Questions for pilots or aerodynamicists

[SlipStream77]

A couple of questions that have been puzzling me recently, maybe PH has a few residents who can answer them.

First, is it necessarily bad practise to fly an aircraft using primarily ailerons and elevator? Obviously when landing and taking off, particularly in crosswinds, rudders are essential, but when at altitude, does it really matter? I can only speak from sims like IL2 and FSX but I would have thought that banking and then applying a little elevator would be enough in most cases and the aircraft wouldn't be slipping too much would it?

My second question is regarding lift. The formula for which is based on several factors, one of which is the velocity of air over the airfoil. Essentially, the faster the airflow, the more lift.

Formula here...

http://en.wikipedia.org/wiki/Lift_(force)

If an aircraft, for example an F15, is in level flight at 5000 feet travelling at 300knots and it then accelerates to 600knots without the controls being touched (Let's assume calm air conditions), surely it is going to gain a lot more lift and thus climb?

The velocity of air is squared in the lift formula so at high speeds, the amount of lift must be immense. Why is it then that when flying at high speed and low altitude, aircraft don't have to assume a nose down attitude to compensate for the extra lift?

Perhaps it's a very subtle attitude at those speeds, but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

[Glade]

As a previously trainee glider pilot...

It's more important to use rudder in gliders and I presime light aircraft to cancel the secondary effect of ailerons.

In large aircraft e.g Hercules the rudder is so massive that very slight movements have a large effect meaning it is used less frequently.

Though my memory is hazy on the subject

[tegwin]

Firstly...

In an average plane the rudder is mainly there to adjust the trim of the aircraft...So if you have a crosswind, the rudder is used to keep the aircraft flying "into the wind" so as to reduce the drag on the aircraft... It is also used in steep turns (turns done using the aerlerons) to help balance out the turn and again keep the aircraft in trim flying in a straight line...

The same principle applies to helicopters (in forward flight)

if you actually try and turn a plane ONLY using the rudder....its not going to work too well!



Your second question... have you considered the effect of drag? The faster you go, the amount of drag on the airframe is squared.....

Edited by tegwin on Tuesday 17th February 20:14

[stuthemong]

SlipStream77 said:

but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

Wow. Great idea. We could change the profile of a wing to generate more lift. We'd need some sort of a.... flap or something, that could move.

Yeah. Let's call them 'flaps'



EDIT: http://en.wikipedia.org/wiki/Flap_(aircraft)

I'm being slightly cheeky, as I'm sure from what you've said you have a bit of knowledge around the subject, and you'll do a 'oh durrrr' moment when you realise what flaps are for. If not, I apologise, and the above link tells all

Edited by stuthemong on Tuesday 17th February 20:16

[tegwin]

stuthemong said:

SlipStream77 said:

but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

Wow. Great idea. We could change the profile of a wing to generate more lift. We'd need some sort of a.... flap or something, that could move.

Yeah. Let's call them 'flaps'

Dude... you should patent that!

[john_p]

Glade said:

As a previously trainee glider pilot...

It's more important to use rudder in gliders and I presime light aircraft to cancel the secondary effect of ailerons.

the nose wanders about all over the place if you don't use the rudder. I think generally the faster you go the less is required, but a secondary effect of roll is yaw so you need to cancel it out. I never found it modelled that well in FS etc.

On a recent flight my instructor went through flying a glider without the rudder and then without ailerons - uncomfortable and hopeless in turbulence but a good lesson in secondary effects!

Edited by john_p on Tuesday 17th February 20:15

[SlipStream77]

stuthemong said:

SlipStream77 said:

but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

Wow. Great idea. We could change the profile of a wing to generate more lift. We'd need some sort of a.... flap or something, that could move.

Yeah. Let's call them 'flaps'

If you read my post again, you will realise, flaps are the opposite of what you would need in the situation I described. To summarise again, at high speed and low level, a fixed wing profile may generate so much lift that a nose down attitude is required to stay at level flight, this would generate excess drag. I was suggesting changing the wing profile in order to reduce lift at speed and thus reduce the need for attitude correction and it's associated drag.

I apologise if my question was badly worded.

tegwin, yes, drag will increase, in exactly the same way as lift increases (the equations are virtually identical), however I'm sure that the drag doesn't counter the lift.

[V8mate]

I think the OP needs one of these...

[HiAsAKite]

Rudders come in handy to correct adverse yaw, as v. often the aileron on the wing that goes 'up' moves more than that goind 'down', resulting in adverse yaw away from the direction of the turn- a touch of rudder prevents this.

