acronym for Vmc factors

[tgrayson]

Originally Posted by wrxpilot

That's what I always thought as well. I subscribe to the heavier a/c having a higher Vmc (because of AOA/P-factor) camp...

Sorry, you saw my post out of context. Greater weight absolutely reduces Vmc; it was only the explanation of "why" that I was taking issue with.

[Fly_Unity]

Originally Posted by GaTechKid

I'm not sure why people believe Vmc for every aircraft was tested at max gross weight but perhaps you've read something I haven't. Piper tested Vmc for the Seminole at a weight of ~2730, which is the most unfavorable weight with regards to directional control. Read below:

[ QUOTE ]
§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.



[/ QUOTE ]

I know this thread is old, but actually FAR 23.149 did not say that "VMC must be determined with the most unfavorable weight" back when Piper Seminole was certified, All V speeds are based on gross weight unless otherwise specified in the POH, and the POH for the Seminole doesnt say which weight VMC was calculated on.

The FAA has revised FAR 23.149 saying that it must be based on the most unfavorable weight AFTER Piper determined VMC speed for the Seminole, so its based off of max weight of 3800 I believe.

[meritflyer]

Originally Posted by Fly_Unity

I know this thread is old, but actually FAR 23.149 did not say that "VMC must be determined with the most unfavorable weight" back when Piper Seminole was certified, All V speeds are based on gross weight unless otherwise specified in the POH, and the POH for the Seminole doesnt say which weight VMC was calculated on.

The FAA has revised FAR 23.149 saying that it must be based on the most unfavorable weight AFTER Piper determined VMC speed for the Seminole, so its based off of max weight of 2800 I believe.

Interesting.

Although, if this is true it still is an advantage since the Vmc value would've been greater if determined at 2800 vs. 3800.

[tgrayson]

Originally Posted by Fly_Unity

I know this thread is old, but actually FAR 23.149 did not say that "VMC must be determined with the most unfavorable weight" back when Piper Seminole was certified, All V speeds are based on gross weight unless otherwise specified in the POH,

Not quite true. The way it used to read was

"The maximum sea level takeoff weight or any lesser weight necessary to show VMC"

Part 25 still reads this way. The thing is, most often it was necessary to use a lighter weight in order to show a Vmc, because at max gross, the aircraft will usually stall first.

Regardless of the regulation, the normal practice for flight testing is and was to use the lightest weight. I have little faith that anything in the POH is accurate in regards to how the test was actually performed. The writers probably just copied the regulation.

[Fly_Unity]

Originally Posted by tgrayson

Not quite true. The way it used to read was

"The maximum sea level takeoff weight or any lesser weight necessary to show VMC"

Part 25 still reads this way. The thing is, most often it was necessary to use a lighter weight in order to show a Vmc, because at max gross, the aircraft will usually stall first.

Regardless of the regulation, the normal practice for flight testing is and was to use the lightest weight. I have little faith that anything in the POH is accurate in regards to how the test was actually performed. The writers probably just copied the regulation.

Interesting, I read through the whole 23.149 CAR 3 (early 23.149), I could not find where it said lesser weight necessary to show VMC. Even the airplane flying handbook says that it didnt regulate what weight it was to be determined at (12-28)

2nd of all, even IF it said that, I believe that reg was a way to test VMC, not base their determination off it. The Seminole would VMC at sea level before it would stall at gross weight, I believe the test pilots tested VMC at a higher altitude, at a light weight, then converted it to a sea level and to gross weight, I think you may be getting confused on the difference what weight they tested VMC at, and what weight VMC is calculated to be, because its obvious that test pilots could not test what VMC speed is at at gross weight at 10,000 feet.

3rd, why would Piper calculate VMC at a light weight, and not even put it in the POH, when CAR 3 didnt even mention weight? that would make their numbers look bad.

My MEI Examiner (June Bonesteel for you PHX guys) was big into this, and actually busted another student for telling her that the Seminole was tested at a light weight...(not the only reason he busted btw) which is the only reason I studied up on it.

Im not sure what part 25 says, and what it has to do with the testing process

[tgrayson]

Originally Posted by Fly_Unity

Interesting, I read through the whole 23.149 CAR 3 (early 23.149), I could not find where it said lesser weight necessary to show VMC. Even the airplane flying handbook says that it didnt regulate what weight it was to be tested at (12-28)

I copied the above from Part 23, prior, to, oh, 1996 or something like that. I haven't checked Car 3. I'll do that when I get home.

