Please help on 4 exam questions

[dynamite dean]

Please couls all those much cleverer pilots help me...

1) what are the advantages of a V tail a/c (eg bonanza) 6 marks

2) why is the nose attitude higher in a sideslip than in a straight glide? 10 marks

3) expalin why we use a rudder to pick up a wing at the incipent without mentioning the use of ailerons (harder) 15 marks

4) what is the effect of sweepback on induced drag 10 marks

okay this is a lot of writing but my south african texts are not enough any help is much appreciated.

[Slasher]

Im not much clevererer and Ive forgoten a lot but yeh Ill have a crack at it:

1. Beech built the V35 with the wings too far bloodey forward. A more forward CG was therefore required and Beechs simplest solution was to ditch the least weight at the furthest arm. So they dumped the fin and rudder and bent the tailplane up.
I did a few hours in the V-tail and saw no advantage with this configuration. Flew well in calm air but the tail bucked and kicked like a paraletic fish when flying in moderate/severe turbulence. Very longitudinaly unstable.

2. I know what it is but I just cant put it into bloodey words! I think Im gettin old. http://www.pprune.org/ubb/NonCGI/frown.gif

3. Out-spin aileron will accentuate the dropped wing by inducing a higher AoA as well as add further induced drag. A downwards-going aileron exaserbates the stall. So use the secondary effect of rudder to pick up the stalled wing.

4. If Im not mistaken induced drag is less due to smaller energy losses in the smaller tip-trailing vortices (esp at lower angles of incidence).
Be aware at low speed induced drag is much much higher when a sweptback wing falls apart and has everything dangling out. AoA which is directley proportional to ID can be very high (ID=1/2RoV2SCd). Given a same Vimd (min drag speed) a typical jets sweepback will have a higher coeficient of induced drag. Note that Vimd typicaly is 1.2Vs flaps down for a straight-wing and around 1.4Vs for swept. Clean, a swept is up around 1.6Vs as opposed to 1.4Vs unswept.

[Stan Evil]

I'd just like to go for question 3.

Rudder should NEVER be used to 'pick up a wing' - that's what the ailerons are for once the wing is unstalled. Let's look at the standard stall recovery:

1. Move the stick forward to unstall the wings (stop the horn or the buffet as appropriate)

2. Simultaneously apply full power and STOP ANY FURTHER YAW WITH RUDDER.

3. Now that the wings are unstalled, roll the wings level with aileron.

4. Ease out of the dive.

Applying lots of rudder when still in the stall is a recipe for disaster. How do you enter an intentional spin? - You stall the aircraft and apply lots of rudder - enough said.

It is true that you must not use aileron while the wings are stalled as this can also lead to autorotation and spin entry but they are perfectly safe as long as there is no stall buffet/warning.

By the way, about 15 years ago the UK RAF removed mention of the rudder from the stall recovery on the grounds that, as long as the stick was moved forward promptly and power applied, the ailerons could be used almost immediately and so the complication of using rudder was avoided.

[Slasher]

Where do you get that one from Stan? A stalled wing (as in a spin) can only be unstalled by opposite rudder. A spin can be entered intentionaly or unintentionaly (inadvertantley letting the speed to drop off while turning final or doing the same while in a steep turn). Once the wings are unstalled yes you use aileron to level them.

Spin recovery procedure:

1. Release back pressure on the stick (or full forward stick if required in some types)

2. Boot in full oposite rudder until the spin stops. Ailerons are neutral.

3. With spin arrested and when IAS starts to increase, smoothley apply back-pressure on the stick to arrest the descent, and input aileron to level the wings.

4. Level off and apply power as required.

Torque effect can seriously jeapardise the recovery if power is introduced too early in the recovery procedure, and especialy if spinning in the direction of the torque.

In a DH82 (slats locked) I generaly put in full oposite aileron entering a spin if I want to really wind it up. For recovery I have to neutralise the ailerons then boot in the rudder.

Maybe Stan these newer toys have possibley eliminated the need for rudder input with full flying fancy ailerons. I wouldnt know because I dont fly them.

PS Still cant put Q2 into words!

[Skycop]

Slasher,

I go with Stan on this one. The original question mentions the term incipient. The wing isn't stalled yet.

He is correct that the RAF are quite insistent that rudder is not used to pick up the low wing at the incipient stall. If you are near the stall and use a bootful of rudder the inner wing slows down just as much as the outer one speeds up (a great disadvantage of these old fashioned joined-up wing aircraft). If the decelerating wing then stalls you have just given the aircraft the perfect excuse to bite you by entering a spin in the direction of the rudder input.

