Vmc in ground effect

[palgia]

I would like to hear what you think about how ground effect changes Vmc. The fact that FAR 23.149 specifies that the aircraft has to be out of ground effect might indicate that is has some effect on Vmc.

I was taught that in ground effect the reduction in induced drag has the same effect as increasing thrust, thus highering Vmc. However this never made too much sense to me, and for this reason I am not teaching this to my students (I just tell them that the effect on Vmc, if any, is trivial).
What I am having a hard time understanding is how a reduction in induced drag in ground effect would affect the airplane in an asymmetrical way. In other words, if the reduction in induced drag is the same on both wings, the asymmetrical thrust would not change (and therefore Vmc would not change). A fellow instructor was speculating that the wing on which the operating enginine is mounted would be producing more lift due to the induced lift by the propwash (therefore producing more induced drag, and thus experiencing a larger drag reduction than the oher wing). However this does not make sense to me. If one wing was producing more lift than the other, we would have a constantly accelerating roll towards the dead engine. Since the wings are level (or at least not rolling), there MUST be a roll-wise balance of forces. Therefore each wing is producing the same amount of lift overall.
The only thing I can think of, it that maybe the distribution of lift on each wing might be different and this might somehow lead to asymmetrical reduction of induced drag. But this is just pure speculation.

So can anyone clear up all the confusion that is going on in my crazy mind?


I am wrong in assuming the effect on Vmc, if any, is almost trivial?

Thanks for your input.

palgia

[Milt]

Vmc in Ground Effect

Don't go anywhere near it unless you want a short cut to the big hangar in the sky.

To determine your Vmc you progressively reduce IAS at a safe height.until you are unable to maintain control. Every pilot who tries that in ground effect has made his/her last flight unless the crash is survivable.

The mc in Vmc means minimum control for you under particular conditions of asymmetry in a particular aircraft under particular conditions.
Having determined the value of your Vmc and having remembered the conditions under which it was determined you are then pre programmed to be able to better handle an engine failure and to make a decision as to whether you can extract yourself from a marginal situation..

Ground effect may enable you to reach the beach from a little way off shore!

Ground Effect will vary with the following

Wing span and shape
Wing sweep
Fuselage shape
Aircraft configuration
Alpha
Distance above ground
TAS
Wind gusts
Ground/water surface regularity
And some others which I haven't thought of at this time.

A RAAF pilot under instruction in a B707 had 2 engines on one side shut down at about 1500 ft. He went below Vmc.
They didn't make it.

[palgia]

Milt,

thank you for taking your time to respond. However, probably because my post was not very clear, you have not answered my question.

I am aware of what Vmc is, the risks associated with flying near or below that speed, as well as how to find it doing a simulated loss of control.

I also know how most of the factors you cited affect Vmc, except for GROUND EFFECT.

The question I posed in my thread was more of a theoretical brain excercise. Its a question that I wouldn't even dream about attempting a simulated loss of control in ground effect. However, I am interested in knowing how ground effect infuences Vmc (the speed at which loss of control occurs).


Many books write that ground effect influences Vmc, but do not explain how.
The FAA mandated that for certification of small aircraft (23.149), the published Vmc must be for an aircraft out of ground effect. (which might indicate ground effect DOES affect Vmc).

Can anyone shed some more light? (refer to my first post)

Thanks,

palgia

[swh]

Rather simplistic ....

Low IAS -> High AOA -> Greater Induced Drag.

Ground effect reduces induced drag by providing a ground plane, so you could possibly go slower in ground effect, than what you could at altitude due to the change in induced drag.

How low...seem to remember 3 wing spans in altitude you are in ground effect, could be wrong.

:ok:

[palgia]

swh,

we are talking about multi engine aircraft here.... single-engine-Vmc in a twin is limited by loss of control due to asymmetric thrust (and not enought rudder force to counteract it), not stalling AOA like in single engine aircarft. (unless you are in a situation where Vmc is below stall speed, like at altitude, but thats another story)

Thanks for contributing though.

palgia



Edited so that it would make sense:rolleyes:

[swh]

palgia,

My post was for a ME aircraft ...

"I was taught that in ground effect the reduction in induced drag has the same effect as increasing thrust, thus highering Vmc. "

I would disagree with this, with the reduced induced drag, the aircraft has more "excess power" as the "power required" is reduced, but the "power available" has not changed.

Ground effect reduces Vmca below red line towards the stall.

:hmm:

[ROB-x38]

G'day palgia.

I think I remember that Vmc is lower in ground effect but to be honest don't know why.

I understand your different lift distibution ideas and therefore maybe different moments due to the induced drag. Along those lines....

