Vmca and flaps

[Charles van Haren]

@FFF: To the best of my knowledge nowhere is stated that these values are measured at or related to ISA conditions. In the Seneca, which has turbo's and thus constant pwr until app. 12.000' this is correct. There is a graphic in the manual in which you can see what happens whit VMCA after this alt. I don't know if there are comparable graphics in non-turbocharges aircraft, but I would like to know what they would say to ISA.
However, if you let your engine have more power at higher density, I will make my rudder more effective at higher density to counter for that!
Second, to the more flaps argument: The only reason I can see to fly with more flaps than recommended for t/o by the manufacturer (in normal flight operations i.e.) would be on the approach or during landing. Therefore, if we are to make a go around, the first thing in my crew concept is "g.a., set pwr for take off, set flaps... where you would fill in the normal number of flaps during t/o or at least a reduction of flaps. In other words, you would reduce your flapsetting ASAP to t/o cnfg. This would again lead to my worst case scenario, which of course relates to all "normal" flight conditions, whith all "normal" emergencies.

@zakka: In my original post I tried to give a correction on the, i.m.h.o., misuse of the VMCA definition by formulaben, and thereafter I tried to give an explanation to why VMCA would be higher with flaps lowered, as I thought this was an assumption that had to be cleared. This last part of my post was just my 2 cents worth, but as I re-read it, I can understand that you would have thought this to be my definition of the truth. I was reacting to one of the thoughts posted earlier in the thread, and missed severall of your comments in your post. Therefore to my statement that flaps would by definition raise VMCA, I stand corrected.

[FlyingForFun]

However, if you let your engine have more power at higher density, I will make my rudder more effective at higher density to counter for that!

I beg to differ.

As a twin instructor (you say in your public profile that you are a FI and CRI, so I assume you instruct on twins), I am horrified that you don't understand this, and the consequences of it.

The rudder will have exactly the same effectiveness, regardless of altitude, at a given IAS. This is because IAS is a measure of dynamic pressure, and control effectiveness is directly related to dynamic pressure. I think you may be confusing IAS with TAS here. If you increase pressure but maintain TAS, then yes, rudder effectiveness will increase. If you increase pressure but maintain IAS, then TAS will decrease, but rudder effectiveness will not change.

I accept that you teach on turbo-charged aircraft, therefore this doesn't apply to your current type. But it is a fact that, as pressure altitude increases (in a normally-aspirated aircraft), so Vmc reduces. The implication of this is that, as we continue to climb, Vmc eventually reduces to a point where it coincides with Vs. As instructors, it is vital that we understand this, since we regularly (at least once with each student) have to demonstrate Vmc - and if we this at too high an altitude, without being aware of the implications, we risk running out of rudder at the same time as we stall, resulting in a spin.

This is a well-known fact. Not directly relevant to our discussion, until you reverse it. The reverse situation is that, as pressure altitude decreases, so Vmc increases. Contrary to your last post, I believe that Vmca is measured at sea-level in ISA conditions, but I don't have documentation to hand to prove it.

But, whether that's true or not, Vmca must be measured in some kind of atmosphere. And, whatever atmosphere it is measured in, it's only necessary to add a millibar or two to that atmosphere to increase Vmc to something above Vmca (albeit only marginally).

Therefore, if we are to make a go around, the first thing in my crew concept is "g.a., set pwr for take off, set flaps...

That's interesting. My company's procedures are quite different. For an all-engine go-around, we raise the drag flap immediately, then wait for a positive rate of climb. Then we raise the gear. Then we raise the flap. Plenty of time for an engine failure to occur in there! (Although, if an engine failure were to occur at such a critical time, the EFATO drill would involve raising gear and flaps immediately after selecting full power, so the situation would only exist for a matter of seconds.)

