Vmca and flaps
Thread Starter
Join Date: Feb 2005
Location: in my pants...
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Vmca and flaps
Hi all!
Quick question: One of the conditions of Vmca are that the flaps have to be in the up position. I'm looking for some explanation on this.
OK, flaps increase directional stability, thus decreasing the Vmca but what's exactely going on?
Thks for your input
Quick question: One of the conditions of Vmca are that the flaps have to be in the up position. I'm looking for some explanation on this.
OK, flaps increase directional stability, thus decreasing the Vmca but what's exactely going on?
Thks for your input
Vmca is a certification speed therefore it is measured under a set of predetermined conditions. The conditions are set to represent those most likely to be encountered in the take off condition. For light twins see BCAR Section K. The parameters may well vary for different certification groups.
If you want to test something in a repeatable way you need to define the test conditions to control the variables that aren't of interest.
Flaps will generally affect the asymmetric rolling moment and also cause an increase in drag which will lead to an increase in asymmetric yawing moment.
Why do you say that flaps reduce Vmc?
HFD
(edited to correct a typo)
Flaps will generally affect the asymmetric rolling moment and also cause an increase in drag which will lead to an increase in asymmetric yawing moment.
Why do you say that flaps reduce Vmc?
HFD
(edited to correct a typo)
Last edited by hugh flung_dung; 1st Feb 2007 at 17:30.
Why do it if it's not fun?
Join Date: Jul 2001
Location: Bournemouth
Posts: 4,779
Likes: 0
Received 0 Likes
on
0 Posts
One of the conditions of Vmca are that the flaps have to be in the up position
FFF
--------------
Join Date: Jan 2007
Location: Portugal
Posts: 8
Likes: 0
Received 0 Likes
on
0 Posts
ok, let see what the definition said:
Vmca : Air Minimum Control Speed is the minimum flight speed at which the airplane is directionally controllable as determined in accordance with FARs. The airplane certification conditions include one engine becoming inoperative and windmilling, a 5-degree bank towards the operative engine, take-off power on the operative engine, landing gear up, flaps in take-off position, and most rearward C.G. For some conditions of weight and altitude, stall can be encountered at speeds above Vmca as established by the certification procedure described above, in which event stall speed must be regarded as the limit of effective directional control.
extract from BE76 Airplane Flight Manual
So based on this definition everything can be clarified... and updated to general definition ...
I hope its help you.
bye
Bruno
Vmca : Air Minimum Control Speed is the minimum flight speed at which the airplane is directionally controllable as determined in accordance with FARs. The airplane certification conditions include one engine becoming inoperative and windmilling, a 5-degree bank towards the operative engine, take-off power on the operative engine, landing gear up, flaps in take-off position, and most rearward C.G. For some conditions of weight and altitude, stall can be encountered at speeds above Vmca as established by the certification procedure described above, in which event stall speed must be regarded as the limit of effective directional control.
extract from BE76 Airplane Flight Manual
So based on this definition everything can be clarified... and updated to general definition ...
I hope its help you.
bye
Bruno
Thread Starter
Join Date: Feb 2005
Location: in my pants...
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
well, i was just hoping for some aerodynamics explanation...
Let's just forget about the definitions of Vmca or the regs and the fact flaps have to be in the TO config for the demo.
How will flaps being lowered or raised affect the Vmca? and i am not interested in just knowing if it will increase or decrease it.
hugh flung_dung started with a good point, i do understand there will be an increase in the asym rolling moment due to the increased lift on the blown wing compared to the failed wing.
Why do you say that the flaps will cause an increase in drag which will lead to an increase in asymmetric yawing moment? The form/parasite drag will be equally increased on both wings but does the decrease in induced drag due to the increase in lift on the blown wing has anything to do with it?
Most of the books/manuals i have referred to just stay vague about it..
Thanks for your help
I'm just trying to come up with a real and valid explanation in front of my students
Let's just forget about the definitions of Vmca or the regs and the fact flaps have to be in the TO config for the demo.
