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DO winglets have effect on Vref when landing?

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DO winglets have effect on Vref when landing?

Old 16th Apr 2014, 00:29
  #81 (permalink)  
 
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Originally Posted by kip View Post
Vmca, min control speed in the air... Below Vs you are not in control...

If you think you can prove opposite you compare apples with pears, but please be my guest try to rotate below Vs and keep it there next time.
Yes, you are. Or can be.

If you're rotating you're not in the air
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Old 16th Apr 2014, 01:29
  #82 (permalink)  
 
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maybe if you consider that vmcA is only valid in the take-off configuration you would understand my post.
My aircraft (a turboprop) has a VmcL equal to stalling speed with landing flap at MLW.

It should come as no surprise at all then that Vmc is very much lower than Vs in the clean configuration. Even the classic Vmca is much less than Vs in the specific circumstances pertaining to an engine failure during take off.

Presumably, the size and design of the rudder/fin have been specified for the most limiting case (in my aircraft, a rated power go around at final approach speed & configuration). Inevitably, Vmc is less than Vs in some other configurations. It may be more than Vs in some configurations, in some aircraft.

This doesn't mean that pilots will or should attempt to fly below Vs! It's just a result of compromise in the design criteria, and the different aerodynamic laws pertaining to max CL versus directional controllability.
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Old 16th Apr 2014, 02:58
  #83 (permalink)  
 
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My question is if Vmca is below VS then how is it demonstrated? It cannot be flown.
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Old 16th Apr 2014, 05:00
  #84 (permalink)  
 
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If the test pilot can't do it, it will be a job for the wind tunnel &/or mathematicians.

These days, more maths and less wind tunnel.

There are people like John Tullamarine or djpil who are more knowledgeable than I about the "how" of airframe development.
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Old 16th Apr 2014, 09:08
  #85 (permalink)  
 
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So please explain me then why a manufacturer would put on a fin that is so big that VMCA is below Vs?
Clearly indicating that they can save weight, materials and thus reduce fuel burn.
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Old 16th Apr 2014, 09:37
  #86 (permalink)  
 
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Originally Posted by Vilas
My question is if Vmca is below VS then how is it demonstrated? It cannot be flown.
Just to illustrate a point I made earlier:



Vmc is normally determined by flight test at minimum weight. At 5 degrees of bank Vmc reduces with increasing weight. That change is small so for practical purposes it may me ignored, which is conservative. So then Vmc is constant with weight.

Stall speed increases with increasing weight. At some weight Vs will be equal to Vmc, and at higher weights it is greater than Vmc.

If the cross-over weight is below the lowest weight that can be achieved with the test airplane, then Vmc is obviously not limiting and the regulator will permit a value determined by analysis to be used instead.

Originally Posted by 737Jock
So please explain me then why a manufacturer would put on a fin that is so big that VMCA is below Vs?
Firstly, you have to consider all weights and flap settings. Secondly, there are other considerations for sizing the fin, for example the decrab maneuver when landing with crosswind.
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Old 16th Apr 2014, 09:57
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Ever flew on a jet that needs more rudder input during a crosswind landing then in an engine failure at V1 at TOGA ?
No thought not....

I completely understand the theory, the practicality is a bit far fetched imho.

Maybe check this link out again: http://www.avioconsult.com/downloads...%20failure.pdf

Seems that VMCA does change, at least on a 707/dc8. Banking into the dead engine makes it all quite exciting.

Your graph is interesting, but unfortunately its not based on actual data. You could put the blue line higher by reducing the size of the fin, to a maximum of 1.2 x Vs. Thus saving weight and fuel.

Last edited by 737Jock; 16th Apr 2014 at 10:21.
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Old 16th Apr 2014, 10:19
  #88 (permalink)  
 
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737Jock,

Please explain me why single-engine airplanes have a fin and rudder.
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Old 16th Apr 2014, 10:21
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Fin is for stability.

Rudder is to compensate for crosswind, engine power etc. It controls the longitudinal axis around the vertical axis.

Your point? Its the same reason why multi-engined aircraft have a fin and rudder.
Although there are aircraft that don't have a fin and rudder.

My question was if you ever needed more rudder on a crosswind landing then on a TOGA V1cut with minimum V speeds in a jet (not talking about fancy improved climb speeds or flex takeoffs)?

Last edited by 737Jock; 16th Apr 2014 at 10:50.
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Old 16th Apr 2014, 10:49
  #90 (permalink)  
 
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Your graph is interesting, but unfortunately its not based on actual data. You could put the blue line higher by reducing the size of the fin, to a maximum of 1.2 x Vs. Thus saving weight and fuel.
My graph is generic and schematic, to assist in understanding what I'm writing. Of course you may put the blue line anywhere you like, but you would create a very unusual airplane if you put it at 1.2 x Vs at maximum takeoff weight. Since V2 must be at least 1.1 x Vmca you would severely compromise takeoff field length requirements at all takeoff weights.
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Old 16th Apr 2014, 10:54
  #91 (permalink)  
 
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Nor would you put it far below, as it would create an unnecessarily large tailfin.

After all V2 is also a minimum of 1.2Vs or 1.13VS1g. (Vs= 0.94 x Vs1g)

If you would aim for VMCA=Vsx1.09 V2min will meet both requirements. The tailfin would be minimum size and VMCA is larger then Vs at MTOW.

Now it all depends on how much VMCA changes with the decreasing weight of the aircraft. If its very little (almost constant) as you say then this is not a problem. If the change is massive then it needs to be changed in order to keep VMCA at a maximum of 1.2vs...
(Actually that is not true as the restriction is that it may not exceed 1.2VS at Mtow. It doesn't have this limitation at lower weights.)

Anyway I understand how VMCA can be below VS, I just don't think that when it is below the difference is massive. And it probably has more to do with the tailfin being designed for multiple variants of the same aircraft.

Last edited by 737Jock; 16th Apr 2014 at 11:28.
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Old 16th Apr 2014, 11:10
  #92 (permalink)  
 
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Numbers Jock.

For B744ER with GE CF6-80C2B1F engines close to standard conditions close to sea level.

Using Full rating thrust, Vs1g @ F20 varies from 160kt @ 412T to 120kt @ 230T whilst Vmca is 120kt and Vmcg is 126kt.

Before you say "gotcha" at the 230T mark, nobody would be at FR at this weight, rather Rtg 1 with Vmca of 117kt or even Rtg 2 with a commensurate reduction in Vmca.

I reiterate, I have no idea how this could be demonstrated but since it is certified data that's what we use.

There's a bit more to tail fin sizing than 1EO ops on the ground but that's a whole new thread drift from the one we are currently involved in.
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Old 16th Apr 2014, 12:32
  #93 (permalink)  
 
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737jock,

Hazelnuts' graph is only generic, as you said. But the point remains true that Vs changes more than Vmc with different configurations or weights.

Here are real numbers from my turboprop. I've included only changes in configuration, keeping weights the same.

Vs @ MLW: F0 104kts, F35 79kts
Vmca @ MLW: F0 94kts, F35 78kts

Why have a wastefully large fin & rudder, you asked. The fin & rudder is designed for the most limiting case. At other weights or configurations, Vs and Vmc won't match up.

My turboprop lacks the high lift devices of a jet, so there is "only" a maximum 10kt split between the two. I would expect the difference between F0 Vs and F0 Vmc to be even bigger in a jet.
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Old 17th Apr 2014, 00:28
  #94 (permalink)  
kip
 
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Ok, I eat my words...

Ok, this made me read up stuff that is too theoretically ;-)

Yes, Vmca can be below Vs - but this is NEVER a speed we can fly, so why bother!?

