effect of flap on vmca

[glenb]

Tried doing a search on here and reading the texts but cant find any sound theories on the effect of flap on vmca. Appreciate any input,cheers.

[chimbu warrior]

I am assuming that you are referring to normal takeoff flap configuration only, and that varies from one type to another.

My 2 cents worth..........it would depend on the aircraft type; aircraft with split flaps (a la C310 & 402) would react differently to say PA-31. Very difficult to give a general answer.

[glenb]

Sorry may not have explained myself clearly. Assume a published figure for vmca with flap in the takeoff configuration. By increasing or reducing the amount of flap, would vmca increase, decrease, or remain constant, and why

[disco_air]

Well what is Vmca governed by?

The yaw moment generated by the assymetric thrust and the rudder's capability to counteract this moment.

Yaw due to assymetric thrust factors:

Thrust line lateral distance from Longitudinal Axis
Power delivered by engine
Drag produced by inoperative engine

Rudder authority (counteracting yaw) factors:

CofG location on Longitudinal Axis
Airspeed (altering lift produced by rudder)
Inherent longitudinal stability of airframe (size of fin etc)

I dont see flap as controlling or altering any of these. If I have omitted anything please post!

Obviously an increase in Flap setting will increase drag (symmetric) and the operative engine will have to produce more thrust and/or pilot have to lower nose to achieve the same airspeed, but Vmca itself would remain the same.

I think.... :p

...Disco

[Pass-A-Frozo]

I think that reducing flap will increase Vmca for the reason you mentioned. Prop slipstream flowing over the flap producing drag towards the live engine.
So, if you have max power on the operating engine(s), raising the flap will increase Vmca.
I think ... ouch... aerodynamics = headache :eek:

[disco_air]

Ahhhh PAF... didnt think of that, but then again.....

1) This assumes that the flaps are even in the slipstream of the engines in the first place (aforementioned type specific analysis)

2) With a relatively increased airflow over the flap behind the operative engine, wouldnt that also mean increased lift on the live engine's side? Increased lift on live engine = Roll towards inop engine = Yaw (secondary effect) thus increase in flap will increase yaw moment therefore increase Vmca.

I see how PAF logic works, so then maybe the two effects cancel each other out to have negligible effect?

I like these :}

...Disco

[chimbu warrior]

Just to really confuse the issue, what about where flaps are OUT of the slipstream, as on most light business jets, or even the Piaggio 166?

Methinks not a simple question, therefore no simple answer.

[Pass-A-Frozo]

Increased lift on live engine = Roll towards inop engine = Yaw (secondary effect)

Wouldn't the secondary effect of yaw only come into play if you had ran out of aileron and started rolling to the dead (ie.. you're below Vmca)

[Jet_A_Knight]

Think about......the conditions under which Vmca is determined for 'certification' purposes (SL/MTOW/Critical eng inop/MCP on live eng/TKO Config etc etc etc etc).

VMCA’s taken from Flight Manual charts are ‘standardised’ (constants for a take-off or an approach)

But the actual VMCA is not a constant and changes with configuration, density altitude, weight, CofG or any variations from those used at certification.

The speed at which controllability becomes an issue does vary. In a nutshell, anything that aggravates that T/D couple, (eg more drag/less drag/less power/critical/non critical engine failure/altitude) affects the actual vmca.

I suggest that:

Flap setting less than that for TKO = less drag = Lower actual Vmca.

Flap setting greater than that for TKO = more drag = higher actual Vmca.

[Pass-A-Frozo]

Given Vmca is predicated on maximum power on the operating engine(s)., I think you have that the wrong way around.

More flap = More drag (countering the assymetric power situation) = Lower Vmca ??

[hoss]

For the Dash 8 the Vmca, Vmcl and Vmcg all decrease with an increase in flap.

Hope that helps :)

[compressor stall]

It depends on the aircraft type.

[TruBlu351]

Another angle to look at it is that for certification, Vmca cannot be greater than 1.2Vs.

So with more flap, Vs reduces, and therefore Vmca too.

[Arm out the window]

Dropping flap increases both drag and lift, progressively more drag than lift the greater the flap angle.

Vmca is all about asymmetric effects, funnily enough, so if we're talking about putting flap down behind a working engine vs. a windmilling one, the answers should become a bit clearer.

Drag: More flap, more drag, and more so behind the working engine, so yaw to the live, lowering Vmca.

Roll: More flap, more lift, so more roll towards the dead engine. Aileron authority also being a factor in asymmetric controllability, this would raise the speed at which control is marginal.

Jet_A, Vmca, as I understand it, is worked out with rearmost CG (least rudder moment), gear down, flap in takeoff configuration (so with different takeoff flap configs, will have different Vmca figures), dead prop windmilling and takeoff power on the live, and up to 5 deg bank to the live - so all things being equal, any other CGs should only lower Vmca (ie more rudder authority, so should be able to get slower before losing it).

