View Full Version : How flaps affect VMCA & VMCG
luckytwotter
26th Jul 2007, 01:22
Top of the mornin' to you all.
I was recently asked this question on an interview and cannot find a proper answer (in the books I've been reading or with the search function of this forum). All that I could stammer out was " With increased T/O flap on my type (DHC8) VMCA will decrease, by about 5kts from flap 5 to flap 15." I cannot figure out an intelligent answer for this so I look to your wisdom oh all knowing ppruners :ok:
As for VMCG on my type it doesn't change with flaps. I would appreciate any help with this, even a nudge in the right direction, where I might find the answer for myself.
Cheers,
LT
razzele
26th Jul 2007, 01:53
How does this sound:-
1-Prop aircraft = propwash.
Increased flaps setting normally increases the ammount of lift.
So if flaps (on the engine operative side) are subjected to propwash there will be an increased rolling moment- hence increased VMCA.
Mad (Flt) Scientist
26th Jul 2007, 03:27
Its a very stupid interview question because it's founded entirely on a single type, not on any characteristic that one could rationally deduce, because there IS NO SUCH RELATIONSHIP.
As previously discussed many times here (do a search - or check out the 'related thread' down the bottom of the page) there are different types which have different relationships between the various VMCs in the various configurations.
Mad (Flt) Scientist
26th Jul 2007, 03:45
Some of the things that MAY be relevant:
Flaps increase directional stability, which generally helps for VMCA, but also sideforce, which hurts.
Flaps may or may not increase dihedral effect and roll power, depending on where the roll control surfaces are, which may help or hinder if VMCA is near the roll control limit.
Flaps (of course) reduce AoA for a fixed speed, which may make the rudder more powerful, helping VMCA
Flaps reduce load on the mainwheels at a given speed, which might hurt VMCG.
Obviously, on a type with powered lift (like a dash 8) flaps can make a big change to all the aerodynamics.
How each of those (and other) effects interact will be very dependent on the specifics of a type design. There's no simple relationship you can reliably state.
Of course, you could say...'well I dunno about other types but I fly a Caravelle, and it doesn't have a published Vmca...next question'.
bookworm
26th Jul 2007, 08:21
How each of those (and other) effects interact will be very dependent on the specifics of a type design. There's no simple relationship you can reliably state.
Doesn't that make it a very stupid multiple choice question and a very good interview question? ;)
Mad (Flt) Scientist
26th Jul 2007, 11:35
Only if the question is regarded as a lead-in to a discussion.
It's evident (from the frequency of these threads, if nothing else) that at least some employers expect a single canned answer to some of these questions; that's stupid.
walkertonio
26th Jul 2007, 11:51
Hi !
I found that for my Airbus 320 :
http://img509.imageshack.us/img509/2388/sanstitreer1.th.png (http://img509.imageshack.us/my.php?image=sanstitreer1.png)
Apparently Vmca doesn't change with flaps position... but Vmcg does :ugh:
bookworm
26th Jul 2007, 13:27
MFS
Couldn't agree more. IMO questions with only one right answer have no place in any interview process. :)
Mad (Flt) Scientist
26th Jul 2007, 13:28
Bear in mind, too, that any actual published data represents what the OEM was both able and willing to achieve in test. Once you get any of the VMCs low enough that they don't affect performance, there's no incentive to keep pushing to see if you can break something; you take what you've got and call it a day.
So you can't infer a great deal about the underlying aerodynamic behaviour from the published numbers. Maybe the 'real' VMCA on the A320 IS affected by flap position, but Airbus didn't need to find out to get decent numbers.
(I know of at least one case of ours where we chose to publish a single VMCA value for two flap configs, even though the 'real' data show a slight difference. We just use the higher, conservative, value for simplicity.)
luckytwotter
1st Aug 2007, 14:40
I'd like to thank everyone for responding. As I suspected after an exhaustive search in different texts there is no absolute answer in this case.
As for the interview question, I don't think they were looking for a black or white answer. Now armed with this new info when or if I am asked this question again I will be able to talk intelligently about it, or I could justy answer liek an engineer "it depends" :E
Again thanks for the help guys,:ok:
Cheers,
LT