This paper http://www.avioconsult.com/downloads...d%20Engine.pdf
Explains very well the correlation...

...and then back to flying the speeds in the box
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Old 17th Apr 2014, 01:54
  #95 (permalink)  
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Harry pushes a particular wheelbarrow .. and that's fine .. his papers are worth reading for general pilot education.

One needs to keep in mind for civil line operations -

(a) Vmc is a problem at very low weights which are not the routine.

(b) beware of empty positioning flights and flights off very short runways where (a) becomes much more pertinent

(c) beware of the routine approach to airline simulator training where the low weight region rarely is investigated. Indeed, I can recall doing so with an operator which routinely used high V2 overspeed on the 732 .. the immediate results for crews so exposed was, shall we say, eyebrow-raising. However, we left them with a much better understanding and competence after a bit of explanation and practice ... I initiated this work specifically because, while using significant overspeed schedules routinely (and in the sim), they had one aerodrome on the network for which the aircraft routinely did a low weight positioning flight .. with min speed schedules.

The military, for reasons beyond me but, I guess, related to the perceived need to be able to play at the boundaries, tends, in my view, to have an unhealthy preoccupation with Vmc. I've frightened myself enough times to leave that region to the QTPs these days ..
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Old 17th Apr 2014, 02:43
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Is it because geese, like pilots, like to keep traffic on their left? I wonder if the ponds they land in are more likely to be left-hand circuits and if they have appropriate noise abatement & "dump" procedures. (I'm actually curious about his answer to the V formation leg thing.)
The question about one leg of a V formation of birds being longer than the other is usually asked by a Captain to a wet-behind-the-ears copilot after saying, "You know something about aerodynamics, right?"

The idea is to trigger a long reply about up-wash, down-wash, wing tip vortex shedding, flapping wing flight theory, complaint wing shapes, and other extended vocalization consisting mostly of BS. When the eager co-pilot finally runs out of wind, the Captain puts on his serious face and gives the answer: "It's because there are more geese on that side".

Captains invariably find this hilarious ................ copilots not so much.
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Old 17th Apr 2014, 03:33
  #97 (permalink)  
 
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Kip
The link you posted certainly helped. What Mustafa stated Vs has nothing to do with Vmca and Vmca is lower by margin raised the question how did they demonstrate the speed as is required by the authorities? It is obvious that it is derived speed if below stalling speed. Actually publishing Vmca below Vs is of no value and is rather misleading that it can be flown.
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Old 17th Apr 2014, 03:38
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Underfire

Why do you claim there is no such thing as wingtip vortices? Would "vortices formed at the wingtip caused by the combination of spanwise and chord wise airflow mixing" be a better description?

I am not saying vortices only form a the wingtip but it is widely accepted this is where they are most prevalent. Any part of the wing trailing edge will have this issue where span and chord wise flow meet, clearly exacerbated by sweep. This in turn increases the downwash, reducing the effective relative airflow which turn affects the total reaction in favour of drag, hence lift induced drag.

Either way, this induced drag is reduced by winglets. The fact that poor winglet design in terms of wing blending can increase interference drag was not part of my discussion or that of the original poster.
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Old 17th Apr 2014, 04:34
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Would "vortices formed at the wingtip caused by the combination of spanwise and chord wise airflow mixing" be a better description?
Simple response is NO, vortices do not form at the wingtip.

One may observe a compression discontinuity at the wingtip, engine cowling, or at a flap edge, but this is not the wake vortex rollup created by the aircraft.

In the next year or so, there will be a radical shift in the understanding of the mechanics of wake generation. I noted opposing arguments from Boeing and Airbus on the issue of wing design, airflow, and vortex generation, which was a bit surprising at this point. Then again, even the 787 has vortex tabs on the wings....
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Old 17th Apr 2014, 15:14
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Under fire,

Well that is certainly radical. What technological breakthrough has led to a better understanding of this situation? I wait with baited breath...
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