Anyway, are you saying that the Seminole is a Car 3 airplane? It's newer than the Seneca and the Seneca was a Part 23 aircraft.

<<2nd of all, even IF it said that, the Seminole would VMC at sea level before it would stall at gross weight, I believe the test pilots tested VMC at a higher altitude, at a light weight, then converted it to a sea level and to gross weight, >>

Why do you think they would do that?

<<I think you may be getting confused on the difference what weight they tested VMC at, and what weight VMC is caculated to be, because its obvoius that test pilots could not test what VMC speed is at at gross weight at 10,000 feet.>>

The flight testing literature is clear that they test Vmc at light weights because Vmc is higher, not because they need a light weight to climb to altitude.

<<3rd, why would Piper test it at a light weight, and not even put it in the POH, when CAR 3 didnt even mention weight? that would make their numbers look bad.>>

Not if all airplanes were tested in the same manner. And that's assuming that the Seminole is a Car 3 aircraft.

<<My MEI Examiner (June Bonesteel for you PHX guys) was big into this, and actually busted another student for telling her that the seminole was tested at a light weight...(not the only reason he busted btw) which is the only reason I studied up on it.>>

Not really a data point.

<<Im not sure what part 25 says, and what it has to do with the testing process>>

Most of you will be flying Part 25 airplanes in the future.

[Fly_Unity]

I have looked for my old copy of 23.149 (pre 1980) I had and couldnt find it, nevertheless Airplane flying handbook says that the the early FAR 23, AND CAR 3 didnt specify weight for VMC, and that all V-speeds are based on gross weights unless otherwise specified in the POH.

Who knows though what piper actually did though. So your saying that they just did it because it was a common practice?

<<<The flight testing literature is clear that they test Vmc at light weights because Vmc is higher, not because they need a light weight to climb to altitude>>>

I agree that they tested it lighter, not because of performance, but because it will NOT VMC at gross weight at their altitude. The question isnt what they tested it at, but what weight, was the caculated 55 knot VMC speed based on.

[BuickCFI]

VMCmust be determined with the most unfavorable weight and center of gravity position and with the airplane airborne and the ground effect negligible,

that is straight out of 23.149, Vmc is to be determined at the most unfavorable weight for Vmc, not for performance. in this case Vmc must be determined at lightest weight

[tgrayson]

Originally Posted by Fly_Unity

I have looked for my old copy of 23.149 (pre 1980) I had and couldnt find it, nevertheless Airplane flying handbook says that the the early FAR 23, AND CAR 3 didnt specify weight for VMC, and that all V-speeds are based on gross weights unless otherwise specified in the POH.

Who knows though what piper actually did though.

You're right, we have no way to for sure. But CAR 3 went away long before 1980. My guess is that Part 23 kicked in sometime in the 60's. The section I quoted was until 1996, but I don't know if there were any other versions prior to that. I have a copy of CAR 3 at home, so I'll look just for grins, but I don't think it applies to this airplane.

Even if we could establish the laws enforce during certification, the FAA provides a lot of liberty to manufacturers in how they meet various requirements, so there are no guarantees.

Originally Posted by Fly_Unity

So your saying that they just did it because it was a common practice?

I'm not sure what "it" you're talking about, but if you mean "light weight", yes, that's been the accepted practice for many years. Always? Well, I obviously can't say that. As you know, the further back you go, the more cavalier they were about safety.

Originally Posted by Fly_Unity

I agree that they tested it lighter, not because of performance, but because it will NOT VMC at gross weight at their altitude. The question isnt what they tested it at, but what weight, was the caculated 55 knot VMC speed based on.

You are correct in that they do extrapolate back to sea level, and there are graphical techniques shown in the flight testing book about how they do that. I don't recall seeing any weight adjustments, though. And it doesn't make sense that they would want to do that. The V speeds that are based on max gross weight are done so to be conservative. A Vmc at max gross would not be conservative.

[tgrayson]

Originally Posted by BuickCFI

that is straight out of 23.149, Vmc is to be determined at the most unfavorable weight for Vmc, not for performance. in this case Vmc must be determined at lightest weight

Fly_Unity is arguing that aircraft certified before 23.149 was written that way might have been tested a max gross weight.

