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Why Is An A/c Engine Only Efficient At High Alt?  
User currently offlineKlemmi85 From Germany, joined Mar 2009, 210 posts, RR: 0
Posted (5 years 8 months 3 days 9 hours ago) and read 8536 times:

Hi there,
I was just wondering why an aircraft engine isn't fuel efficient in low altitude but much more at high altitudes.

I don't have exact figures now, but I think I read somewhere, that i cruise, the fuel burn is only a quarter of the amount used during takeoff. Of course, in cruise your're not at maximum takeoff thrust, but there has to be something else?!

50 replies: All unread, showing first 25:
 
User currently offlinePackcheer From United States of America, joined Nov 2008, 333 posts, RR: 0
Reply 1, posted (5 years 8 months 3 days 9 hours ago) and read 8532 times:

Basically, the air is thinner, the engine doesn't work as hard (intakes less fuel to move same amount through thinner air)


Things that fly, Girls and Planes...
User currently offlinePolymerPlane From United States of America, joined May 2006, 991 posts, RR: 3
Reply 2, posted (5 years 8 months 3 days 7 hours ago) and read 8479 times:

I don't know about more or less efficient (efficiency measured as thrust/fuel ratio).

Airplane is more efficient because at higher altitude it encounters thinner air, thus less drag. It needs less thrust to move the airplane as compared to lower altitude.

Cheers,
PP



One day there will be 100% polymer plane
User currently offlineKlemmi85 From Germany, joined Mar 2009, 210 posts, RR: 0
Reply 3, posted (5 years 8 months 3 days 6 hours ago) and read 8432 times:

So it's all about the thinner air and thus resulting less drag ?

Okay, if that's what it's all about, thank you very much guys  Wink


User currently offlineSoku39 From United States of America, joined Nov 2000, 1797 posts, RR: 9
Reply 4, posted (5 years 8 months 3 days 6 hours ago) and read 8429 times:

No, that's not it, much of it has to do with how much colder it is. Generally they try to fly right up until the tropopause, once you get into the stratosphere it starts to warm up again. All combustion engines are more efficient at colder temps.


The Ohio Player
User currently offlineLHRjc From Netherlands, joined Apr 2006, 1964 posts, RR: 19
Reply 5, posted (5 years 8 months 3 days 5 hours ago) and read 8406 times:

In a jet aeroplane fuel flow is directly related to the thrust produced. Jet thrust is limited by 2 things, the TGT (Turbine Gas Temperature) limit and RPM limit. As the throttles are advanced both RPM and TGT increase, whichever limit is hit first will dictate how the maximum thrust varies with temperature. Therefore, when the air is hot, the TGT limit is hit 1st, whereas when the air is cold, the RPM limit is hit 1st. Jet thrust will decrease in the lower density air found at higher altitudes.

So, we want to operating in the most efficient RPM range, (80%- 97%) and by flying up high for low temperatures and the best thermal efficiency which is helped by the low intake temperature.

In the cockpit we can see 1 of 2 (or both) gauges, EPR and/or N1. EPR is the ratio of jet pipe pressure to inlet pressure, whereas N1 is the speed of the fan. In an A320 for example, the IAE engine is controlled by EPR changes, the CFM controlled by N1 changes.

Hope that helps...



"Our 319's are very reliable. They get fixed very quickly."
User currently offlinePlaneWasted From Sweden, joined Jan 2008, 538 posts, RR: 0
Reply 6, posted (5 years 8 months 3 days 4 hours ago) and read 8343 times:

Isn't jet engines less efficient when the airplane is flying at slower speeds? Planes usually fly slower at low altitudes...
I know that the Concorde's engines actually were the most efficient of all jet engines when the plane was new. But only at supersonic speeds.

Edit: Or is the the cold temperatures that make the engines more efficient? According to the thermal physics I've studied larger temperature differences between intake air and engine core gives higher efficiency.

[Edited 2009-04-20 12:51:55]

User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 7, posted (5 years 8 months 2 days 22 hours ago) and read 8251 times:



Quoting Klemmi85 (Thread starter):
I was just wondering why an aircraft engine isn't fuel efficient in low altitude but much more at high altitudes.

It's considerably less efficient at high altitudes. Several posts above are mixing efficiency, thrust, and fuel flow. At high altitudes the engine is less efficient, producing less thrust, and has much lower fuel flow. The overall fuel burn for a particular trip is minimized at high altitude because you can go faster for a particular amount of thrust due to lower drag.