Stall speed vs. Vmca is obviously also a major factor - due to the varying lift/drag effects of flap, some configurations will be more hazardous than others due to the reduced margin between Vmca and stall speed with the associated risk of spinning at low level.

[Richo]

Page 25 of Russ Evans, Understanding the light twin engine aircraft" (copyright)

Flap on Vmca
"The rule to remember is that more efective flap will increase Vmca"

The reason is quite long but sumarised as follows:

Flap on good engine side creates more lift, because it has more flow (airflow + prop wash) which has to be countered by roll with aileron.
Aileron use means increased AoA and drag on the side of the failure. Ie down going aileron = increased AoA (drag)

Now if you were at any other speed above Vmca, you would have rudder available to correct the yaw. But you don't as at Vmca you have all the rudder in already.

The book goes on to mention that some aircraft (type not mentioned)will have an increase from 89kts (TO setting) to over 145kts with full flap.

Reducing flap also increases Vmca, as you need to go faster to achieve lift for takeoff in the first place.

Its a very good book for ME Instructors and people learning the trade.

Richo

[Bula]

increase thrust = incrased drag .... yeah ok but I think most people are leaving out that an increase in thrust also produced an increase in lift created by the wing where the live engine is operating. This rolling moment creates a yawing moment.

Now, with certification requirements allowing the use of 5 degrees of AOB to assist in lateral controllability, the increase in rolling moment will cause the ailerons to reach full movement prior to VMC(a). This will cause the aircraft to loose directional control at a higher airspeed.

Therefore the use of any flap will cause an INCREASE in actual VMC(a) however is allowed for during certification with the use of "TAKE-OFF FLAP" during the VMC(a) demonstration.

[GUARD]

You lower flap, you increase drag!

If you are countering Yaw with rudder at Vmca of say 65kts without flap, then Vmca occurs at 65kts. If you increase drag ( by adding flap and consequently more yaw on one side ) then you will suffer the effects of Minimum Control at a slightly higher speed than the published 65kts because you've added that drag.

At least thats how I explained it as a M.E. Instructor. At least that made sense to me and my students.

Guard:)

[Pass-A-Frozo]

increase thrust = incrased drag .... yeah ok but I think most people are leaving out that an increase in thrust also produced an increase in lift created by the wing where the live engine is operating. This rolling moment creates a yawing moment.
Now, with certification requirements allowing the use of 5 degrees of AOB to assist in lateral controllability, the increase in rolling moment will cause the ailerons to reach full movement prior to VMC(a). This will cause the aircraft to loose directional control at a higher airspeed.
Therefore the use of any flap will cause an INCREASE in actual VMC(a) however is allowed for during certification with the use of "TAKE-OFF FLAP" during the VMC(a) demonstration.

Wouldn't that depend on whether with the critical engine failed, the limiting factor is rudder or aileron?

For example.. if rudder is the limiting factor with the critical engine - then taking flap would lower Vmca as you still have more aileron to "give".

[the_regulators]

"A possible increase" seems to be the accepted answer given the lack of data from type to type.

Try the ME text written by J Chesterfield, tis a well regarded read.

[Pass-A-Frozo]

I would say a possible decrease depend on type :)

So who is volunteering to go do a practicle experiment!

Slow till just starting to lose directional control and lower flap :E

[Led Zep]

You got me thinking, PAF, even if it is slightly OT - don't the Cessna 400 series POH tell you not to raise flap in the event of an EF?
I've never flown one myself, but heard words to that effect. :confused:

[gaunty]

C300 and 400 Series all have split flaps systems and DO NOT require flap for TO, with the exception of the C404 Titan, C441 which have Fowlers and do.

In almost all cases except for very light weights Vmca is BELOW Vs in straight and level flight, which is as it should be I.E the aircraft should quit flying in straight and level flight before it all turns upside down.

Stray from the straight and narrow at your own peril.:ugh:

You get around 4KTS lower with Flaps15 but incur around -200fpm drag penalty and there are no take off data for Flaps15.

Just for the record

For C300-400 Series around
Engine windmilling = -400fpm
Gear Down = 350fpm
Flaps 15 = -200fpm
Flaps 45 = -800fpm

I dont imagine similar Piper or Beechcraft types are better.

[Pass-A-Frozo]

I guess (as people have mentioned) it depends on type. Vmca on a C130 is above Vs (In most if not all cases I'm aware of.. I'm happy to be corrected). However you raise a good point with rate of climb. Not much point lowering flap to lower Vmca if the extra drag is going to drag you into the ground.

[Bula]

Pass-A-Frozo fair point but if you lowered more flap would that not give a disproportionate incrase in roll and yaw vs aileron effectiveness. If the rudder is at the stops the ailerons must stop the roll aswell as maintain straight flight (stop the yaw)........


"There be some massive turning paddles" :ok: ... i'd hate also to think of the nose high attitude.... CRINGE... :uhoh: