False behavior in high Levels
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Hi FBMS Community,
i have a Question about the behavior of the AC in high Flightlevels.
Mikes ago I talked to a Airliners-Pilot and to one of my Flight-Teachers. The Conversation went over a lot of Topics, one of them was the Speed-Behavior in the higher Atmosphere.
All of you know the Problem with the Speed-Declaration if you fly higher. If I fly at 30.000ft, I need much more Engine-Power than at 20.000ft for holding my Speed. Sometimes I need the Afterburner for staying at 350kn.
Thats not realistic! If you gonna fly at 30.000ft you dont need 110% Power to hold your Cruise-Speed of 350kn. Thats the Statements of the Pilot and my Teacher.
The Reason is, that the Air-Resistant falls in high Alt. - and he does that in x^2. So, if theres a much lower Resistance than at 20.000ft you would not need more Fuel for holding the Speed.
Is this right? - I think so. Could I fly false? - I dont know.
Best regards Guys!
Salute
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I thought the higher the more fuel efficient it was hence air in less dense so requires less power?.
I’m no pilot but that’s what I thought
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Turn your airspeed to TAS, you will see that your true airspeed has increased greatly as the air around you has become less dense. Calibrated Airspeed makes a lot of sense to use due to it basically sensing the amount of air your wings etc. need to work (and hence what you need to know to not stall/overspeed the plane) but it doesn’t give you a full idea of your true airspeed/ground speed. Mach number is also a good source of info for traveling speed. Capibrated is basically fly the airplane speed.
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Indeed, CAS (calibrated air speed) and TAS (true airspeed) are completely different.
CAS is TAS at a specific pressure, temperature, and humidity, I think 1013 hPa, 20°C, and “dry” air.Basically, CAS is the conversion of a pressure difference in a “virtual” speed. This pressure difference is between the basic air pressure and the pressure felt in an “arrest point” in the aircraft, where all the air speed is stopped and converted into a pressure. The faster you are, the more the pressure difference is.
So if the air is less dense, for a same true airspeed, the pressure difference will be less and your CAS will be less.
The CAS is still a useful info, because aircraft behaviour depends on the pressure exerced by the air on it. Your corner velocity will ALWAYS be between 330 & 440 kts CAS, for ex, no matter the altitude.
Instead of watching the CAS, watch the Mach number. Mach 1 correspond to a speed which doesnt change as much with altitude (from 660 to 570 kts, max ; depends only in the temperature, not the air density). You will have a better idea of your TRUE airspeed.
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If you reach 30.000 ft in one steep climb the tubine is and stays ‘underfed’ it needs air to burn the fuel.
If you climb to 23.000 ft first and stay there to pick up speed you can go up to 35.000 ft and fly fuel efficient without much fuel flow but fast.
Now if you give much fuel to the turbine (AB) and gain speed again you can climb higher again (empty jet - 50.000 ft) stay there and get mach 2.0Also while climbing and easing off the climb: rotate roll the jet and try to fly zero-G-curve to pick up speed faster - never push on the stick and if you pull up again
do it gently.With many weapons you have to climb slowly to keep the speed - and you can not gain much speed compared to an empty jet. Best is to climb slowly but with full power.
At 35.000 ft you should roll the jet 180° and bring the nose down let the nose come down , roll 180° again and wait before reducing thrust. There you have less drag but efficient mil power speed with full weapons.
Thats how I remember it was long long time ago when I flow flew the virtual jet in BMS 4.32 2012. -
Is this right? - I think so. Could I fly false? - I dont know.
In short, he is not right. I’m too tired to explain longer. Maybe tomorrow.
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CAS is TAS at a specific pressure, temperature, and humidity, I think 1013 hPa, 20°C, and “dry” air.
Almost correct.
International Standard Atmosphere (ISA) conditions are at 15° C. (can be found here) -
Almost correct.
International Standard Atmosphere (ISA) conditions are at 15° C. (can be found here)Indeed ! Thanks for the precision.
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Almost correct.
International Standard Atmosphere (ISA) conditions are at 15° C. (can be found here)… Almost correct. International Standard Atmosphere (ISA) conditions are at 15° C , 1013.25 hPa … at sea level
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Thanks for the fast Answers!
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I never thought of switching to TAS or GS. I knew the difference between the Speed-Types but didnt think of them in relation to my Problem. You solved it very precisely. Thanks
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This post is deleted! -
so whoa whoa whoa whoa
whoa
what? i didn’t know anything about this. how do you switch to TAS? i’ve never seen anything about this in all the MFDs i’ve switched to. you’re telling me you’re actually faster at high altitudes? i’ve always stuck to angels ~20 because i thought it was the most efficient cruising altitude (as a median between cruising speed and fuel use) am i totally wrong about this?
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20.000 ft is pretty low. Not efficient in consumption and not at all tactical wise.
For a AG striker, I cruise around 26.000 ft. And for a Air to Air, around 35.000 ft.