Quoting Klemmi85 (Thread starter):
I don't have exact figures now, but I think I read somewhere, that i cruise, the fuel burn is only a quarter of the amount used during takeoff. Of course, in cruise your're not at maximum takeoff thrust, but there has to be something else?!

The major change with altitude is thrust...as you ascend, density and pressure are dropping, so thrust drops. Fortunately, drag drops off faster than thrust, so your overall burn drops off considerably at altitude. This isn't because the engine is gaining efficiency, it's because the engine isn't sucking as much fuel because required thrust is dropping.

Quoting PlaneWasted (Reply 6):
Isn't jet engines less efficient when the airplane is flying at slower speeds?

If you're going very very slow, the pressure at the fan face is is lower than atmospheric because the fan has to suck air in. As you speed up, you start to get "ram pressure" in the inlet. However, for subsonic aircraft, ram pressure is a fairly negligible effect up until you hit the transonic region.

Quoting PlaneWasted (Reply 6):
Edit: Or is the the cold temperatures that make the engines more efficient? According to the thermal physics I've studied larger temperature differences between intake air and engine core gives higher efficiency.

This is complicated by the fact that they Brayton cycle, as implemented in a jet engine, isn't actually closed. The temperature recover happens outside the engine (as the exhaust returns to atmospheric). But you're absolutely right that any thermodynamic cycle gets better as the difference between the hot and cold temperatures goes up.

Tom.


User currently offlineKlemmi85 From Germany, joined Mar 2009, 210 posts, RR: 0
Reply 8, posted (5 years 8 months 2 days 15 hours ago) and read 8114 times:

Aaah  Wink I knew there was more than just thinner air  Wink Thank you guys for explaining.

User currently offlineVinniewinnie From United States of America, joined Nov 2005, 803 posts, RR: 0
Reply 9, posted (5 years 8 months 1 day 14 hours ago) and read 7915 times:

So could manufacturers make more environmentally friendly aircraft by optimising the design for lower altitudes?

Is there a possibility that hasnt been searched for/discovered?


User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 10, posted (5 years 8 months 22 hours ago) and read 7800 times:



Quoting Vinniewinnie (Reply 9):
So could manufacturers make more environmentally friendly aircraft by optimising the design for lower altitudes?

I'm not sure that would work. Going lower requires more thrust, which means more fuel...the change in fuel flow with thrust is a lot bigger than the change in fuel flow with efficiency. The fundamental problem comes about because of the thrust range that's required...enough thrust for takeoff will always be vastly overpowered for cruise, which means the engines are running off-maximum at cruise with the related efficiency hit.

Flying slower could have fairly huge benefits, since drag goes with speed squared, but there are other costs involved that go up with decreasing speed (crew time, asset utilization, etc.).

One interesting option would be much longer runways, which would reduced the amount of thrust required, which would allow the engines to run harder at cruise, which would improve their efficiency. That that's probably a logistical nightmare.

Tom.


User currently onlineZBBYLW From Canada, joined Nov 2006, 1993 posts, RR: 6
Reply 11, posted (5 years 8 months 22 hours ago) and read 7794 times:

Your example mentioned take off vs cruise. Due to the acceleration of take-off the engines would be creating alot more thrust and burning more fuel because of that. While in level flight the engines do perform better at higher elevation and that was already covered.

Another little thing of info is that in some aircraft it may be more fuel efficent to cruise around at say 18,000 feet then at 36,000 feet for instance because the fuel required to climb from 18,000 to 36,000 may be more then the extra fuel burned at a less efficent altitude. This only happens on shorter flights. After any signifigant cruise this is no longer a factor.

[Edited 2009-04-22 19:11:31]


Keep the shinny side up!
User currently offlineJetMech From Australia, joined Mar 2006, 2699 posts, RR: 53
Reply 12, posted (5 years 8 months 19 hours ago) and read 7770 times:



Quoting Klemmi85 (Thread starter):
I was just wondering why an aircraft engine isn't fuel efficient in low altitude but much more at high altitudes.



Quoting Tdscanuck (Reply 7):
Several posts above are mixing efficiency, thrust, and fuel flow. At high altitudes the engine is less efficient, producing less thrust, and has much lower fuel flow.