[BuickCFI]

Originally Posted by tgrayson

Fly_Unity is arguing that aircraft certified before 23.149 was written that way might have been tested a max gross weight.

doh, i read the whole thing too, i was just not fully awake yet.

anyway, for sure the seminole was tested under 23.149 becauase it was certified March 10, 1978 according to the data sheet for it

[tgrayson]

Originally Posted by BuickCFI

anyway, for sure the seminole was tested under 23.149 becauase it was certified March 10, 1978 according to the data sheet for it

That was my guess. However, 23.149 did change in 1996. And at one time, it was merely copied from the former CAR 3 regulations, so the regulation has evolved. Tough to say how it read in 1978.

[BuickCFI]

Originally Posted by tgrayson

That was my guess. However, 23.149 did change in 1996. And at one time, it was merely copied from the former CAR 3 regulations, so the regulation has evolved. Tough to say how it read in 1978.

i was looking at the data sheet for the PA-30 and PA-39 twin commanche's and the PA-30 has a higher Vmc speed (it was under CAR 3 regulations) BUT it also has a critical engine, whereas the PA-39 looks like it has counter-rotating props. The PA-39 was certified under Part 23(i think, but i might have misread)

edit: nevermind, looks like they were all certified under CAR 3, how does that work when the FAR's were in effect in 1965 IIRC?

[tgrayson]

CAR 3:

§ 3.111 Minimum control speed (Vmc). (a)
A minimum speed shall be determined under the
conditions specified below, such that when any
one engine is suddenly made inoperative at that
speed, it shall be possible to recover control of
the airplane, with the one engine still inoperative,
and to maintain it in straight flight at that speed,
either with zero yaw or, at the option of the
applicant, with a bank not in excess of 5 degrees.
Such speed shall not exceed 1.3 Vs1, with:
(1) Take-off or maximum available power on
all engines,
(2) Rearmost center of gravity,
(3) Flaps in take-off position,
(4) Landing gear retracted.
(b) In demonstrating this minimum speed,
the rudder force required to maintain it shall not
exceed forces specified in § 3.106, nor shall it be
necessary to throttle the remaining engines.
During recovery the airplane shall not assume
any dangerous attitude, nor shall it require
exceptional skill, strength, or alertness on the part
of the pilot to prevent a change of heading in
excess of 20 degrees before recovery is complete.

[caliginousface]

Originally Posted by tgrayson

<<heavier aircraft is more stable and has more inertia; >>

You hear this a lot and it's wrong on several fronts:

1) Inertia will potentially only affect the *rate* at which control is lost, but will not determine *whether* it's lost.
2) Even so, an aircraft rotates around the CG, therefore the weight alone will not affect the nature of rotation around this point.

So what else is the answer besides the added weight contributing to the HCL?

Is it just more force being applied to the point of rotation?

[jawright]

Originally Posted by dakovich

thats the one we used at flight school

L
C
W
B

F
A
T

C
O
W
S

We use that same one...at least that's the one I was taught. "LC Will Buy Fat Cows."

[Doug Taylor]

LCWB is a very popular acronym for an Air Force Academy graduating class.

[fish314]

Originally Posted by Doug Taylor

LCWB is a very popular acronym for an Air Force Academy graduating class.

Ah, yes. I had an instructor from the class of '79!

[tgrayson]

Originally Posted by caliginousface

So what else is the answer besides the added weight contributing to the HCL? Is it just more force being applied to the point of rotation?

The larger the HCL, the more of a sideslip to the good engine you have and the greater the directional control.

It's more of a paper reduction that a real one. In reality, you're going to be shooting for zero sideslip and won't be able to take advantage of the greater weight; it will only reduce your climb rate.

In all, better to be light, rather than heavy, no matter what it does to Vmc.

[aloft]

Originally Posted by Doug Taylor

LCWB is a very popular acronym for an Air Force Academy graduating class.

Class of '79, IIRC.

[SmitteyB]

Originally Posted by flyguy

Actually, "Most Unfavorable Weight" would be a lighter aircraft. Vmc decreases as weight increases.

Originally Posted by Poster

Care to explain that? As I understand it, more weight = higher angle of attack = higher P-Factor = higher Vmc

Most unfavorable weight- I think by this they're assuming you understand with increased weight you will get a decrease in performance. Therefore unfavorable weight would obviously be MGW.

[tgrayson]

Originally Posted by SmitteyB

Most unfavorable weight- I think by this they're assuming you understand with increased weight you will get a decrease in performance. Therefore unfavorable weight would obviously be MGW.

Performance doesn't play any role in Vmc. You'll fly into the ground with good directional control.

