VMC-Max WT or Light Weight?

[Checks]

Is VMC determined using Max Gross weight or most unfavorable weight? I have heard it two different ways. The ALLATPS study book says the Semonile had VMC determined at 2730 lbs which would be a weight lighter than max gross(most unfavorable).

Question #2-
VMC is determined OGE. I am having a brain fart and which condition would VMC be higher-IGE or OGE?

Thanks!

 

[Snoopy58]

Generally, a published VMC is a "worst case" number -- it may be slightly better than charted, but it shouldn't be worse. So it would be determined at a "worst case" weight & CG. To the extent that "your milage may vary" (and you aren't using a computer to calculate your VMC at this moment to the tenth of a knot), you may be slightly better off (i.e. you retain directional control at a marginally lower speed than the published number).

My recollection is that in/out of ground effect doesn't actually change the VMC speed, as much as it changes the READING on the airspeed indicator. Thus, the change in the VMC read on the airpseed indicator would be a function of whatever error the indicator gets in ground effect. As I recall, the C-130 had one chart for in ground effect and another chart for out of ground effect. As to which number is greater, the Herk VMCA went up I think about 4 knots out of ground effect, but I don't know that this would be the same in other planes (and it's been a while since I've looked at the performance numbers). I DO recall that there were separate charts for different pitot/static systems -- Rosemont vs (whatever the other one was). Read your performance charts (and associated notes) closely to see what your plane is based on (if there even is a correction for ground effect).

Obviously, VMCAs are determined by test pilots at altitudes WELL OUT OF ground effect!

Hope that helps,

Snoopy

 

[Shieki]

It's been a while since I instructed in the Duchess, but I believe I remember teaching that Vmc would be slightly lower in ground effect because the relative increase in lift (or efficiency of the lifting surfaces) that occurs IGE allowing the same plane to stay airborn at a lower angle of attack. Reduce the angle of attack and reduce the factors that increase Vmc (torque effect, spiraling slipstream, a-symetrical thrust, and p-factor, ect.) I agree with Snoopy58 about the worst case. I cannot recall the reg that stipulates the configuration that must be used to calulate Vmc but there is one, and any current multi instructor should be able to refer you to that. The conditions that I do recall are:

1. Max Gross Weight
2. Critical Engine Windmilling
3. Flaps in T.O. position
4. Max Sea Level Power on operative engine
5. Standard Atmospheric Conditions
6. Most ADVERSE C.G. (depending on the A/C as to aft,forward,
etc.
7. Out of Ground Effect

The rest escape me at the moment, but remember that these conditions are used to calculate the highest Vmc for your airplane, so it safe to say that if you change any of these conditions (reduce power, feather prop, IGE) your going to decrease the Vmc.

Word to the Wise. Try not to get drawn into any too indepth conversations about what affects Vmc. You will think yourself into a corner. A lot of people try to argue about Flaps and Cowel Flaps and Air Vents and such. It's not worth it to me and it makes my brain hurt. Hope this helps.

 

[Shieki]

I think number one should be Max TO weight instead of max gross. Does the Seminole have a Max TO weight that is lower than Max Gross Weight? This could account for why your manual shows it calculated at a weight lower than gross.

 

[Groundskprwilly]

Vmc factors....chapter 14 of the Aircraft Flying Hand Book.

It's FAA and good info.

 

[Crizz]

I read that in the seminole the difference is negligible. My lesson plans are formulated as such and my MEI DE seemed to agree.


However.. verbatim p. 14-4 in the Airplane flying handbook.

While in strait and level flight, the airplane weight will not affect Vmc; however, banking into the operating engine creates a horizontal component of lift. This component pulls the airplane into the operating engine, counteracting adverse yaw, requiring less rudder deflection. The heavier the airplane, the stronger the horizontal component of lift, and the lower Vmc becomes.

Still sort of ambiguous.

 

[jetalc]

Only Three Factors Affect Vmc - FAA

OK - let's set this straight once and for all. There are only three factors that the FAA states that affect Vmc. CG, Density Altitude and Angle of Bank. The laundry list of factors that supposedly "affect Vmc" may or may not depending on the aircraft, and is a list of the configuration that a multiengine aircraft (not centerline thrust) must be in when it is originally certified - they must state to the general public what Vmc is without being able to "cheat."