I think there are two seperate entities at play that combine to give overall "efficiency", these being thermal efficiency and propulsive efficiency.

Thermal efficiency in simple terms is the useful work output divdied by the energy input from combusting fuel. Propulsive efficency with respect to aerospace vehicles propelled by reaction type engines is the vehicle velocity divided by the exhaust gas velocity.

Thermal efficiency can be estimated from either the compression or temperature ratio across the engine. I seem to remember that the thermal efficiency tops out at around 40% when calculated by either method for the following reason;

Quoting Tdscanuck (Reply 7):
This is complicated by the fact that they Brayton cycle, as implemented in a jet engine, isn't actually closed.

Below is the diesel cycle plotted on Pressure vs Specific volume and Temeprature vs. entropy diagrams.

http://www2.cemr.wvu.edu/~smirnov/mae320/figs/F9-5.jpg

The area inside the curves represents the amount of useful work that one has the potential to extract from the process. The larger the area, the more work one can get out of the cycle. However, as the amount of cycle work increases, one would find that the thermal efficiency of the cycle starts to decrease towards zero.

Alternatively, one could make the area inisde the curve smaller in an attempt to increase the thermal efficiency, but then the cycle work would approch zero. Either extreme is of no practical use, thus we must choose the optimum balance between thes two mutually exclusive factors to get the best compromise. Unfortunately, this means we have to accept less than 100% thermal efficiency and maximum cycle work. I suspect the Brayton cycle may have similar characterisitcs.

Propulsive efficiency may be affected by the engine / airframe combination. An airframe with high drag not only slows the aircraft, but also demands a higher exhaust gas velocity from the engine to make more thrust to hold a given aircraft velocity, which decreases propulsive efficiency. A streamlines airframe may have the opposite effect.

A compromise where the combination of thermal and propulsive efficiencies are optimum would probably give the best overall "efficency" that one is seeking. Again, the extremes are of no practical use. One may have a 100% thermally efficent engine, but if it stays at idle for an extended period whilst the airrcaft sits with the park brake set at the end of the runway, the propulsive efficency will be zero, and all the enrgy will be going to waste.

Regards, JetMech



JetMech split the back of his pants. He can feel the wind in his hair.
User currently offlineDocLightning From United States of America, joined Nov 2005, 20334 posts, RR: 59
Reply 13, posted (5 years 8 months 19 hours ago) and read 7763 times:



Quoting JetMech (Reply 12):

Below is the diesel cycle plotted on Pressure vs Specific volume and Temeprature vs. entropy diagrams.

AAAARRRGH!!!!!! *goes into a seizure of flashbacks from HS physics class*


User currently offlineBaroque From Australia, joined Apr 2006, 15380 posts, RR: 59
Reply 14, posted (5 years 8 months 10 hours ago) and read 7678 times:



Quoting DocLightning (Reply 13):
Quoting JetMech (Reply 12):

Below is the diesel cycle plotted on Pressure vs Specific volume and Temperature vs. entropy diagrams.

AAAARRRGH!!!!!! *goes into a seizure of flashbacks from HS physics class*

The trouble with those diagrams is that they induce large slugs of entropy within the brain of the physicsophobes.

But keep going Jetmech, there is probably a (faint) hope of educating us mechanically unwashed.

Remember:

Heat won't flow from the cooler to the hotter,

You can try it if you like,

But you far better notta.


User currently offlineDocLightning From United States of America, joined Nov 2005, 20334 posts, RR: 59
Reply 15, posted (5 years 8 months 7 hours ago) and read 7642 times:



Quoting Baroque (Reply 14):

The trouble with those diagrams is that they induce large slugs of entropy within the brain of the physicsophobes.

I completely dominated physics (and calculus). And despised every second of it.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 16, posted (5 years 7 months 3 weeks 3 days 4 hours ago) and read 7279 times:

You must consider the design point for the engines. They're designed to operate efficiently at a relatively high RPM. At a high RPM at sea level, they produce a lot of thrust in order to enable the aircraft to take off and climb. Flying straight and level at this high RPM setting would mean overspeeding, or at least blasting through the skies at a speed much higher than the speed for aerodynamically economical cruise. Reducing the RPM by throttling the engines would mean taking them below the design RPM where they are the most efficient and a severely increased specific fuel consumption.

What to do, what to do?

Bring the engines to a higher altitude, where the thinner air will make them produce the right amount of thrust for level flight at or near the design RPM. You want to find that sweet spot where you have the minimum amount of aerodynamic drag (in actual truth, a bit more to find a good compromise between speed and efficiency), and where this drag corresponds to the thrust generated by running the engines at their most efficient RPM setting.

High also mean a lower EAS for a given TAS, converting to less drag at a higher speed and less energy used per track mile to overcome drag.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6536 posts, RR: 54
Reply 17, posted (5 years 7 months 3 weeks 3 days ago) and read 7235 times:



Quoting Tdscanuck (Reply 7):
The major change with altitude is thrust...as you ascend, density and pressure are dropping, so thrust drops. Fortunately, drag drops off faster than thrust, so your overall burn drops off considerably at altitude.

No, drag doesn't drop. When cleaned up and flown at a resonable speed, then drag is always around 5 or 6% of the weight of an airliner. That is when flown at the speed at which the plane has the best L/D (lift to drag ratio).

Drag will be roughly the same going M=0.8 at 35,000 feet or M=0.4 at sea level, and you will need the same thrust for level flight.

But since you go twice as fast at at 35,000 feet on the same thrust, then you save half of the fuel up there.

Quoting Tdscanuck (Reply 10):
Flying slower could have fairly huge benefits, since drag goes with speed squared, but there are other costs involved that go up with decreasing speed (crew time, asset utilization, etc.).

No, it's a lot more complicated than that. If you fly an ordinary airliner at, say, M=0.6 at cruising altitude, then you will need to fly it with a very high angle of attack and your, drag will be very high, and your fuel efficiency goes to hell. You will need a much larger wing to fly it efficiently at such a slow speed, and then your drag would still be 5-6% of the weight.

Airliners are designed to cruise with best fuel efficiency around M=0.75 to M=0.8 give and take a little depending on load (payload and fuel load) and assigned flight level. Up to 10% slower for some RJs, especially the BAe-146/ARJ.

That speed was chosen because going much faster, then transonic airflow will be present at certain places as air is acellerated around the plane. and totally different and very unfavorabble drag rules apply.

When RJs are optimized for slightly slower speeds, then it is because they mostly have a generously sized wing to boost short field performance. In addition, for sinplicity they often do away with the most efficient high lift devices. Instead the have a wing airfoil section which has a high lift without high lift devices. Such airfoil sections will encounter transonic drag at slower speed than airfoils sections used on long range planes.

Airliners generally have a max sea level thrust which is 25 to 33% of MTOW, which is roughly five times what is needed for level flight. At 35,000 feet most turbofan engines will have lost three quarters of that thrust due to the thin air. So there isn't much power left for further climb until you have got rid of some fuel and that way become less heavy. Fortunately fuel flow at max thrust will also be cut by roughly 75% maintaning roughly the same SFC (specific fuel consumption) which is some 0.5 lb fuel for one lb thrust in one hour give and take a little depending on engine type and operating conditions.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 18, posted (5 years 7 months 3 weeks 2 days 20 hours ago) and read 7206 times:



Quoting Prebennorholm (Reply 17):
No, drag doesn't drop. When cleaned up and flown at a resonable speed, then drag is always around 5 or 6% of the weight of an airliner.

Yes, drag drops. The weight-dependent part of the drag is the induced drag, but that's only one piece of the overall drag. Form drag and skin drag are also in there, and they drop with altitude (at equal speed).

Tom.


User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3597 posts, RR: 66
Reply 19, posted (5 years 7 months 3 weeks 2 days 8 hours ago) and read 7126 times:



Quoting Tdscanuck (Reply 18):
Quoting Prebennorholm (Reply 17):
No, drag doesn't drop. When cleaned up and flown at a resonable speed, then drag is always around 5 or 6% of the weight of an airliner.

Yes, drag drops. The weight-dependent part of the drag is the induced drag, but that's only one piece of the overall drag. Form drag and skin drag are also in there, and they drop with altitude (at equal speed).

Tom.

No, if you fly the same L/D and the same weight at FL150 and FL350, the drag in units of force is the same since:

D = Weight* (D/L)

An advantage you have of being at the higher altitude is the higher TAS for the same EAS as Prebennorholm says.

Of course, it may be a bit difficult to get an exact L/D match between the two altitudes but you'll come pretty close if you fly the same CL (same EAS) for a relatively incompressible Mach (say below 0.75M).