Use of rudder can also be used (especially with art stability) to reduce/move dutch roll outside of the usual flight spectrum..

with regards to the speed vs lift question- ignoring that at 600knts airspeed you start to get areas of supersonic flow over parts of the fuselage and wings (assuming you're at low enough altiude.. airspeed vs Mach reduces with altitude as the air gets thinner..but anyway)./.... there would usualy be a slight correction in angle of attack to counteract this.. but as someone else mentioned at 600knt much more drag would be produced as well as lift..


The supersonic 'patches' at speeds close to supersonic become really interesting from an aero point of view.. lift generated at supersonic speeds (and the behavious of air) is fundementally different due to the presence of shock and expansion waves, which can v. interesting affects if they coincide with control surfaces..

The centre of lift also moves rearwards at supersonic speeds, which can lead to a nose pitch down motion as the centre of gravity usually does not move...this combined with shocks in the wrong places can lead to irrecoverable dives at speeds aroudn MAch 1 as some of early supersonic flight pioneers found out

Concorde got round this 'shift of centre of lift/pressure) by pumping fuel to rearwards to create a corresponding rearwards shift in centre of gravity...

I'll shut up now...

[navier_stokes]

http://books.google.co.uk/books?id=DPZYUGNyuboC&am...

Page 144 is typical of a wing section.

Lift coefficient does not vary much with Reynolds Number (velocity), except at the extreme angles of attack. Getting into the transonic region is a different phenomenon.

Edited by navier_stokes on Tuesday 17th February 20:45

[eharding]

tegwin said:

Firstly...

In an average plane the rudder is mainly there to adjust the trim of the aircraft...So if you have a crosswind, the rudder is used to keep the aircraft flying "into the wind" so as to reduce the drag on the aircraft... It is also used in steep turns (turns done using the aerlerons) to help balance out the turn and again keep the aircraft in trim flying in a straight line...

Whilst some aircraft have a rudder trim control, typically the concept of 'flying in trim' involves adjusting the elevator trim to reduce stick forces for a given power, attitude and configuration. Crosswinds on takeoff and landing will require rudder inputs, but in the cruise 'crosswinds' are accounted for by adjusting your heading based on known or forecast winds and airspeed to achieve the desired track. Most modern types have an aileron design which reduces the adverse yaw when banking to a vestigal minimum. In vintage or aerobatic types this certainly isn't the case. In the Pitts, it's arguably the primary flying control - lose the elevator and there is a possibility of landing using the trimmer (probably not without some excitement in an S1, but certainly achievable in an S2), lose aileron authority and the rudder can induce enough roll on it's own to compensate....lose the rudder, and I'd be over the side PDQ.

tegwin said:

if you actually try and turn a plane ONLY using the rudder....its not going to work too well!

Works perfectly well in the Pitts, and reasonably well in other aerobatic types I've flown - excellent for disguising the fact you've stuffed up a rolling turn.

[HiAsAKite]

navier_stokes said:

said intelligent stuff....

No guessing from your username what you do for a living then...

Edited by HiAsAKite on Tuesday 17th February 20:50

[Crimson Tide]

Think of it this way. An F15 will be designed to have a high cruising speed, so the wing setting angle will probably be calculated to allow the aircraft have zero pitch at quite a high speed. Above that speed, a small amount of pitch downwards will cancel the extra lift. Below that speed, the same but in reverse. If you go really far below that speed in order to land, you'll have pitched up so far you won't be able to see ahead, so you lower some flaps to give you the same amount of lift for a given angle of attack.

You mentioned that the velocity is squared which would massively increase the lift for a given speed increase. True in theory, but the coefficient of lift is directly proportional to the angle of attack, which is always a relatively small number. Let's imagine that your value of Q in the lift equation doubles, and the angle of attack is four degrees. It would only take a reduction in AoA of two degrees to halve CL and return the resulting lift to the original value. However, the appearance of the aircraft as it went past would barely have changed enough to notice the difference.

It's also worth mentioning that an aerofoil will have a zero-lift angle, usually a couple of degrees below the horizontal.

Is that just gibberish?

Edited by Crimson Tide on Tuesday 17th February 20:56

[bitwrx]

SlipStream77 said:

Perhaps it's a very subtle attitude at those speeds, but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

Funny you should say that. I got a mate who's doing a PhD in that exact thing at the mo. I'll let you know how it's going in a few years or so.

[esselte]

stuthemong said:

Yeah. Let's call them 'flaps'

:snigger:

[Papoo]

SlipStream77 said:

A couple of questions that have been puzzling me recently, maybe PH has a few residents who can answer them.