This is directly from the FAA's Flight Training Handbook in Chapter 16, and is actually pretty clear:

Vmc Demonstrations


Every light twin engine airplane checkout should include a demonstration of the airplane's engine out minimum control speed. The engine out minimum control speed given in the FAA approved Airplane Flight Manual, Pilot's Operating Handbook, or other manufacturer's published limitations is determined during original airplane certification under conditions specified in the Federal Aviation Regulations. These conditions normally are not duplicated during pilot training or testing because they consist of the most adverse situations for airplane type certification purposes. Prior to a pilot checkout, a thorough discussion of the factors affecting engine out minimum control speed is essential.

Basically, when one engine fails the pilot must overcome the asymmetrical thrust (except on airplanes with center line thrust) created by the operating engine by setting up a counteracting moment with the rudder. When the rudder is fully deflected, its yawing power will depend on the velocity of airflow across the rudder - which in turn is dependent on the airspeed. As the airplane decelerates it will reach a speed below which the rudder moment will no longer balance the thrust moment and directional control will be lost.

During engine out flight the large rudder deflection required to counteract the asymmetric thrust also results in a "lateral lift" force on the vertical fin. This lateral "lift" represents an unbalanced side force on the airplane which must be counteracted either by allowing the airplane to accelerate sideways until the lateral drag caused by the sideslip equals the rudder "lift" force or by banking into the operative engine and using a component of the airplane weight to counteract the rudder induced side force.
In the first case, the wings will be level, the ball in the turn and slip indicator will be centered and the airplane will be in a moderate sideslip toward the inoperative engine. In the second case, the wings will be banked 3 to 5 degrees into the good engine, the ball will be deflected one diameter toward the operative engine, and the airplane will be a zero sideslip.

The sideslipping method has several major disadvantages: (1) the relative wind blowing on the inoperative engine side of the vertical fin tends to increase the asymmetric moment caused by the failure of one engine; (2) the resulting sideslip severely degrades stall characteristics; and (3) the greater rudder deflection required to balance the extra moment and the sideslip drag cause a significant reduction in climb and/or acceleration capability.

Flight tests have shown that holding the ball of the turn and slip indicator in the center while maintaining heading with wings level drastically increases Vme as much as 20 knots in some airplanes. (Remember, the value of Vmc given in the FAA approved flight manual for the airplane is based on a maximum 5 degree bank into the operative engine.) Banking into the operative engine reduces Vmc, whereas decreasing the bank angle away from the operative engine increases Vmc at the rate of approximately 3 knots per degree of bank angle.

Flight tests have also shown that the high drag caused by the wings level, ball centered configuration can reduce single engine climb performance by as much as 300 FPM, which is just about all that is available at sea level in a nonturbocharged light twin.
Banking at least 5 degrees into the good engine ensures that the airplane will be controllable at any speed above the certificated Vmc, that the airplane will be in a minimum drag configuration for best climb performance, and that the stall characteristics will not be degraded. Engine out flight with the ball centered is never correct.

The magnitude of these effects will vary from airplane to airplane, but the principles are applicable in all cases.

NOTE - A bank limitation of up to 5 degrees during Vmc demonstration is applicable only to certification tests of the airplane and is not intended as a limit in training or testing a pilot's ability to extract maximum performance from the airplane.

For an airplane with nonsupercharged engines, Vmc decreases as altitude is increased. Consequently, directional control can be maintained at a lower airspeed than at sea level. The reason for this is that since power decreases with altitude the thrust moment of the operating engine becomes less, thereby lessening the need for the rudder's yawing force. Since Vmc is a function of power (which decreases with altitude), it is possible for the airplane to reach a stall speed prior to loss of directional control.

It must be understood, therefore, that there is a certain density altitude above which the stalling speed is higher than the engine out minimum control speed. When this density altitude exists close to the ground because of high elevations or temperatures, an effective flight demonstration is impossible and should not be attempted. When a flight demonstration is impossible, the check pilot should emphasize orally the significance of the engine out minimum control speed, including the results of attempting flight below this speed with one engine inoperative, the recognition of the imminent loss of control, and the recovery techniques involved.

