VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeo

[newcrew]

why the bit in the title?

thanks for the help

cheers



FAR PART 23.149

FAR 23 (the certification requirements for typical general-aviation13 airplanes) gives a very specific definition of VMC, namely:

FAR 23.149 Minimum control speed.

(a) VMC is the calibrated airspeed at which, when the critical engine is suddenly made inoperative, it is possible to maintain control of the airplane with that engine still inoperative, and thereafter maintain
straight flight at the same speed with an angle of bank of not more than 5 degrees. The method used to simulate critical engine failure must represent the most critical mode of powerplant failure expected in service with respect to controllability.

(b) VMC for takeoff must not exceed 1.2 VS1, where VS1 is determined at the maximum takeoff weight. VMC must be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible, for the takeoff configuration(s) with--
(1) Maximum available takeoff power initially on each engine;
(2) The airplane trimmed for takeoff;
(3) Flaps in the takeoff position(s);
(4) Landing gear retracted; and
(5) All propeller controls in the recommended takeoff position throughout.



Note that none of these definitions require that the airplane exhibit a positive rate of climb at VMC. Also note that during a VMC demonstration, the pilot is not required to optimize the climb rate or to maintain zero slip -- although zero slip may be an advantage if it can be achieved.

[Mad (Flt) Scientist]

Part of your title is cut off, I assume you're referring to the "where VS1 is determined at the maximum takeoff weight" part.

Basically, it's a requirement that VMC not be so high that it drives the takeoff speed schedule at all weights. You're allowed to have a VMC which is high and limits V2, but not EVERYWHERE.

Part 25 is similar IIRC.

There's no positive rate of climb requirement for VMC because there are other requirements which deal with having the ability to climb at takeoff speed schedules; this is just a controllability requirement which provides one of many constraints on the speed schedules.

[newcrew]

thanks for the reply

i am a bit slow at this early hour and have still not got the jist of it...

not sure why they would only want a limit on part of the speed regime?
any chance of a practical example

looking forward to your reply

[Mad (Flt) Scientist]

Not having ready access to the various NPRMs that led to the rule as it is today, the following is more "reverse logic justification" than anything else, but....

Imagine an aircraft with a stall speed varying from 80kts at minTOW to 120kts at MaxTOW (which, all other things being equal, implies a TOW range from 64/144max to max - not far off 50%, good enough for government work). Assuming old-style V2 minima (to make the maths easier) we get a V2min of 1.2Vs or 96kts to 144kts. The stall speed ratio being the same, the alpha will be about the same, and any tests at one weight will be aerodynamically (if not inertially) representative.

Now, if our Vmca is nice and low - like stall speed for minTOW, say (nice big rudder) - then Vmca will not (all other things being equal) influence V2min nor V2. So the light weight and heavy weight takeoffs are conducted at very similar stall speed ratios - nice repeatable characteristics.

Now, if our Vmca suddenly is instead the max permissible - 1.2Vs1 - then it becomes 144kts. Since 1.1Vmca is another V2min criteria, we now find that V2min is EVERYWHERE 158.4kts. Which means our maxTOW case is flown at 1.2*1.1=1.32Vs, and that our minTOW case is being flown at a whopping 158.4/80 ..... almost 2.0*Vs.

While not in and of itself a problem, such a huge variation in V2 expressed as a stall speed ratio will make minTOW and maxTOW cases rather different beasts. Bear in mind that a 5-10% increase in MTOW is generally considered 'significant' for testing - and that's assuming the speed spreads are still quite reasonable. For this aircraft a pilot may conduct two VERY different takeoffs on the same plane, just because of changing the weight (keeping the minTOW aircraft on the ground before Vr is going to be fun, for one thing!)

There's no single p"point at that reason" to explain why there's a max on Vmc - it just happens to keep designs 'reasonable'.