FFF
----------------

[englishal]

This is the FAA's certification requirements for Vmc:

1) Aft CG (at rear CG limit)
2) Trim in takeoff position
3) Out of ground effect
4) Maximum takeoff weight at sea level (or at a weight that produces most unfavorable Vmca)
5) Maximum available power or takeoff power on operating engine
6) Flaps (wing) in takeoff position
7) Flaps (cowl) in takeoff position
8) Landing gear retracted
9) Standard atmospheric conditions (29.92”, 15 deg. C, sea level)
10) Bank up to 5 degrees into operating engine
11) Prop windmilling (or feathered if equipped with autofeather)
12) No more than 150 lbs. of rudder pressure required
13) Must be able to maintain heading within 20 degrees


Flaps up and Vmc will increase
Gear down and Vmc will decrease
Fwd CofG and it will decrease
Cowl flaps closed and it will increase
> standard temp and it decreases
No bank and it increases
In ground effect and it decreases

etc.....

Regarding the Seneca II, this actually generates most power at 12,000'. Rated per side to 200HP at sea level, 215 Hp at 12,000 (which is why in FAA land you need a High Performance endorsement to fly one )

[italia458]

I'm surprised at the lack of knowledge here and the ridiculous quotes people are using.

Granted I'm bringing the thread back to life years later but the correct Vmc testing standards were published in 1996, years before 2007, so why are incorrect versions of the FARs being quoted?

The current version is: § 23.149 Minimum control speed.

(a) VMCis 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) VMCfor takeoff must not exceed 1.2 VS1, where VS1is determined at the maximum takeoff weight. VMCmust 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.

Now, when aircraft were certified they most likely were using previous versions of 23.149 and possibly where it said that it was to be completed at MTOW, among other things. However, that's not the current version and I would be teaching my students the current version as well as letting them know that aircraft that have been certified before the current version date would have been tested differently.

Touching on the point of being the "worst case"... Vmc is determined for a set configuration. Nowhere does it say that it is required to be the worst case for the aircraft. You'd probably be able to pick that up just by looking at the previous versions where they say that it should be at MTOW and the first versions don't even mention weight and then the current one says most unfavourable weight. They can't all be right kids!

As FFF has stated, there are times where actual Vmc would be higher than what the POH states.

The reason I came here was to find some solid evidence about what happens with regard to Vmc when flaps are either retracted or extended. I personally think that there are too many variables to state what will happen as a rule because each airplane is so different. It makes sense that fowler flaps would increase the lift and drag symmetrically along the wings. Both the lift and drag would increase from 0 degrees to full deflection. The lift part coupled with the blown side would create more lift, banking the aircraft towards the dead engine. The opposite aileron used to counteract this would produce drag on the dead wing side which would increase Vmc.

As surprising as this might be, wings produce induced drag and parasite drag. The flaps produce the same. At small deflections the induced drag is in the majority and when increasing towards 40 degrees or even 60 degrees, parasite drag is now a big player. What's the difference between flap at 60 degrees at the trailing edge of the wing or a flap opening up to 60 degrees at the middle bottom of the wing? Both will slow the airplane down due to parasite drag.

Induced drag will decrease with an increase in speed so the induced drag on the blown wing will be lower than the dead wing, increasing Vmc. However, the parasite drag at large flap settings will produce more drag on the blown side than the dead side because of the higher velocity on the blown side. The question is where is the equal point and how much do these factors actually affect the Vmc speeds of the aircraft? That depends on a whole bunch of things. That's my proposition.

It would be good to note that there are multiple factors with regard to single engine flight. Three basic ones I can think of are stability, control and performance. Vmc is only talking about performance. The reason the 5 degree limit is there is so that manufacturers don't "abuse" the rule and state an airplane has a very low Vmc by using 10 degrees of bank into the live engine. The 5 degrees has nothing to do with performance. If you want performance you want zero sideslip which is different between aircraft but as a rule of thumb it's ~2 degrees for non critical engines and ~3 degrees for non critical engines. So, decreasing the bank from 5 degrees will increase the Vmc but will increase the performance, i.e.: climb rate. And what about weight? It's said to decrease Vmc. That's true if the angle of bank remains the same, the side component of lift will be greater which helps oppose the asymmetric thrust. But what if you decreased the bank so as to maintain the same side component of lift? What happens to the zero sideslip? Well, if your Vmc is calculated at 5 degrees of bank, it would go towards the zero sideslip, improving performance. But the extra weight would be a penalty in your climb performance. I think the overriding factor would be the weight, therefore reducing your climb performance but like above, it really depends on the aircraft.