A couple of assumptions:

- a left engine failure
- an imaginary centre of pressure for each wing (a point where the lift can be assumed to act through on the wing)
- the induced drag acts through the centre of pressure (can you do that!? :oh: )

I'd guess the centre of pressure would be more inboard on the right wing due to the propwash and lift over the nacelle -> the centre of pressure would be more outboard on the left wing. If you can assume the induced drag acts through the centre of pressure, climbing out of ground effect would increase the induced drag, and with the longer moment along the left wing the net result would be an increased overall moment swinging the nose to the left -> a higher Vmca.

Any thoughts palgia?

Like you pure speculation! :confused:

[FlyingForFun]

Palgia,

Please excuse me writing from a position of ignorance, but I think you may be reading between the lines when there is nothing between the lines to read. You say:The FAA mandated that for certification of small aircraft (23.149), the published Vmc must be for an aircraft out of ground effect. (which might indicate ground effect DOES affect Vmc).I would suggest that this quote does not imply that ground effect affects Vmc at all. The FARs tell you how to find Vmc, and they tell you to calculate it out of ground effect for no other reason that to do so in ground effect would be dangerous. You can not imply from this that ground effect either does or does not affect Vmc, IMHO.

(As for whether it does affect it or not, and how it might affect it, I don't know enough to argue for or against the other replies, but I'm reading them with interest!)

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

[ft]

Also speculating, but here goes.

Vmc is the speed at which you no longer have the control authority to handle the assymetric thrust caused by a failed engine.

If we assume that this is not for maximum/takeoff power, but rather for a power setting which will get you out of the situation intact without exchanging altitude for speed (i e enable you to accelerate in level flight), then we are on to something.

In ground effect, you will need less power to accelerate out of the low-speed region. Less power in the engine out scenarion means less torque caused by thrust assymetry and a lower Vmc since you’ll need less airflow over your control surfaces to counter the assymetric yaw.

Note that I’m talking actual Vmc here, not the FAA approved one...

Cheers,
Fred

[Oktas8]

There is an asymmetry of lift with one engine inoperative, assuming that the slipstream passes over the wing. This might allow for asymmetry of induced drag, which could give rise to reduced VMCA when IGE.

The live engine (propeller not jet?) generates slipstream over part of the wing & therefore more lift, while the windmilling engine generates disturbed air over part of its wing.

In practice asymmetry of lift is counteracted with aileron towards the live engine to maintain some specified angle of bank.

There is an asymmetry of induced drag associated with this aileron use - failed engine wing is operating at an increased AoA due to displaced aileron. However, I would have thought it negligible. Perhaps in some aircraft types the aileron drag asymmetry has a measurable effect on VMCA in ground effect, hence the restriction.

FlyingForFun - in my interpretation, Part 23 doesn't make comments based on the safety or otherwise of particular flight test manoeuvres. Perhaps it should!

cheers to all,
O8 :)

[palgia]

Thanks to everyone who replied.

swh,

I still cannot see how excess power can affect Vmc. Considering Vmc is the slowest speed where the asymmetrical thrust can be counteracted with rudder force, I cannot see how having more overall excess power can affect any of these variables. Maybe you could expand on what you wrote and illustrate your thought process... (I tend to be a little slow:uhoh: )

Rob,

I like your theory a lot. It definately makes sense.


Ft,

I read our post several times but can't seem to grasp the concept. Or rather, I understand what you are saying, but I can't convince myself. Maybe the reason being that in the FAA's Vmc (which is is what I learned and teach) there is no mention of performance. In fact, we teach that Performance and Directional Control are on the 2 sides of a balance(you give up one for the other). In other words, the FAA Vmc is the speed at which a heading change of less than 20 deg can be maintaned with 150lb max rudder pressure, prop windmilling, TO config, max 5deg bank, ect. There is no mention of performance. In fact, for small aircraft certification, there is no single-engine positive climb requirement, not even at Vyse (and therefore certainly not at Vmc). In other words, performance is not the limiting factor of Vmc, but control is(I don't reacall performance playing any role in the Vmc arena, I believe its all about airspeed, rudder angle of attack, cg, bank angle and yawing moments, with maybe some stabilizing keel effect from the gear, depending on its position relative to the cg).
So reducing drag the same amount on both sides (and the same distance away from the longitudinal axis) will increase performance, but will not change neither asymmetrical thrust, not rudder force, thus not altering Vmc.



