Vmc vs OAT

[Feno]

Hello,

Vmc decreases as OAT increases.
I take it that the main governing factor for that is the lower thrust/power delivered by the live powerplant due to the lower air density, hence the lower resulting yawning moment at higher OATs.

Question:
Within an increased OAT environment, can't one imagine that the subsequently reduced air density also reduces the aerodynamic controls efficiency which then might need to be compensated by an increased dynamic pressure requirement equating to a greater CAS? This would then reverse the Vmc vs OAT relative evolution.

Any numbers out there that might indicate how possible that can be or not ?
Or, are certification requirements locking things so that it can't happen ?
I was thinking of relatively light underpowered aeroplanes in a high OAT environment

[italia458]

Feno,

Within an increased OAT environment, can't one imagine that the subsequently reduced air density also reduces the aerodynamic controls efficiency which then might need to be compensated by an increased dynamic pressure requirement equating to a greater CAS?

Not really. The aerodynamic controls respond to dynamic pressure - EAS is a direct measure of dynamic pressure. CAS is equal to EAS if there are no compression effects. Below 250kts and 10,000' compression effects are very small and generally don't need to be considered. Since VMC on most airplanes is less than ~150 kts (under standard testing conditions) you really don't need to consider compression effects.

For example: @ 10,000' and 250 kts EAS, CAS = 252 kts. @ 5,000' and 250 kts EAS, CAS = 251 kts.

[Feno]

The aerodynamic controls respond to dynamic pressure - EAS is a direct measure of dynamic pressure.

Yes ok.
I get it now.
No matter how air density varies, required dynamic pressure remains 'about' the same for controllability while at the same time powerplant thrust/power may vary considerably resulting in the subsequent yawning moment variation being the main influencing factor.

Thx for that