When you're thinking about Vmc, think about the "control" of the airplane. Disregard performance factors such as zero sideslip until you're ready to start thinking about the performance of the airplane.

EDIT: When I'm reviewing material or studying I always try to ask "why" something is the way it is. Or why the regulation states something. Going back to the point of this not being the "worst case" scenario... when is the airplane at full takeoff power, gear retracted and flaps in the takeoff position? Right after takeoff, which is also where you're passing through Vmc at full power. I can't think of any other time in normal flight that you'd be passing Vmc at a high power setting. It's also where an engine failure has a significant chance of happening since you're working the engine quite hard. And "significant" meaning more so than the other phases of flight. I didn't talk to the person who made the regulation but it makes sense to test Vmc in this configuration here.

[Desert185]

Takeoff flaps is the key to this. That might mean flaps up or not up. Vmc has to be above stalling speed. If not, there is really no Vmc(a). I believe the B-N Islander falls into this category and of course the C-336/337, for obvious reasons.

Keeping in mind the conditions and aircraft configuration established for measuring Vmc published in the certification requirements, actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.

In doing an unplanned Vmc demo in the Twin Otter simulator, exceeding the 50# torque limit will result in a higher demonstrated Vmc and loss of directional control, which is why there is a 50# torque limit when upgrading the engines from -27's to -34's. Vmc in the Twin Otter is with flaps 10, not up, BTW.

[Big Pistons Forever]

Personally I think too much emphasis is placed on VMC, and not enough on Vyse, in the training for the ME rating. Every light twin will have a negative rate of climb at, or any where near, VMC. If you are close to the ground and any significant speed below Vyse, than you are in a unrecoverable situation.

Whether the VMC goes up or down slightly with configuration changes is irrelevant to the safe operation of a multi engine aircraft. If you are actually flying close to VMC in an actual one engine inoperative situation then you have grossly mishandled the aircraft.

From an operational stand point the critical point in every takeoff is that period when the aircraft is below Vyse and with the gear and flaps still down. Until the aircraft is cleaned up with Vyse on the ASI and healthy positive rate of climb, the only sane course of action is to close both throttles and land straight ahead.
VMC has nothing to do with this.

[Desert185]

Big Pistons Forever:

You are correct, although VMC has to be understood as part of the process. A good example of why you are making a good point is a 3-engine ferry takeoff in a Herc. 2-engine Vmca is 136 KIAS with high rudder boost and 166 KIAS with low rudder boost. Rotation speed is less than that, so there is a period of time when the loss of a second engine could result in a failure to continue controlled flight.

Even though I haven't flown a Herc since 1987, those speeds are still ingrained. Like I said above, Vmc has to be understood as part of the process.

[Big Pistons Forever]

Originally Posted by Desert185

Big Pistons Forever:

You are correct, although VMC has to be understood as part of the process. A good example of why you are making a good point is a 3-engine ferry takeoff in a Herc. 2-engine Vmca is 136 KIAS with high rudder boost and 166 KIAS with low rudder boost. Rotation speed is less than that, so there is a period of time when the loss of a second engine could result in a failure to continue controlled flight.

Even though I haven't flown a Herc since 1987, those speeds are still ingrained. Like I said above, Vmc has to be understood as part of the process.

Since I am type rated on the DC6 and L188 I am quite familiar with the 3 engine case and should add that Vmcg is also a very significant limitation, particularly for the L188. However I would suggest that these very specialized situations are not very relevant to flying a light piston twin during initial training for the Multi engine rating.

[italia458]

actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.

Desert, you've got it backwards. Higher DA, lower power, higher temp, lower pressure will all get Vmc to be lower since Vmc is an indicated (calibrated) speed and all of those things will reduce engine power.

BPF, I agree Vyse could actually be considered the more important value here. However, it's important to be teaching students correctly the first time. There is a lot of misinformation out there about Vmc. Just because it's possibly not as important as Vyse doesn't justify continuing to teach it incorrectly. As an instructor I've come across MANY areas that are incorrectly taught and it's frustrating when you're trying to explain to your senior instructor that they're wrong... most of them will quote things they were told with no research or reference or even any logical reasoning! My instructor who was teaching me for the flight instructor rating told me that I should teach that all forces are balanced while the airplane is in a level turn. He justified it because it's "easier for some students to understand". Btw, the forces in a turn are NOT balanced, if they were then the airplane wouldn't turn. The correct diagram shows less forces... wouldn't that be easier to understand?! I'm getting off topic a bit but this is one of the main reasons why we have so much misinformation out there!

