Most unfavorable weight in regards to VMC

Fly_Unity

Fly_Unity

Well-Known Member
Manufactures determine VMC speed based off CFR 23.149 which states:

"(5)The most unfavorable weight in the range of takeoff weights."

I always taught that this weight was a light weight, and that anything heavier helped decrease VMC speed.

That being said, we got an examiner who is adamant VMC speed is based at Gross weight. Two statements he uses to back that up is:

1, All V speeds are calcualated at max gross weight unless otherwise noted in the POH.
2, "Most unfavorable weight" that is mentioned is most unfavorable in regards to performance, not VMC.

So the question remains: Is "Most Unfavorable weight" heavy or light in regards to 23.149? Source needed.




 
Found the answers I needed with a little research. Currently it is "light", however early 23.149 regs (which at one point was mearly a copy of CAR 3) in which a lot of twins were certified did not specify weight, and thefore many manufactures "could" have used Max Gross weight in their testing.

Source. Airplane Flying Handbook 12-28
 
All of the above is correct... :-)

I am not expert on certification, but I'll make an attempt to relay what I have been taught.

First, Vmc does vary inversely with weight. Very slightly, but it does. Vmc is also slightly lower with gear and flaps extended, but, as with weight, the performance penalties far outweigh the benefits of lower Vmc

In certification, it makes sense that they need a specific weight to test it at. A random number that involves the Empty Weight of the tested airframe and the weight of the test pilot doesn't make much sense. What if Beech's test pilot is a triathlete, and Piper's test pilot needs to lay off the cheeseburgers? Another aspect of this is that they are also testing Vyse (Blue line) and publishing a "climb" performance number, which must be done at max gross. So, my guess in all this, is that if you dig through part 23 with a fine toothed comb, you will find some way that they are able to backdoor into testing Vmca at max gross.

In the long run, publishing Vmc is a bit of a joke anyway, as it can change drastically with air density.
 
ElihuSmails said:
Is it really light weight? Seems to me that MTOW is "least favorable" because of the performance penalty.
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MTOW is "good" for maintaining directional control (it's barely noticeable, but it is there)

MTOW is bad for climb performance, obviously.


Blueline and Vmc are calculated based on the absolute worst case scenario. You just raised the gear after lifting off from a sea level airport at MTOW with an aft CG.
 
USMCmech said:
MTOW is "good" for maintaining directional control (it's barely noticeable, but it is there)

MTOW is bad for climb performance, obviously.


Blueline and Vmc are calculated based on the absolute worst case scenario. You just raised the gear after lifting off from a sea level airport at MTOW with an aft CG.
Click to expand...

That's what I was always taught, and what I've always taught. Just seems a bit misleading to use the words "most unfavorable". It's like they've got government employees writing these things!
.
 
USMCmech said:
Blueline and Vmc are calculated based on the absolute worst case scenario. You just raised the gear after lifting off from a sea level airport at MTOW with an aft CG.
Click to expand...

Actually under "current" regs (23.149), Vmc is based on the "most unfavorable weight in the range of takeoff weights" in regards to directional control. Not performance.

You just raised the gear after lifting off from a sea level airport with 30 mins of fuel, and one person in the airplane, with an aft CG.

Source:

AFH 12-28

CFR 23.149

AC-8A: FLIGHT TEST GUIDE FOR CERTIFICATION OF PART 23 AIRPLANES
Page 60 and Section 48(c)(1) states that 'The critical loading for Vmc testing is generally minimum weight and maximum aft c.g.; however, each airplane design should be evaluated independently to be assured that tests are conducted under the critical loading conditions.'

Minimum weight, they say.

AC 25-7A (change 1) Flight Test Guide for Certification, page 144. It states, 'VMCA is normally determined at minimum weight in order to minimize the stall speed and because static VMCA decreases with increased weight if a 5-degree bank angle is used.' There are a couple of reasons why VMC is higher at decreased weight.
 
I am an expert on certification, and have done numerous Vmc certifications in various king air mods. It is done as light as we can get and still be at the aft cg. We typically have less than 20 minutes of fuel to get it done and it is almost always done very early in the morning to get calm air and directly overhead an airport.

Hammertime said:
All of the above is correct... :)

I am not expert on certification
Click to expand...
 
IIRC, the POH for the Piper Seminole specifically states the weight Vmc was tested at- (2730 lbs @ 1500' msl) which turns out to be Empty Weight of the aircraft plus a standard 170 lb pilot plus approximately 5 gal of usable fuel.
 
Jimflyfast said:
I am an expert on certification, and have done numerous Vmc certifications in various king air mods. It is done as light as we can get and still be at the aft cg. We typically have less than 20 minutes of fuel to get it done and it is almost always done very early in the morning to get calm air and directly overhead an airport.
Click to expand...


AND you stayed at a Holiday Inn Express last night ;)
 
Jimflyfast said:
I am an expert on certification, and have done numerous Vmc certifications in various king air mods. It is done as light as we can get and still be at the aft cg. We typically have less than 20 minutes of fuel to get it done and it is almost always done very early in the morning to get calm air and directly overhead an airport.
Click to expand...
So, at least from a directional control perspective, any realistic configuration of weight and balance will have more favorable directional control results than the certification test. Interesting. Thanks.
 
Autothrust Blue said:
So, at least from a directional control perspective, any realistic configuration of weight and balance will have more favorable directional control results than the certification test. Interesting. Thanks.
Click to expand...

Correct. And, from a strictly directional control point of view, remember that we are limited to 5 degrees for certification. Nothing says 5 degrees is the limit in the real world if you are losing directional control.
 
Jimflyfast said:
Nothing says 5 degrees is the limit in the real world if you are losing directional control.
Click to expand...

This, and, just to note, weight plays a role only because of the addition of bank angle. Just know that adding more bank means sacrificing performance. Of course if losing a few feet means staying upright....
 
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