Very slow at high altitude

[Cebo]

Why is the F-5 so slow at higher altitudes? I am flying at 35k, levelled flight, no external load, 100% rpm and the air speed indicates shows 350 knots, switching to the external view(F2) confirms the low speed. it shows me around 600km/h. even engaging the afterburner does not push me to a faster speed:(

[VTJS17_Fire]

You address two different points.

 

A) The speed you see in the cockpit and external view is your indicating airspeed. This is the speed, which indicates the performance of your aircraft. The indicated airspeed is normally lower at high altitudes than at low level, because of the thinner air.

 

What counts, is our ground speed. The F-5 hasn't any indicator for ground speed (AFAIK).

 

 

B) With the afterburner enabled, there should be an acceleration. Maybe a bug?

[probad]

air gets thinner as you go up, indicated airspeed is going to be affected. you're likely to be over mach 1 at 350kias @ 35k. note you have a machmeter inset to your ias dial.

 

f-5 is a nominally supersonic aircraft but it's really most comfortable at high subsonic. you also happen to be hitting the transonic zone, and your little learjet engines will struggle to push enough air to get you past it.

if you want to push any plane it's going to involve a little more than simply seating the throttle at zone 5.

hint: gravity is your best friend.

 

here it's near the top speed of m1.6 @ 35000 and the ias is "only" 500kt

Edited September 19, 2016 by probad

[Cebo]

The speed you see in the cockpit and external view is your indicating airspeed.

Ah, i thought the external view shows the ground speed, thanks for clarifying.

 

air gets thinner as you go up, indicated airspeed is going to be affected. you're likely to be over mach 1 at 350kias @ 35k. note you have a machmeter inset to your ias dial.

I have noticed the discrepancy between knots and mach. I think the machmeter showed around mach 1, which is 1064km/h or 574 knots at this altitude. Does this mean in higher altitude I should ignore the knots on the gauge or external views and just look at the machmeter to get a sense for the true speed?

How does the aircraft determine the mach number? Does it use an different sensor than for the knots which apparently gets affected by the air pressure?

[probad]

both ias and mach are useful. ias is how much air you have to work with and mach is how the air behaves.

 

example, you can be going at high mach but at high enough altitude you have so little air (low ias) that your plane will still stall out (coffin corner). the u-2 is tricky for this because it's mach limit isn't very far from its stall speed at the altitude it operates, so the pilot needs to walk the tightrope between the two metrics.

 

the wikipedia article on mach explains nicely what mach is and how its calculated.

Edited September 19, 2016 by probad

[Cebo]

example, you can be going at high mach but at high enough altitude you have so little air (low ias) that your plane will still stall out (coffin corner)

makes absolutely sense, thanks:)

[VTJS17_Fire]

Ah, i thought the external view shows the ground speed, thanks for clarifying.

It was, until a patch a few weeks ago. There was the new GUI for the external views introduced and with them, the IAS in the external view.

[Art-J]

While we're at it, hit Ctrl-Y (if I recall correctly) in external view to switch between IAS, TAS, metric and imperial.

[Cebo]

It was, until a patch a few weeks ago. There was the new GUI for the external views introduced and with them, the IAS in the external view.

 

Ah, this was what confused me. I was sure I saw higher numbers on the external view before flying the F-86 at high altitude. That is exactly what make me think the F-5 is so slow. Will check if Ctrl-Y does the trick, thanks heaps!

[plaiskool]

Yo Cebo,

 

 

Here, if i may, my two cents.

 

Try now to fly a mach number when above 20Kft, Knots are useless and not representative.

RUle of thumb average Jet working speed: mach 0.85/0.9

 

 

 

try this:

 

-Go 20Kft

-Go full burner straight level

-Intercept mach 0.9 then, and only after getting mach 0.9

-Climb, and control your speed using the pitch and never descend below 0.9 mach.

 

If you conserve at or > mach 0.9 til 45Kft, you should be able to increase speed because you have proper mach, proper AOA, thus proper DRAG that thrust can overcome to accelerate.

 

Skool

Edited September 19, 2016 by plaiskool

[Cebo]

While we're at it, hit Ctrl-Y (if I recall correctly) in external view to switch between IAS, TAS, metric and imperial.

Yes, Ctrl-Y does toggle between IAS and TAX on external view, thanks!