My problem with Prebennorholm's statement is that it's unlikely you'll get the same L/D for 0.4M and 0.8M because at the higher Mach you may be into the drag rise so the Drag term will now include a Dcompress term that won't be there at 0.4M



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3309 posts, RR: 13
Reply 20, posted (5 years 7 months 3 weeks 2 days 7 hours ago) and read 7115 times:
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Quoting OldAeroGuy (Reply 19):
No, if you fly the same L/D and the same weight at FL150 and FL350, the drag in units of force is the same since:

D = Weight* (D/L)

Yes and no. If you are the same L/D, then yes, but when you are cruise, your L/D changes since the air is thinner and your lift remains the same. You may be confusing maximum L/D with actual L/D for a given real-world flight dynamic.

Parasitic drag is a function of air density, velocity, area, and drag coefficient.

D = (1/2)*(rho)*(v^2)*(A)*(Cd) where "rho" is air density. Therefore, as density decreases, the drag force will decrease. And you therefore need less thrust for a given velocity than you would at ground level.

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3597 posts, RR: 66
Reply 21, posted (5 years 7 months 3 weeks 2 days 6 hours ago) and read 7099 times:



Quoting ANITIX87 (Reply 20):
Yes and no. If you are the same L/D, then yes, but when you are cruise, your L/D changes since the air is thinner and your lift remains the same. You may be confusing maximum L/D with actual L/D for a given real-world flight dynamic.

Parasitic drag is a function of air density, velocity, area, and drag coefficient.

D = (1/2)*(rho)*(v^2)*(A)*(Cd) where "rho" is air density. Therefore, as density decreases, the drag force will decrease. And you therefore need less thrust for a given velocity than you would at ground level.

No, I'm not confused.

For an incompressible drag polar (Mach below 0.7), for a given CL there is a given CD that is virtually invariant with altitude. [I say virtually since as altitude increases the skin friction Cd (part of Cd parasitic) increases due to decreased Reynolds number.]

For best efficiency, an airplane wants to fly at (L/D)max. On an incompressible drag polar, there is a unique CL for (L/D)max and a corresponding CD. For best range, fuel mileage etc, the airplane wants to always fly at the constant CL for (L/D)max.

To fly at the same CL for the same weight at various altitudes, dynamic pressure must be constant. This introduces the concept of Equivalent Airspeed or EAS.

The relationship between True Airspeed (TAS) and EAS is:

TAS = EAS * (1/rho)^.5 where rho is the ratio between the actual density and sea level density.

If the airplane is at FL150 or FL350 and flying at an incompressible Mach no at the CL for (L/D)max for the same weight, then dynamic pressure (and EAS) is the same for both altitudes.

Since CD will be the same and dynamic pressure is the same, then Drag will be the same.

However, at F350, TAS will be higher because rho is lower at FL350 than at FL150 and airframe efficiency will increase due to the higher airspeed. See the Breguet range equation.

This discussion is a bit simplified since best efficiency for high speed aircraft is obtained when flown at (M(L/D))max but the general principles still hold.

Airplanes climb not to reduce drag at constant weight but to gain airspeed when flying at the same drag.

Concurrently, airplanes climb as weight reduces to operate at the CL that produces the best L/D or ML/D. Drag does drop under these circumstances, but only because weight (ie lift) is also reducing.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineANITIX87 From United States of America, joined Mar 2005, 3309 posts, RR: 13
Reply 22, posted (5 years 7 months 3 weeks 2 days 5 hours ago) and read 7089 times:
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Quoting OldAeroGuy (Reply 21):
No, I'm not confused.

Apparently not. Sorry, haha. I understand what you meant now. I thought you were simply stating that drag is always the same since it's a function of L/D regardless of speed or altitude.

TIS



www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4
User currently offlineOldAeroGuy From United States of America, joined Dec 2004, 3597 posts, RR: 66
Reply 23, posted (5 years 7 months 3 weeks 2 days 5 hours ago) and read 7085 times:



Quoting ANITIX87 (Reply 22):
Quoting OldAeroGuy (Reply 21):
No, I'm not confused.

Apparently not. Sorry, haha. I understand what you meant now. I thought you were simply stating that drag is always the same since it's a function of L/D regardless of speed or altitude.