How will flaps being lowered or raised affect the Vmca? and i am not interested in just knowing if it will increase or decrease it.
hugh flung_dung started with a good point, i do understand there will be an increase in the asym rolling moment due to the increased lift on the blown wing compared to the failed wing.
Why do you say that the flaps will cause an increase in drag which will lead to an increase in asymmetric yawing moment? The form/parasite drag will be equally increased on both wings but does the decrease in induced drag due to the increase in lift on the blown wing has anything to do with it?
Most of the books/manuals i have referred to just stay vague about it..
Thanks for your help
I'm just trying to come up with a real and valid explanation in front of my students
Why do it if it's not fun?
Join Date: Jul 2001
Location: Bournemouth
Posts: 4,779
Likes: 0
Received 0 Likes
on
0 Posts
Most of the books/manuals i have referred to just stay vague about it..
We've already discussed the increased rolling effect due to slipstream. Also, the CofG might change marginally when flaps are lowered. The CofP will certainly change, which I would think would affect Vmc by altering the effectiveness of the rudder. The flaps themselves may have a slight stabilising or de-stabilising effect too. I doubt that any of these would make any appreciable difference, but the number of factors involved would, I would think, make it very hard to say - in general terms, at least - exactly what difference the flaps would make.
FFF
-----------
How will flaps being lowered or raised affect the Vmca?
Why do it if it's not fun?
Join Date: Jul 2001
Location: Bournemouth
Posts: 4,779
Likes: 0
Received 0 Likes
on
0 Posts
...shows a complete lack of understanding of the definition of what Vmca is
FFF
----------
Thread Starter
Join Date: Feb 2005
Location: in my pants...
Posts: 20
Likes: 0
Received 0 Likes
on
0 Posts
Right on FFF... was pretty much a typo... i am as well not an expert in aerodynamics and just trying to clarify one point.. which is the flaps effects on Vmc...
Join Date: Mar 2005
Location: House
Age: 54
Posts: 89
Likes: 0
Received 0 Likes
on
0 Posts
There are a lot of factors contributing to effect of flaps on Vmca. On some airplanes it will increase others it will decrease. Let me try and give some examples of some of the factors:
1. Type of flaps: Split flaps for example, will generally have a lowering effect on Vmca (BilliBob, I know Vmca is always the same, but for the sake of the argument blah blah blah) if deployed as they primarily produce drag, and almost zero lift, therefore they will have a stabilizing factor. If you have flaps that generates high amount of lift you might experience increased roll towards the dead engine due to slipstream. On some airplanes you might experience selecting flaps 20-25 will add 1 or 2 knots to the Vmca, but going for full flaps, it might decrease a couple knots as the last step of flaps is mainly drag on many flap types.
2. Position of flaps: If your engines are located on the tail, there won't be much of a slipstream over the flaps and therefore lower Vmca with flaps out. So to put 1 and 2 together, you will see that amount of power, position and type of flaps are some factors.
3. Counter or Co-rotating engines: Most types of flaps enables you to fly at a lower angle of attack at a given airspeed - hence you can say, that p-factor or asymmetric blade effect will decrerase with flaps down. With co-rotating propellers, that will be an advantage (lower Vmca because thrustline moves closer to the center), but with counter-rotating propellers it will be a disadvantage, as there is no critical engine, and with a lower AOA the thrustline will move AWAY from the center.
The conclusion is that the effect of flaps has many variables. In any case it will not change the Vmca drastically on most airplanes. Only very few POH's state Vmca with and without flaps. Sometimes it is stated in the Type certificate data sheet. I have copied this from the type certificate data sheet of a BAE HS748:
Airspeed limits: Vmo (Maximum Operating)
From sea level to 15000 feet 225 kts.
Above 15000 feet 215 kts.
Va (Maneuvering) 155 kts.
Vfe (Flap Speeds)
Flap deflection 7 1/2° 180 kts.
Flap deflection 15° 180 kts.
Flap deflection 22 1/2° 140 kts.
Flap deflection 27 1/2° 120 kts.