[Desert185]

Quote:
actual Vmc can be at a higher airspeed given the higher DA, lower power, higher temp, lower pressure, CG, etc. Vmc can also be at a lower airspeed at a lower DA, very cold temp, etc.
Desert, you've got it backwards. Higher DA, lower power, higher temp, lower pressure will all get Vmc to be lower since Vmc is an indicated (calibrated) speed and all of those things will reduce engine power.

You're right. Shouldn't have been in such a hurry.

[Desert185]

BPF:

Originally Posted by Desert185
Big Pistons Forever:

You are correct, although VMC has to be understood as part of the process. A good example of why you are making a good point is a 3-engine ferry takeoff in a Herc. 2-engine Vmca is 136 KIAS with high rudder boost and 166 KIAS with low rudder boost. Rotation speed is less than that, so there is a period of time when the loss of a second engine could result in a failure to continue controlled flight.

Even though I haven't flown a Herc since 1987, those speeds are still ingrained. Like I said above, Vmc has to be understood as part of the process.

Since I am type rated on the DC6 and L188 I am quite familiar with the 3 engine case and should add that Vmcg is also a very significant limitation, particularly for the L188. However I would suggest that these very specialized situations are not very relevant to flying a light piston twin during initial training for the Multi engine rating.

Any example to enhance understanding and transfer of knowledge to the student is relevant, IMO. I would rather impart related experience to reinforce the importance of Vmca (to include Vmcg, if applicable), and the relationship to Vyse/V2 speeds than parse the information. As usual, the devil is in the details.

[Big Pistons Forever]

Originally Posted by Desert185

Any example to enhance understanding and transfer of knowledge to the student is relevant, IMO. I would rather impart related experience to reinforce the importance of Vmca (to include Vmcg, if applicable), and the relationship to Vyse/V2 speeds than parse the information. As usual, the devil is in the details.

Actually I do use the case for 4 engine transport category aircraft as an example of a case where Vmcg and Vmca actually has operational relevance before I return to emphasizing the factors which have operational relevance to the aircraft being actually operated for the training flight (usually a Pa 44 in my case), where Vmc is only a theoretical consideration in a normal takeoff, or any other phase of flight for that matter.

I am all for students having a sound understanding of the relevant theory, but not all theory is equally important. In the case of Vmc I think entirely too much time is spent on configuration trivia and pedantry over definition and far too little time on the practical operational significance of the published Vmca and the concept of Vmc in general. In other words it is assigned an importance in the training syllabus that exceeds its value.

I find there is a discouragingly large number of ME instructors who can site chapter and verse on Vmc but also think it is possible to fly away from a engine failure just after liftoff in your average piston twin......

[italia458]

I am all for students having a sound understanding of the relevant theory, but not all theory is equally important. In the case of Vmc I think entirely too much time is spent on configuration trivia and pedantry over definition and far too little time on the practical operational significance of the published Vmca and the concept of Vmc in general. In other words it is assigned an importance in the training syllabus that exceeds its value.

Even though I believe that teaching the correct/current definition is important, I definitely agree with your comment. What Vmc means while operationally flying needs a bit more spotlight time.

[Pull what]

I'm surprised at the lack of knowledge here and the ridiculous quotes people are using.

You havent learned much in 12 months then!

[Pull what]

I find there is a discouragingly large number of ME instructors who can site chapter and verse on Vmc but also think it is possible to fly away from a engine failure just after liftoff in your average piston twin......

And more to the point quite a lot of very inexperienced PPLs are flying around thinking the same.

[Pull what]

The reason I came here was to find some solid evidence about what happens with regard to Vmc when flaps are either retracted or extended.

