local highs and lows will determine the wind in which a GA aircraft flies. You want to get on the side of the pressure system that works for you. The clockwise high should be to your north and the counter-clockwise low should be to your south for a westerly flight. Of course, lows carry fronts and you want to avoid them.
Click to expand...
That's a good reminder to fine tune a flight plan for the day of flight conditions.

Nit pick: crossing a front can vary from no-way-in-hell to where-was-the-front-I-missed-it.
 
I get queasy in hot bumpy air, so in the summer anyway I'll give up groundspeed to get to smooth cool air whenever I can.
 
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