Vlo (Landing Gear Operation)
Operation 160 kts.
Extended 160 kts.
Vllo (Landing Light Operation)
Operation 140 kts.
Extended 140 kts.
Vmc (Minimum Control Speed)
82 kts. (Flaps 0 < 22 ½ O)
81 kts. (Flaps = or >22 1/2°)
Notice only 1 knot difference.
I hope this answers your question, otherwise I suggest you ask the CI at your flying school, as I am sure he will know
1. Type of flaps: Split flaps for example, will generally have a lowering effect on Vmca (BilliBob, I know Vmca is always the same, but for the sake of the argument blah blah blah) if deployed as they primarily produce drag, and almost zero lift, therefore they will have a stabilizing factor. If you have flaps that generates high amount of lift you might experience increased roll towards the dead engine due to slipstream. On some airplanes you might experience selecting flaps 20-25 will add 1 or 2 knots to the Vmca, but going for full flaps, it might decrease a couple knots as the last step of flaps is mainly drag on many flap types.
2. Position of flaps: If your engines are located on the tail, there won't be much of a slipstream over the flaps and therefore lower Vmca with flaps out. So to put 1 and 2 together, you will see that amount of power, position and type of flaps are some factors.
3. Counter or Co-rotating engines: Most types of flaps enables you to fly at a lower angle of attack at a given airspeed - hence you can say, that p-factor or asymmetric blade effect will decrerase with flaps down. With co-rotating propellers, that will be an advantage (lower Vmca because thrustline moves closer to the center), but with counter-rotating propellers it will be a disadvantage, as there is no critical engine, and with a lower AOA the thrustline will move AWAY from the center.
The conclusion is that the effect of flaps has many variables. In any case it will not change the Vmca drastically on most airplanes. Only very few POH's state Vmca with and without flaps. Sometimes it is stated in the Type certificate data sheet. I have copied this from the type certificate data sheet of a BAE HS748:
Airspeed limits: Vmo (Maximum Operating)
From sea level to 15000 feet 225 kts.
Above 15000 feet 215 kts.
Va (Maneuvering) 155 kts.
Vfe (Flap Speeds)
Flap deflection 7 1/2° 180 kts.
Flap deflection 15° 180 kts.
Flap deflection 22 1/2° 140 kts.
Flap deflection 27 1/2° 120 kts.
Vlo (Landing Gear Operation)
Operation 160 kts.
Extended 160 kts.
Vllo (Landing Light Operation)
Operation 140 kts.
Extended 140 kts.
Vmc (Minimum Control Speed)
82 kts. (Flaps 0 < 22 ½ O)
81 kts. (Flaps = or >22 1/2°)
Notice only 1 knot difference.
I hope this answers your question, otherwise I suggest you ask the CI at your flying school, as I am sure he will know
Join Date: Sep 2006
Location: Pergatory
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
That is an unanswerable question and shows a complete lack of understanding of the definition of what Vmca is. For a specific aeroplane type, Vmca never changes and can be determined only with the flaps in the take-off position.
For practical use, actual Vmca is constantly changing as the aircraft goes through its flight profile. To answer the question (which IS answerable), flap position shouldn't have much effect on Vmca, ATBE. Flame away...
Join Date: Aug 2004
Location: Sittard, the Netherlands
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
I'm sorry formulaben, but I don't agree.
VMCA is a MINIMUM controll speed, i.e. the speed above which you are certain during flight that you can maintain directional controll during an engine failure.To establish this minimum number, the manufacturer takes a worst case scenario, i.e. configuration just after t/o, and then sees at what speed you can no longer maintain direction. This configuration has been given above. This means that for a given type the VMCA is always the same. However, given your configuration and flight profile, it is possible that you are able to maintain direction at a lower speed, but that is not what the definition is about.
The rest of the story goes for a light twin prop, where a big part of the lift is generated by the propwash. If you would have flaps set for t/o, and you would have an engine failure, this would mean that the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement.
As seen before, if this difference is really noticable, remains the question, but this would be the theoretical approach.