Don’t bother with TAS in medium / high level. The only moment you should work in TAS is for low level coordinated attack like popup, TOSS, etc… Fly with mach. -
flying fuel efficient is an art. it only starts with these simple things. if you can fight fuel efficient you get into it.
select ‘5’ - CRUS in the DED and your optimum speed and altitude is displayed.
it depends on your weapons loadout / drag.
for an empty jet it is much higher. a packed jet would indeed be cruising best around 22 - 24 thousand feet.the airspeed readout is selected on the right console (the HUD panel - TAS true air speed /CAS calibrated air speed /GND ground speed switch)
won’t make you faster though. -
@Cik:
so whoa whoa whoa whoa
whoa
what? i didn’t know anything about this. how do you switch to TAS? i’ve never seen anything about this in all the MFDs i’ve switched to. you’re telling me you’re actually faster at high altitudes? i’ve always stuck to angels ~20 because i thought it was the most efficient cruising altitude (as a median between cruising speed and fuel use) am i totally wrong about this?
Yes. At higher alt for same TAS you have to set higher RPM but the specific fuel consumption (SFC) is decreasing up to 30-39k feet depending on your loadout. Just check your speed and fuel flow rate and calculate the nm/lbs SFC. The CAS is good because shows the amount of lift and drag forces on airframe, but your TAS and Mach numer indicates your real airspeed. In short if you wish the same turn rate at 1k or 30k on a certain speed you have to reach the same CAS. Only problem that on 1k feet the CAS and TAS are literally the same but on 30k feet the CAS and your thurst is much smaller. This is why happened 95% of air combat below 25k feet in RL since 1953.
These are one of the most basic things in aviation without this is impossible to fly well and AC or do air combat…
Check the performance envelope curves as well as the CAS-TAS relation.
https://info.publicintelligence.net/HAF-F16-Supplement.pdf
http://www.hochwarth.com/misc/AviationCalculator.html -
yes. i just didn’t know TAS existed. fighting at high altitudes seemed nigh-impossible when i’ve tried it, because turning is so sluggish and your fuel consumption (and thus, i assume raw thrust output) are so low. you bleed speed in even small turns like nothing. this is why i burn up to angels 30~ or whatever for BVR engagements but otherwise stay lowish. i like to be able to actually manage a decent turn if i have to. i’ll have to look at your stuff though, thanks for the info.
so is there a “optimal altitude” for cruising to mission then? if it’s true that CAS only really matters for maneuvers, and TAS is your “real” airspeed that determines how fast you are going overland, is there a specific altitude that allows you to cover lots of ground while still staying at the 30000~ fuel output of about 3000 (as opposed to angels 15 at like 10000~)
and i assume when you say “specific fuel consumption” you mean efficiency of engine output as in TAS attainable per unit of fuel consumption per second?
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There are many types of specific fuel consumption. Engine manufacturers for comparison give thrust/(weight of fueal*h) value which provide info about how much fuel is required to produce X thurst for one hour. Of couse this kind of SFC is depending on many variables.
This is the engine data of MiG-25. Yes, the SFC is decresing with incresing speed but this does not mean that total FC is smaller because thrust is also increasing with speed. Thrust x SFC = total fuel consumption
http://www.leteckemotory.cz/motory/r-15/R-15 was a very special engine here is AL-31.
http://www.leteckemotory.cz/motory/al-31/You can see how depend the thrust and SFC on alt.
There is no optimal altitude generally it depends what is your priorty. If you fly at high you can save lots of fuel but eng. range of heavy SAMs also increasing until a certain alt because it is worth to climb higher the missile to fly with smaller drag. If you fly lower you can reach faster the more dense air where the BVR missile looses very quickly energy and your manoverability is also increasing. The tyical combat cruising alt is the last 20-30 year was 15-25k feet except for TFR capable low level attackers.
During ODS after first week of campaign the med alt. flying was typcal because the heavy SAMs were mostly destroyed, the risk of facing with few survivors of SA-2/3/5/6/etc was smaller then facing the AAA/MANPAD/SHORAD which are pop up threast and can be anywhere, and in that time MAWS against IR missiles were not available.
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alright, thanks for the lecture molni. it was helpful. sorry if i’m asking too many questions, how do you toggle CAS/TAS/mach readouts? is the mach readout a seperate thing? i’m not sure if i’m stupid not to have seen it. is it somewhere in the MFDs?
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@Cik:
alright, thanks for the lecture molni. it was helpful. sorry if i’m asking too many questions, how do you toggle CAS/TAS/mach readouts? is the mach readout a seperate thing? i’m not sure if i’m stupid not to have seen it. is it somewhere in the MFDs?
For selecting CAS/TAS/GS; already answered a few post above here:
@Cik
the airspeed readout is selected on the right console (the HUD panel - TAS true air speed /CAS calibrated air speed /GND ground speed switch)
won’t make you faster though.Also check the BMS manual in de Docs folder on your hard drive. Page 53, section 1.2.5.3 HUD panel.
The mach number can be found on your center pedestal mach meter , see same manual page 39 section 1.2.3.9 Machmeter.
It can also be read on your HUD low left. See same manual page 98 section 1.5.1 HUD Settings -
@Cik:
alright, thanks for the lecture molni. it was helpful. sorry if i’m asking too many questions, how do you toggle CAS/TAS/mach readouts? is the mach readout a seperate thing? i’m not sure if i’m stupid not to have seen it. is it somewhere in the MFDs?
Ctrl+Shift+v as I can remember the three position swich in on the righ panel next to the joystick from left to radar alt swich. Mach number is always displayed on HUD except the dogfight override mode.