[VDEE7]

So, Lets get this straight. You dont bank toward the good engine You'll have degraded performance due to the side-slipping and less rudder authority leading to higher VMC.
You bank 2-3º or whatever necessary to obtain zero side-slip (ZSS), and you'll have the best performance and fair VMC. You bank more than whats recommended for ZSS, and you'll have lower VMC and degraded performance again due to slipping toward the good engine!

Now, Are the performance numbers in the PA44 or any A/C for that matter, published based on ZSS bank angle or the maximum 5º the FAA allows? Because that would make a difference in climbing performance!

Also, I understand how horizontal lift is a factor in maintaining directional control, but how can we compare it between light and heavier aircraft?
I mean off course the heavier one would have higher HCL but thats because the mass is bigger and the aircraft is having to generate more lift even in level flight. The heavier aircraft might be producing more lift but guess what!.. it's still in level flight!! cuz its HEAVY, it needs bigger force to accelerate it. so that horizontal lift is not just being used to gain directional control only. I think what I'm trying to say is.. whether it's light or heavy, whatever force it takes to get the airplane to turn at certain rate. again.. thats just what I think, I might be wrong I'll have to see what TG says .

What about the Higher AOA higher P-Factor theory?? is the yawing moment really that great to cause a big change in VMC?
Is that what makes Flaps UP (or T/O position) and out of Gound Efect factors give us higher VMC?
Thanks.

[nosehair]

Originally Posted by tgrayson

The larger the HCL, the more of a sideslip to the good engine you have and the greater the directional control.

It's more of a paper reduction that a real one. In reality, you're going to be shooting for zero sideslip and won't be able to take advantage of the greater weight; it will only reduce your climb rate.

This term "paper reduction" might help explain it.

A heavier aircraft will provide more HCL when the banking technique is used. The more bank, the more directional control. Remember, we are talking about directional control, not climb performance.

The 3-5 degrees used by the manufacturer is all that is allowed for certification purposes, but you can use more. Imagine using 15 or 20 degrees, and no rudder. This wouldn't be good for climb performance, but it illustrates the effectiveness of using some bank for directional control.

If push comes to shove, and you are losing directional control with the recommended 3-5 degrees bank, and full rudder, I would keep cranking in bank until I have directional control. I would rather descend into terrain straight ahead relatively level, than roll upside down losing directional control.

[tgrayson]

Originally Posted by VDEE7

Now, Are the performance numbers in the PA44 or any A/C for that matter, published based on ZSS bank angle or the maximum 5º the FAA allows? Because that would make a difference in climbing performance!

The regs don't specify the condition, so I'd expect that the manufacturer is going to take every opportunity to maximize their performance figures, which would be at ZS. That said, a test pilot will only be able to maintain zero sideslip if he can determine it. The only real way to do that (that I know of) is with a yaw string; in lieu of that, they maybe be using 5 degrees as an approximation to ZS. One test pilot I discussed this with seemed to equate ZS with 5 degrees, but when I pointed out that you'd be slipping the other way, he agreed that this was probably the case. I highlight probably because it indicated that he didn't know, which suggested they weren't making a serious effort to determine the ZS bank angle. This is likely to vary among test pilots and manufacturer, so I'm inclined to say "No way to know."

I think what I'm trying to say is.. whether it's light or heavy, whatever force it takes to get the airplane to turn at certain rate. again.

If I understand what you're saying, you're arguing that the higher HCL is needed in order to accelerate a heavier airplane sideways. Although a higher force is needed to accelerate a heavier airplane sideways at the same rate, the rate at which we accelerate isn't important. All that affects is how long it takes to establish a steady sideslip, not whether one will occur or not, nor its magnitude. The Vmc maneuver is from an equlibrium condition, where the sideslip has already been established, regardless of how long it took. With a particular bank angle, the aircraft will accelerate sideways until the drag is equal to the lift force.

What about the Higher AOA higher P-Factor theory?? is the yawing moment really that great to cause a big change in VMC?

Seems unlikely to be significant, particuarly since the direction in which it would change Vmc is opposite what we know to occur.

Is that what makes Flaps UP (or T/O position) and out of Gound Efect factors give us higher VMC?

We don't know that those things make Vmc higher in any particular airplane. Some of the criteria for the Vmc measurement are just for consistency of testing among all airplane. However, flaps have been shown to increase directional stability in some airplanes (which would reduce Vmc). This is why many manufacturers suggest using partial flaps when landing in a crosswind.

I've never seen any tests or theory that shows that GE has any particular effect on Vmc. I could offer arguments either way, but until you have test data