I hope you can all live with this explanation, if not, just fire at will
VMCA is a MINIMUM controll speed, i.e. the speed above which you are certain during flight that you can maintain directional controll during an engine failure.To establish this minimum number, the manufacturer takes a worst case scenario, i.e. configuration just after t/o, and then sees at what speed you can no longer maintain direction. This configuration has been given above. This means that for a given type the VMCA is always the same. However, given your configuration and flight profile, it is possible that you are able to maintain direction at a lower speed, but that is not what the definition is about.
The rest of the story goes for a light twin prop, where a big part of the lift is generated by the propwash. If you would have flaps set for t/o, and you would have an engine failure, this would mean that the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement.
As seen before, if this difference is really noticable, remains the question, but this would be the theoretical approach.
I hope you can all live with this explanation, if not, just fire at will
Why do it if it's not fun?
Join Date: Jul 2001
Location: Bournemouth
Posts: 4,779
Likes: 0
Received 0 Likes
on
0 Posts
Charles,
Please forgive me for taking apart your post piece-by-piece and pointing out your errors.
Formulaben's post is, on the whole, correct. The only bit of his post I disagree with is where he says: "actual Vmca is constantly changing". I agree with his sentiment in this statement, but his terminology is very slightly wrong. What he means is "Vmc is constantly changing".
However, if you are going to criticise someone for a minor terminology error, you'd better understand the terminology. You say:
That is not correct. It is true that Vmca relates to configuration just after take-off, but there is no assumption anywhere (except by you) that this is the worst case scenario. There are sure to be worse cases, and it is not true that you are guaranteed to be able to maintain directional control during an engine failure above Vmca if you are configured in one of these worse cases. I'm being quite picky here, more so than I would normally, but only because your criticism of Formulaben's post was even more picky than I am being now. And a small error in terminology such as Formulaben has made is unlikely to have any impact on his ability to fly a multi-engine aircraft safely, whereas a lack of understanding of what a number given in a POH actually means just might impact safety, albeit it's very unlikely in this case.
So what about selecting slightly more than take-off flap? Let's not use landing flap, because landing flap often has a high element of drag - but say 20 degrees of flap? On a type like the Duchess, where no flap is used for take-off (and therefore Vmca is measured with flaps fully up), using 20 degrees of flap would, by your own argument, give more lift on the wing with the working engine, increase the effect of the asymmetry, and increase Vmc to something above Vmca - hence we have a possible situation (a go-around, maybe?) where a speed higher than Vmca is needed to maintain control.
To repeat - nowhere is it stated that Vmca is a "worst case" figure, or a figure above which there is a guarantee that you will be able to maintain control. It is a specific case relating to the take-off configuration, but that's about all.
Although your argument about increased lift and increased Vmc sounds good, and I'm sure it's correct, it's not the full story. Zakka has posted, just a couple of posts above, a quote from the POH of an aircraft which states the oposite - for the type he's discussing, Vmc actually goes down when flaps are lowered. So a blanket statement about Vmc increasing with flaps is not true in every case. That's why myself, Zakka and others have specifically avoided giving a blanket answer.
I think I just did! 
FFF
--------------
Please forgive me for taking apart your post piece-by-piece and pointing out your errors.
I'm sorry formulaben, but I don't agree
However, if you are going to criticise someone for a minor terminology error, you'd better understand the terminology. You say:
VMCA is a MINIMUM controll speed, i.e. the speed above which you are certain during flight that you can maintain directional controll during an engine failure.To establish this minimum number, the manufacturer takes a worst case scenario, i.e. configuration just after t/o
the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without
To repeat - nowhere is it stated that Vmca is a "worst case" figure, or a figure above which there is a guarantee that you will be able to maintain control. It is a specific case relating to the take-off configuration, but that's about all.
Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement.
I hope you can all live with this explanation, if not, just fire at will
FFF
--------------
Join Date: Sep 2006
Location: Pergatory
Posts: 1
Likes: 0
Received 0 Likes
on
0 Posts
The only bit of his post I disagree with is where he says: "actual Vmca is constantly changing". I agree with his sentiment in this statement, but his terminology is very slightly wrong. What he means is "Vmc is constantly changing".