Vmc is greater when the center of gravity is at the rearmost allowable position. Since the airplane rotates around its center of gravity, the moments are measured using that point as a reference. A rearward CG would not affect the thrust moment, but would shorten the arm to the center of the rudder's horizontal "lift" which would mean that a higher force (airspeed) would be required to counteract the engine out yaw. Figure 16-8 shows an exaggerated view of the effects of a rearward CG.

http://www.faatest.com/books/FLT/Chapter16/Vmc%20Demonstrations_files/imageI1M.jpg Questions?

 

[A Squared]

Uhhhh, , jetalc,

I gotta point out that the FTH, or at least your excerpt does *not* say those are the only things that affect VMC.

Those may be the only ones discussed, but the book doesn't say they are the only factors.

The amount of power applied to the operative engine has a *big* effect on VMC.

The weight of tha aircraft also affects VMC.

Don't assume that because the FTH only discusses those factors, that those are the only factors that affect VMC. One does not necessarily follow the other.

Remember, the FTH is the book which promotes the fallacy that the air over the upper surface of the wing has to travel faster to meet the same air at the trailing edge, so don't take the FTH's contents as source of ultimate truth. it's not. It has big flaws.

 

[Lead Sled]

OK - let's set this straight once and for all. There are only three factors that the FAA states that affect Vmc. CG, Density Altitude and Angle of Bank.

FAA or not, that is an incorrect statement - available power is also a factor. (Controllability and VMC limitations is one of the prime factors when it comes to flat-rating the powerplants on some pretty common turboprop twins ie the MU2 series.)

What difference does it really make? Aren't you looking for recognition, recovery and avoidence? If you do your VMC demos at max gross weight, how are you getting there? Stuffing eager young students into the seats? I hope not.

Just some thoughts.

Lead Sled

 

[jetalc]

Uhhhh, , jetalc,

I gotta point out that the FTH, or at least your excerpt does *not* say those are the only things that affect VMC.

Those may be the only ones discussed, but the book doesn't say they are the only factors.

The amount of power applied to the operative engine has a *big* effect on VMC.

The weight of tha aircraft also affects VMC.

Don't assume that because the FTH only discusses those factors, that those are the only factors that affect VMC. One does not necessarily follow the other.

Remember, the FTH is the book which promotes the fallacy that the air over the upper surface of the wing has to travel faster to meet the same air at the trailing edge, so don't take the FTH's contents as source of ultimate truth. it's not. It has big flaws.

I'm perfectly aware of the flawed logic in the FTH - my question was more geared toward somebody finding SOMETHING in writing from a credible source that addresses the factors that affect Vmc. You can come up with your theories about the different factors (BTW, the 'available thrust' on the operating engine certainly would fall under density altitude logic - although there might be other factors that would limit or increase the available thrust on the operating engine), but until somebody comes up with something other than Part 23 (which has little in my mind to do with definitively stating that those factors affect Vmc in all multiengine non-centerline thrust aircraft. You don't need to lecture me on the fallacies of the FAA's thinking - I'm fully aware of it. In an attempt to help students learn 'the right things' I think it's a normal request to ask that you document YOUR answers...Part 23 doesn't cut it...

Thanks for your response.

 

[JetBlast2000]

I’m not quite sure why you would limit your discussion of Vmc. I have always taught the 10 factors that affect Vmc and used 23.149 along with the FAA material. I do make a distinction between factors and limitations as discussed in part 23. The DE we used liked it taught in this manner. It important that students have all the info…
JB2k

 

[A Squared]

Jetcalc,


OK, so you're perfectly aware of the FTH's flaws. Hmmmm alrighty, what was your point again?


"my question was more geared toward somebody finding SOMETHING in writing from a credible source that addresses the factors that affect Vmc."


Uhhhh no you didn't have a question, you were going to "set this straight once and for all." That's not a question.

As far as sources, let's take weight. Greater weight lowers VMC. You can read that in Kershner's "Advanced pilot" , Gardner's "The Complete Multi-engine Pilot" and The FAA's "Airplane Flying Handbook" which has replaced the FTH. (perhaps because of the latter's inaccuracies and omissions?)

