Vmu inversely proportional to air density ??

[stickN'rudder007]

Hello...
Just reviewed some performance class A notes and I see in a graph that Vmu decreases with increasing air density . This doesn’t make much sense ,because Vmu is Calibrated airspeed and as such , should therefore be unaffected by density .
Is there something less apparent with regard to density that I am missing ?
thanks

[Old Smokey]

What you've assumed is true stickN'rudder007, Vmu (which is expressed as a CAS) should remain unchanged, but............

If you read further into your notes, the author may well be referring to TAS at Vmu, which WILL decrease with increased Air Density. The TAS is directly related to the Ground Speed, which is directly related to the runway distance used, thus, from a Runway Performance aspect, the Ground Speed at Vmu will reduce with increasing density, which is significant.

Now here's a nit-picking one, Vmu is actually related to EAS, not CAS, but at typical Takeoff speeds and altitudes there's only a slight difference. Nevertheless, for a given EAS, CAS will decrease with increasing Density. Consider for example an aircraft with a Vmu of 130 KEAS for a given weight and configuration at ISA Temperatures -

At Sea Level : Vmu = 130 KEAS = 130.0 CAS
At 5,000 ft : Vmu = 130 KEAS = 130.1 CAS

So at the higher density Sea Level Airport, Vmu is 0.1 Kt CAS lower, nit-picking indeed!

Regards,

Old Smokey

[stickN'rudder007]

Thank you for the reply Old Smokey.
The textbook was definately trying to show that Vmu is inversely proportional to density as an IAS , because the graph it self had altitude and/or temp Vs IAS and its effect on Vmu and some other speeds . Even though the graph itself had no values , the slope of the Vmu line was significant enough to assume that density variations have a definate effect on Vmu . Unfortunately there is no explanation about this and basically any further reference to Vmu stops there. I dont even recall our intsructor commenting on that when i was doing the ATPLs.
Basically the textbook is the one by Oxford aviation training , book 6 , page 13-10 , ( just mentioning it in case you happen to have it , or any one else that reads this post )
Now with regard to EAS , i understand your point , but as you said this is nit-picking .
Im considering an indirect way in which density may affect Vmu.... Since Vmu occurs at the most nose-up attitute that the aeroplane can achieve , thrust will have a substantial vertical vector , so the higher the density , the more thrust , the more this vertical component will be , the less the Vmu .
just a thought basically.

[Old Smokey]

Now your last point is valid stickN'rudder007, I had not factored that in. Yes, if the increased air density does lead to increased thrust, it will reduce Vmu.

On the other hand, at lower (typical) Takeoff altitudes and not so high temperatures (depending on the engine rating), if the engine is 'Flat Rated', there will be no difference in thrust. Most engines that I'm familiar with are flat rated up to about 30°C, so, below that it would make no difference, above that, Yup, it WILL make a difference.

Good thinking processes, you make a good aeronautical detective.:ok:

Regards,

Old Smokey

[Ignition Override]

Pardon the question, but why do the "authorities" believe that knowledge of flight test data is required for a pilot to know how to rotate at Vr and keep a minimum of V2 speed? Our takeoff data cards have two extra remarks: for a field elevation over about 4,000', we add one knot. For temps over about 37* we also add one knot to the airspeed indicator.

Do they actually believe that understanding data from test pilot schools (Edwards AFB/NAS Pax. River) will make us better pilots?

I've only flown transport category turbofans for years and years, and this truly baffles me.:hmm:

[stickN'rudder007]

Hello again.
Assuming the assumption was correct in the first place , it may be that what the graph is trying to show is meant to be general . The Vmu line does stop suddenly at some point , it may be the flat rating cut-out point or low temp sea level conditions.
Now with regard with the comments of the last poster , i am not going to argue that knowledge of this or anything of this depth will necessarily make someone a better pilot , but i did find throught the years that whetever you know comes back as something usefull one way or another. But even more so , there are just some things that you feel like knowing in depth for it to make sence . If i read anywhere that a particular speed as CAS /IAS varies with density , then it does give me a definate reason to find out how .
Regards
stickN'Rudder007