Join Date: Aug 2004
Location: Sittard, the Netherlands
Posts: 3
Likes: 0
Received 0 Likes
on
0 Posts
return fire!!
I'm sorry, but I have to defend myself here...
VMCA is a minimum controll speed. That meens that the actual controll speed during flight will change, I agree fully, but it will always be lower than VMCA, so if you keep over this speed, you will be able to maintain directional control for sure, no matter what configuration you are in. Maybe when you let your speed drop below VMCA you get lucky, but I wouldn't base my life on the luck of the draw...
And as this is the heart of the discussion and the definition of VMCA, that is why I was "picky"...
Further I have not the slightest wish to say anything about the flying capabilities about anyone here on the board, so if I gave you that impression, I humbly apologise.
That never was my intention, I save my opinion on your flying untill I have actally seen you perform, but then I will be strict and firm in my verdict!!!
Howerver, I stay with my opinion that VMCA is based on a worst case scenarion, although I give you points for the fact that it is never stated anywhere this explicit. And the Seneca in which I did most of the training normally also has 0 flaps for take-off, so that point is also out of the discussion. But, just after rotation, low speed, high angle of attack, one engine windmilling, one t/o pwr, a.s.o., give me an example where there is actual a worse case? I will prove you that any case is better than this (purely looking at directional controll, that is).
eagerly awaiting your response...
I'm sorry, but I have to defend myself here...
VMCA is a minimum controll speed. That meens that the actual controll speed during flight will change, I agree fully, but it will always be lower than VMCA, so if you keep over this speed, you will be able to maintain directional control for sure, no matter what configuration you are in. Maybe when you let your speed drop below VMCA you get lucky, but I wouldn't base my life on the luck of the draw...
And as this is the heart of the discussion and the definition of VMCA, that is why I was "picky"...
Further I have not the slightest wish to say anything about the flying capabilities about anyone here on the board, so if I gave you that impression, I humbly apologise.
That never was my intention, I save my opinion on your flying untill I have actally seen you perform, but then I will be strict and firm in my verdict!!! Howerver, I stay with my opinion that VMCA is based on a worst case scenarion, although I give you points for the fact that it is never stated anywhere this explicit. And the Seneca in which I did most of the training normally also has 0 flaps for take-off, so that point is also out of the discussion. But, just after rotation, low speed, high angle of attack, one engine windmilling, one t/o pwr, a.s.o., give me an example where there is actual a worse case? I will prove you that any case is better than this
(purely looking at directional controll, that is).eagerly awaiting your response...
Why do it if it's not fun?
Join Date: Jul 2001
Location: Bournemouth
Posts: 4,779
Likes: 0
Received 0 Likes
on
0 Posts
just after rotation, low speed, high angle of attack, one engine windmilling, one t/o pwr, a.s.o., give me an example where there is actual a worse case?
1) As per your example, but with a different amount of flap selected. (In the case of the Duchess or the Seneca, I don't know whether selecting a small amount of flap would, indeed, increase Vmc, but you said yourself that it would and I agree that it might).
2) As per your example, but at a sea-level airfield, on a colder-than-ISA day, and with high pressure. In these conditions, the good engine will develop more power and more thrust than it would in ISA conditions, hence more asymmetry (thanks, Englishal!).
Both of these are realistic scenarios - the first relates to a go-around, the second speaks for itself.
FFF
------------
Join Date: Mar 2005
Location: House
Age: 54
Posts: 89
Likes: 0
Received 0 Likes
on
0 Posts
The rest of the story goes for a light twin prop, where a big part of the lift is generated by the propwash. If you would have flaps set for t/o, and you would have an engine failure, this would mean that the wing with the working engine and the flaps down, would give you more lift than without the flaps. Therefore the speed to maintain direction would theoretically be higher with flaps than without, therefore the flaps in t/o position requirement
Zakka