Don't fly fixed wing aircraft - they all have this serious design defect. They can't stop while you think! Also, don't fly anything that may involve using your @rse for an undercarriage.

[Straight]

Hi DD. Q #3 seems to get things started and I must react as Stan has it dangerously wrong!
A C172 will have full aileron authority to roll the airplane even in a full stall situation.
As you progress to the bigger puppies, this is not the case anymore.
What we are talking about is Angle off Attack. Each aircraft has an AoA cross over point, a speed where the rudder authority overrides aileron authority. So at high AoA’s you roll with rudder!
The reason to unload (the yoke forward) in stall excercises is to get the AoA away from Clmax to a point that all controls act in their normal way (no secondary effects anymore), this might even be to 0G!

Lets assume we are flying an approach, as this is the phase of flight where the speed lowers and the AoA rises. When one wing drops (for instance due to windshear or we’re messing things up and flying much too slow), the natural reaction would be to pick it up by aileron input. The aileron on the low wing goes down, thus increasing the camber of the wing and so the AoA, but what happens on the high wing is even more hair raising, the spoilers come up, whoops……gone is the lift.
So the only way out is by putting in rudder to get the wings level and unloading the aircraft to get the AoA down (of course in this scenario you have to keep an eye on not hitting the dirt).

Another interesting point is the runaway rudder. The only way out is to lower the AoA so that ailerons and not the rudder roll the aircraft again. After that get your speed above the cross over point to maintain aileron authority.

These are all unnatural actions to us. It’s nice to have a forum like this where we can get our minds in the right direction before we encounter these phenomena in real life

[UhrCleared-Takeoff]

Called me old fashion Slasher, but I thought that it's use of the elevator that unstalls the wing in a stall or spin.
Anyone who tries recovering from a spin just by using the rudder could be in for a very nasty surprise!
Doesn't use of opposite rudder help to reduce the yawing moment in the direction of the spin and moving the stick forward reduces the angle of attack to recover from the spin?

[Tor]

If I may add that the question, made by JAA, says "why we use a rudder to pick up a wing". So in their mind (and mine too) you should use the rudder to "pick up" the wing. Also remember that the question is made for commercial aviation (G/A?), not supersonic fighters that may have completly different aerodynamics.

I'm completely with Slasher on this one.

Stan - you wont get into a spin if use use the rudder to the correct side. If the A/C drops the left wing, there is no way you enter a spin if you use right rudder (and have the ailerons centered). However, if you by mistake use left rudder you'd most likely spin if the A/C is prone to spin (eg. the Socata TB-series).

It's worth noting that most light A/C won't spin at all. Try and get a C172 to spin - it's almost impossible. When doing my CPL I did some spin excercises in the C172 and tried everything. Power on, nose 30 deg ANU, crossed controls and what did it do?.. Stalled nicely rolled a little bit, dropped the nose and the came out a couple of hundred feets lower at a 90 deg offset course. I did get it to spin though (1 1/2 turns :)) and as soon I let go of the crossed controlls it came out nicely.

The CAA wants to see you using the rudder to pick up the wing when you do your CAA-check.

Tor http://www.aviators.dk/log/ubb_noncgi/wink.gif

[Dick Whittingham]

1) There is a list somewhere, probably written by Beech. In practice, none. Who else uses them?
2)You're yawing. If you were yawing wings level the nose would be off to one side. Roll on bank, as in a sideslip, and the nose goes up.
3)Secondary efect of rudder is roll. Use with care. Coarse use of rudder near the stall can cause autorotation. Some aircraft maintain roll control by aileron deep into the stall, all aitcraft vary but in general at the incipient spin stage use rudder as an anti-spin control to stop the wing going down while you reduce alpha. Be very, very careful with swept wings at the stall, Rudder can wipe you out.
4)Elliptical straight wings have minimum induced drag.Swept wings have a high induced drag. This gets worse as alpha rises. Nose high approach on a swept aircraft is really draggy.

[wysiwyg]

A V-tail gives longitudinal and directional stability with less wetted area, doesn't it?

regards
wizzy

[supermunk]

One of the theories was that a V tail causes less interference drag because it's only attached to the fuselage at two points.