No problem. It's an interesting topic to discuss.



Airplane design is easy, the difficulty is getting them to fly - Barnes Wallis
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 24, posted (5 years 7 months 3 weeks 2 days 1 hour ago) and read 7059 times:



Quoting OldAeroGuy (Reply 19):
No, if you fly the same L/D and the same weight at FL150 and FL350, the drag in units of force is the same since:

Yes but, as you noted, you can't practically fly at the same L/D at two altitudes that different so it's not a realistic comparison. At the same speed, you definitely won't have the same L/D at the different flight levels.

Tom.


25 OldAeroGuy : I don't agree with you here. It's quite possible to have nearly the same L/D at different altitudes if you're operating at the same CL. It may entail
26 Rwessel : I think he's saying that at airliner airspeeds (IOW, IAS=250+kts), that at the higher levels (FL350 was mentioned), that you're almost certainly push
27 OldAeroGuy : Yes, if you include compressibility effects at the higher altitude, you may get a decrease in L/D. But if weight is the same, then Drag will increase
28 Timz : You're saying at a given true airspeed and weight, total drag is higher at higher altitude? The same airliner, at the same weight, at, say, 460 knots
29 Tdscanuck : Yes, but if you're operating that the same Cl then you're *not* operating at the same speed. The translation from Cl to L is q, which is proportional
30 OldAeroGuy : Please see the table below: FL000 FL250 FL350 Press 2116.2 785.32 497.96 Density 0.002377 0.001065 0.000737 a - kts 661.47 601.95 576.42 delta 1 0.37
31 Baroque : But lovely if you copy it to word, insert tabs and convert to table. Fascinating!
32 Tdscanuck : I'm talking TAS. Should have made that clear from the beginning. For the purposes of this initial discussion, I'm not sure why you'd ever want to use
33 OldAeroGuy : You use EAS because the main theme of this discussion is Efficiency as described by the thread title. For best efficiency, you want to operate near t
34 Tdscanuck : I agree we're talking efficiency, but all of your major efficiency measures (L/D, SFC, range, time to destination, fuel to destination, etc.) are peg
35 OldAeroGuy : Actually, it's a mix. Design CL and its associated L/D are keyed to EAS. SFC is also related to EAS as q at the fan face is a factor in this term. Th
36 Timz : I'll try the question again. An aircraft at 460 knots TAS at FL250, compared to the same aircraft at the same weight at 460 knots TAS at FL350. At wh
37 Flipdewaf : Its very much dependant in the aircraft as they all have different drag polars but I would say that it would have higher drag at FL 250 as the EAS wo
38 Prebennorholm : If we talked 0.45M vs. 0.9M, then sure that effect would be huge. But I doubt that compression is a noticeable issue at 0.8M on a well designed moder
39 OldAeroGuy : The question is kind of like asking "How long is a piece of string". The answer needs to be qualified with the statement "It depends". In this case t
40 Tdscanuck : Compression is absolutely noticeable at M 0.8. There's a gap between when compressibility effects start to show up and when shock waves start to show
41 JetMech : I've read several texts that suggest one should start accounting for compressibility as early as Mach 0.3! Regards, JetMech
42 Tdscanuck : Agreed. There's three major "break points" of interest...when the compressible and incompressible solutions start to diverge (typically somewhere in
43 OldAeroGuy : The question though is what equal weight? And you're right that the key question is how close you're to optimum conditions. Please see the two follow
44 OldAeroGuy : Compressibility effects at 0.3M are usually only important at angles of attack near stall. Near stall, you can have upper surface sonic velocities wi
45 JetMech : Interesting! I suppose that this would be more apparent on thinner airfoils with smaller leading edge radii? Regards, JetMech
46 OldAeroGuy : Sonic upper surface velocities can occur on both thin airfoils with small nose radii and thick airfoils with large nose radii. The difference is that
47 Tdscanuck : Fantastic analysis! Always good to put facts and data to the discussion. Thank you very much for this. Tom.
48 Timz : (I inserted spaces to make the table readable-- if they're in the wrong places, speak up.) So you're saying the table applies reasonably well to a 77
49 David L : ... i.e. when rushing to the bar after last orders are called.
50 OldAeroGuy : Sorry, but the table looks the same in your reply, thanks to the editing quirks of A.net. Baroque had the best suggestion in Reply 31. No, you're tak
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