Finding those took about 5 minutes, just grabbing books from my bookshelf. If, as you say, you have been looking for 15+ years without finding that, I would have to ask, where exactly have you been looking? You ever try looking in books?

In addition to those references, If you had a strong enough Physics background to understand free body analysis, I could draw you a vector diagram which would show pretty conclusively that weight does indeed affect VMC.

 

[ROB-x38]

And for another viewpoint from over here on the other half of the earth higher aircraft weight = lower Vmc for the reason quoted by Crizz.

 

[jetalc]

Jetcalc,


OK, so you're perfectly aware of the FTH's flaws. Hmmmm alrighty, what was your point again?


"my question was more geared toward somebody finding SOMETHING in writing from a credible source that addresses the factors that affect Vmc."


Uhhhh no you didn't have a question, you were going to "set this straight once and for all." That's not a question.

As far as sources, let's take weight. Greater weight lowers VMC. You can read that in Kershner's "Advanced pilot" , Gardner's "The Complete Multi-engine Pilot" and The FAA's "Airplane Flying Handbook" which has replaced the FTH. (perhaps because of the latter's inaccuracies and omissions?)

Finding those took about 5 minutes, just grabbing books from my bookshelf. If, as you say, you have been looking for 15+ years without finding that, I would have to ask, where exactly have you been looking? You ever try looking in books?

In addition to those references, If you had a strong enough Physics background to understand free body analysis, I could draw you a vector diagram which would show pretty conclusively that weight does indeed affect VMC.

Well, I thank you for your objective and obviously informed explanation. If you have time in your busy schedule (assuming you're not admonishing a student for actually 'questioning' mainstream thinking), is there any chance you can explain why increased weight would change the IAS at which you would lose directional control?

 

[jetalc]

A Squared, have a nice day.

 

[PHX767]

A practical look at Vmc

Perhaps the best way to look at the factors affecting Vmc is to look at the certification requirements individually.

I.e; In discussion with a student, I would begin with: Aircraft gross weight - Certified at max TOW. Since you cannot be heavier than that, what happens to Vmc at lighter weights? Goes up or down?

Address each certification item in that manner and that should eliminate any ambiguity.

And then... How about the factors that make the critical engine critical?

 

[ROB-x38]

G'day jetalc

The variation of the minimum control speed (Vmc) with weight is closely related to the 5 degree AoB limit set down in the rules when determining Vmc.

As I'm sure you're aware when you bank "five to the live" or "raise the dead" there is a small horizontal component of lift (a force) which is added to the force produced by the rudder. The horizontal component of lift supplements the force produced by the rudder and aids in maintaining directional control to counteract the assymetric thrust/drag. So for a given assymetric thrust condition, when a bank is introduced the force produced by the rudder can be less which translates to a lower Vmc.

There are a couple of reasons the regulations impose the 5 degree AoB limit for determining Vmc:

Firstly banking 5 degrees has a negligible impact on the vertical lift generated to oppose weight (the exact amount can be worked out with trigonometry) which means elevator forces are minimal.

Secondly if the 5 degree rule didn't exist it would be possible for manufacturers to publish a Vmc figure which was much less. Obviously, ignoring stall speeds, if you were able to bank the aircraft to 30 degrees, that horizontal component of lift which is supplementing our rudder is now much greater, so the rudder force required is even less which, again, translates to a lower Vmc. The problem with this is the non-practical or unrealistic handling methods which would needed to be used in the event of engine failure (ie: significant angles of bank, large elevator forces etc...)

Hopefully some of the above makes sense, and if it does it should be clear that for a heavier weight that horizontal component which is helping our rudder is going to be greater, which means less force needed from the rudder which means less airflow over the rudder which means lower Vmc.

 

[jetalc]

Rob -


Thanks - it's been a while since I've looked at this stuff, and I appreciate the answer. Yes, it makes sense, as do the other factors - I just hadn't seen the FAA admit that there was a direct correlation between those factors and the actual Vmc speed. It's not that I don't understand Vmc, or that I don't have my own theories for how each of those factors affects Vmc...it's just that I hadn't seen where the FAA had put its neck on the line and actually come out and said it.

Thanks for the explanation - it's obvious you put a lot of effort into your response, and I appreciate it.