As for spins, it's catch 22. usually you use rudder to stop the rotation, but picking up a wing with rudder before the spin is fully developed carries a government health warning. However... an old adage with the F4 was "When it buffets, use your boots", the F4 is of course a different sort of aircraft

[John Farley]

DD (not 34DD)

Your Q3

“ explain why we use a rudder to pick up a wing at the incipient stall without mentioning the use of ailerons (harder) 15 marks”

Just a thought, but the “without mentioning the use of ailerons” could be key

IF that is what the question said then I read it quite differently to the others who have posted on the topic. I see it as an attempt to get you to write about dihedral effect and explain why the use of rudder will make an aeroplane roll.

(Could be just me though. Glad I don’t have to pass the exams these days)

Q1 IMHO You have been given the points for this (less weight and less drags various) The reason that these ADVANTAGES are not used by many aircraft is because there are disadvantages but they don’t ask for those so lets not get into that.

Q 2. When you sideslip the wing will be a tad less efficient (reduced thickness chord ratio) So to generate the same lift as before you will have to use a bit more angle of attack – assuming you do not increase the speed – IE yank the nose up.

Q 4 There is but one effect of sweepback on induced drag (ID) – it increases it.

As there are 10 marks for this, they probably expect you to punt an explanation.

ID is of course totally lift dependent. No lift no ID. Lift being the upward force on the wing resulting from less pressure above it compared to that below it, THIS PRESSURE DIFFERENTIAL HAS TO ACT AT 90 DEGREES TO THE WING SURFACE. (apologies I’m not shouting just don’t know how to do the clever bold stuff etc) Because of this the more you increase the angle of attack the more you tilt the lift force backwards and increase its drag element.

Now to add the sweepback bit

For a given wing aerofoil section, the greater the sweepback the thinner the thickness chord ratio becomes (that is why we use sweepback of course – to kid the air it is seeing a thinner wing and so delay the onset of Mach effects without actually making the wing thinner structurally or reducing valuable internal volume) All other things being equal reducing the thickness chord of a wing will reduce the slope of the Lift versus AOA curve. A reduced slope in turn means that you need more AOA for any desired increase of lift. I.E. a swept wing needs more AOA for a given lift than a straight one. So the lift vector gets tilted backwards more on a swept wing hence more induced drag


Slasher.

Agree the sense of your words but I would not be happy to go into bat with your formula for ID!

From my words above you will see that I would be looking for an expression that calculated the rearwards (along the flight path) component of lift. For a symmetrical wing aerofoil this would be Lift x Sine AOA plus a term for the tip losses. For a more normal non symmetrical section you would have to add a little to the AOA value to take account of the actual zero lift angle

Of course I find I am wrong more and more often these days.

JF

[4dogs]

Folks,

Having seen how some of these questions get written, I suggest that Q3 is being over-read. Poor choice of words does occur, particularly as the question writer may not be the most knowledgeable person around.

The old stall recovery procedure that I was taught and subsequently taught was very definitely not to pick up the wing with rudder. It said:

"Rudder to prevent further yaw"

for very sound reasons. Firstly, preventing further yaw by the use of rudder removed one necessary condition for autorotation. Secondly, the use of ailerons at Clmax does two things - one, it increases the local alpha on the downgoing wing and may compromise the unstalling process by prolonging the stall, and two, it creates adverse yaw in exactly the wrong direction thus enhancing the chances of autorotation. Thirdly, attempting to pick up the lower wing with rudder at high alpha is an invitation to over-correct and thus create a pro-autorotation yaw the other way. Fourthly, depending on the airframe, generating significant rolling moments by gross use of rudder may well exceed the design load of the back end.

My personal view: only ever enough to prevent, never enough to correct.

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Stay Alive,

[email protected]

[Captain Cessna]

Doesn't the F117 have a V tail? Maybe Beech wanted the bonanza to have a lower radar return too :)

[Ignition Override]

V-tail Bonanzas were known in the US as the "doctor killer", especially in thunderstorms.

[This message has been edited by Ignition Override (edited 21 November 2000).]

[twistedenginestarter]

It's a while since I did a stall but in C152 you instinctively apply rudder to catch the falling wing. Forget the theory. Believe me your leg will move by itself.

Thus in a 777 you keep your feet firmly on the floor.

You could answer the question by saying at the start of a spin only the rudder and elevator retain correct aerodynamic functionality. The rudder is the one for picking up wings.

[Vmu]

Stall recovery checklist:

1. Reduce Angle of Attack

Checklist completed


When the wings are no longer stalled, all controls can be used in the normal way on any normal aircraft.


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"Recovery was marginal..."

[twistedenginestarter]

Wing Drop Checklist

When wing drops apply rudder to level the plane.

End of Checklist.

If you let the wing drop it becomes difficult to unstall because you are now in massive sideslip.