Oktas8,

I will reply to your post after some more pondering.


palgia


PS. Just thought of something, maybe my theory (wings not rolling= equal amount of lift on each wing) is wrong. If the wings are not rolling, all it means is that all the rolling moments cancel out. The moment depends on TWO things, force (in this case lift) and arm. Therefore we could have a wing producing 2000lb of lift 1 meter from the longitudinal axis, while the other wing is producing only 500lb of lift using ailerons that are 4m away. The overall rolling moments cancel out. Does that make sense or am I going insane?

[Traffic]

Palgia

Don't lose any sleep over this. This is the same as wondering whether the glass is half full or half empty or whether the house burned up or burned down.

Vmc is a constant in an environment that changes not the other way around.

Ground effect reduces upwash and downwash of the airflow. This cushioning effect produces more lift and lowers the induced drag but this does not necessarily correlate with control speeds.

Vmc like all other critical V numbers are basically IAS dependent.

I always believe that if you are in ground effect you are arriving or about to die so who really cares.

Cheers

[willbav8r]

Ground effect = more lift.

Vmc = loss of directional control due OEI and operating engine at max power (10 factors of Vmc of which out of ground effect is one).

So here we are in ground effect, left engine goes quiet. We have just taken off from a soft strip, so we rotated 5 knots over red line (silly, but I'm being hypothetical). Full power is being issued by the co-operating engine. Because we are in ground effect with a negligible ROC, we have high AOA (because it is warm and we have density alt of 3,000). Eventually if we pitched for Vmc, because of ground effect (which increases lift and decreases stall speeds) we would be faced with a lower Vmc speed?

Incorrect methinks, because, as we approach Vmc, the increased lift on the wing with the co-operating engine is going to roll faster (loss of directional control)?

But we have reduced the stall speed because of ground effect, ergo we might have a lower Vmc. So does this mean that we could stall before reaching Vmc? Slightly worse than wandering to the left.....I can tell you that stalling with OEI and one at full chat would not make me a happy bunny.......

Intersting stuff.

[sycamore]

Palgia, just throwing a new hat in the ring......
IGE,reduction of ID will allow a reduction in incidence to maintain a particular height; for a prop. a/c this will probably result in a small reduction in the asymmetric turning moment(downgoing-blade effect),but probably little effect on rudder yawing ability at full deflection.So, it is possible to reduce speed a little until one is again balanced ,ie incidence again increased at lower speed to maintain ht. and asymmetric thrust again balanced by full rudder. Can`t confirm as yet as I am trying to get hold of a certification TP....I`ll also ask about effect on jets as well.......

[swh]

palgia,

With more excess power in ground effect you can reduce the power on the live engine, this in turn requires less rudder, Vmca reduces.


:ok:

[sycamore]

swh, you miss the point; the configuration for Vmca is full power on the operating engine(s) , and full rudder.An increase in power will only raise Vmca, assuming rudder power is not increased.

P.I have checked, and whilst theoretically Vmca will reduce IGE(< half-span), it is not tested as it is considered inappropriate and dangerous.

[alf5071h]

Within the certification definition of Vmc then it would be incorrect to focus on induced drag and power. Vmc is defined at full power. Any reduction in drag would enable a faster climb out of ground effect (GE), thus it may only be a transient effect.

From a theoretical viewpoint the change in lift characteristics between the lower wing vs the higher wing (5 deg bank) in GE could change the lateral-directional control input required. Thus more or less aileron implies more or less rudder would be needed for directional control depending on aircraft design / configuration; there may be a similar change for rudder / tail plane effectives at small bank angles. Also there may be changes in prop swirl airflow or reflected thrust line for a jet that may change the control input required to balance the asymmetric forces. Whatever change there may be in thrust (thrust does not increase if drag reduces) it is the change in yaw moment that the thrust causes and the ability to control yaw, that are the important aspects.

From experience of Vmc tests at a safe altitude and separately establishing ground effects, I suspect that the net effect is to reduce Vmc. Some high wing aircraft, particularly with high tails show a double GE, indicating differences between wing and tail being in/out of GE. Thus there may well be a difference in down / up going wing. Low wing aircraft can also suffer from ‘bubble burst’ in GE which indicates a rapid loss of lift as speed decreases towards stall in GE, again indicating a change in the wing lift characteristic that may also apply to the aileron effectiveness.

For those concerned about control during takeoff and climb fear not: V2, may not be less than 110% Vmc. Vr may not be less than 105% of VMC; or the speed that allows reaching V2 before a height of 35 ft above the take-off surface. Extracts from JAR 25 Section 1 (FAR Similar).

Some references to asymmetric flight here: http://uk.geocities.com/[email protected]/alf5071h.htm
See PSM+ICR and Asymmetric flight at low airspeed.
ALF