Anything that increases the drag will increase VMC-putting flap down always increases drag so it will always increase VMC

[Desert185]

Pull what: Anything that increases the drag will increase VMC-putting flap down always increases drag so it will always increase VMC

Well then how do you explain why Vmc in the Twin Otter, for example, is Flaps 10, 64 KIAS and Vyse is Flaps 10, 80 KIAS (reduced 4 KIAS/1000#)?

Obviously, drag has increased with flaps 10, but generally stating that a drag increase will increase Vmc maybe isn't totally accurate. I know that in a four-engine jet manuevering speed is much higher with flaps up. Of course, rudder travel is generally limited at higher airspeeds, so there is obviously a balance between flap position, rudder travel and Vmc for those aircraft. In the large aircraft I have flown, Vmc is always lower with flaps in the takeoff position, which could be as much as 50% flaps in one particular type.

[italia458]

Anything that increases the drag will increase VMC-putting flap down always increases drag so it will always increase VMC

Pull what... I'd love to see where you got that information. Vmc is regarding controllability. Drag is not a direct factor in controllability so you can't state a general rule like that.

[Pull what]

My apologies Italia and Desert-actually I think you right-thanks for challenging that.

[Pilot DAR]

Perhaps I have some learning to do... I am well aware that Vmca maneuvering close to the ground is very high risk, and the cause of too many accidents, particularly when un planned. However, when carefully planned, at altitude, why the worry?

have to demonstrate Vmc - and if we this at too high an altitude, without being aware of the implications, we risk running out of rudder at the same time as we stall, resulting in a spin.

This caught my eye, and I find myself wondering about it's basis. I have been taught that the safest twin, is one where Vmca and Vs are reached at about the same time.

Any light twin to which this discussion applies, would have demonstrated compliance to the following:

Sec. 23.205

Critical engine inoperative stalls.

(a) A multiengine airplane may not display any undue spinning tendency and must be safely recoverable without applying power to the inoperative engine when stalled. The operating engines may be throttled back during the recovery from stall.
(b) Compliance with paragraph (a) of this section must be shown with--
[(1) Wing flaps: Retracted and set to the position used to show compliance with Sec. 23.67.]
(2) Landing gear: Retracted.
(3) Cowl flaps: Appropriate to level flight critical engine inoperative.
(4) Power: Critical engine inoperative and the remaining engine(s) at 75 percent maximum continuous power or thrust or the power or the thrust at which the use of maximum control travel just holds the wings laterally level in the approach to stall, whichever is lesser.
(5) Propeller: Normal inoperative position for the inoperative engine.


I've done this test a number of times, and always found the subject aircraft very compliant. The odd time, a wing has dropped, but was very recoverable, particularly when the power was reduced on the operating engine.

Does the fact that the rudder is hard over as the stall is approached suggest a spin, should things continue, if the aircraft is otherwise symmetrical because of asymmetric power?

Wouldn't it be a reasonable expectation that the candidate pilot be able to keep things under control by reducing power on the operating engine, and managing the yaw and pitch of the aircraft to prevent either a stall or a spin, while flying at the Vmca limits? The aircraft will give a warning of an impending stall, and if the stall is prevented, a spin cannot occur.

Providing that there is adeqaute space between the aircraft, and the ground, are we that concerned about directional control at Vmca? In the worst case, we don't always need to fly straight, if allowing a gentle turn allows control to be maintained otherwise? For my experience, when attemping to fly an assymetric twin slower than Vmca (when it is higher than stall speed) the aircraft does not suddenly flop over on it's back, it just starts a gentle turn. Obsticle clearance aside, a gentle turn is not that big a problem, if it keeps you in control otherwise while you sort the plane out?

I offer the following photos, taken by me while testing a Piper Navajo. Near gross weight, forward C of G, Flaps were at 15 degrees, wheels up, left engine stopped and feathered, right engine 75% power. The aircraft was in stable, straight flight, with an intermittent stall warning, but no impending change in controllability. (it was stable enough that I could take photos while flying!)





I also stalled the aircraft in this configuration right after I took these photos, and it had no tendancy to spin (though I did reduce power right away, and keep it straight with rudder).

I know that Vmca training is percieved as high risk, and many accidents have resulted from poorly executed single engine flying. Is it wise, however, to avoid training into these regimes of flight, when the conditions can be well controlled?