First, is it necessarily bad practise to fly an aircraft using primarily ailerons and elevator? Obviously when landing and taking off, particularly in crosswinds, rudders are essential, but when at altitude, does it really matter? I can only speak from sims like IL2 and FSX but I would have thought that banking and then applying a little elevator would be enough in most cases and the aircraft wouldn't be slipping too much would it?

My second question is regarding lift. The formula for which is based on several factors, one of which is the velocity of air over the airfoil. Essentially, the faster the airflow, the more lift.

Formula here...

http://en.wikipedia.org/wiki/Lift_(force)

If an aircraft, for example an F15, is in level flight at 5000 feet travelling at 300knots and it then accelerates to 600knots without the controls being touched (Let's assume calm air conditions), surely it is going to gain a lot more lift and thus climb?

It would, except it's fly-by-wire computers see to that automatically. Fighters are dynamically very unstable, to maximise agility, so fly by wire systems are a now a necessity.

slipstream77 said:

The velocity of air is squared in the lift formula so at high speeds, the amount of lift must be immense. Why is it then that when flying at high speed and low altitude, aircraft don't have to assume a nose down attitude to compensate for the extra lift?

They do. It's not necessarily nose down pointing below the horizon (although it can be), but the wings must reduce their angle of attack. This is achieved by a low-nose attitude, rather than altering the wing's angle of incidence.

slipstream77 said:

Perhaps it's a very subtle attitude at those speeds, but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

If you take a lesson in a small aircraft, you'll definately see that the difference in attitude is actually quite noticable, as it can be with the AC's weight. Watch a fully laden FedEx aircraft on finals, and it looks like it's pointing to the stars - an increase in angle of attack to compensate for low speeds (and in this case, MASSIVE payload).

Drag increases massively with airspeed, you're right. The total drag curve looks extremely similar to the bottom of the Nike 'swoosh', except it curves upwards slighly as airspeed increases. The lowest point is called V -min drag (VMD). Airliners use a speed found on this curve to determine what the best speed is for their efficiency. It isn't VMD, because the journey would take too long, burning excessive fuel, and it isn't to the far right of the curve, because drag increases so much. So, it's about 1.3 vmd typically - the best balance of minimal drag & good airspeed. VMD is for when your airliner becomes a glider!

Swingwings are more about to increase something called 'critical mach number' - the speed at which airflow over the surface reaches mach 1. Though this is pertinent to your question - when they are 'un-swung', they have a larger profile, so as to create more lift at lower speeds.

As for changing profiles - fowler flaps, as seen on airliners 'grow' out of the back of the wings, as well as changing the camber by curving down. Massive increases in lift are noticed with this, but also with drag, so they have their place - max lift at low speed (take-off & landing).

Hope this helps.

Edited to erm... re'format'

Edited by Papoo on Tuesday 17th February 21:08

[Bushmaster]

WW2 Spitfire pilots were advised that the rudder was not necessary in flight.

I posted a link to the Spitfire pilots' notes on the ' Cool Plane Pictures' thread.

[SlipStream77]

Thanks for the replies guys

I knew PH would have the answers!

[Papoo]

With regard to high-speed rudder use - a little bit of rudder can be applied via the rudder trim, in order to counter imbalances in flight.

Rudder also is the output of the 'yaw damper', which is to counter something called 'Dutch roll', another phenomenon altogether, caused by various stabilities and instabilities!

[IforB]

SlipStream77 said:

A couple of questions that have been puzzling me recently, maybe PH has a few residents who can answer them.

First, is it necessarily bad practise to fly an aircraft using primarily ailerons and elevator? Obviously when landing and taking off, particularly in crosswinds, rudders are essential, but when at altitude, does it really matter? I can only speak from sims like IL2 and FSX but I would have thought that banking and then applying a little elevator would be enough in most cases and the aircraft wouldn't be slipping too much would it?

My second question is regarding lift. The formula for which is based on several factors, one of which is the velocity of air over the airfoil. Essentially, the faster the airflow, the more lift.

Formula here...

http://en.wikipedia.org/wiki/Lift_(force)

If an aircraft, for example an F15, is in level flight at 5000 feet travelling at 300knots and it then accelerates to 600knots without the controls being touched (Let's assume calm air conditions), surely it is going to gain a lot more lift and thus climb?

The velocity of air is squared in the lift formula so at high speeds, the amount of lift must be immense. Why is it then that when flying at high speed and low altitude, aircraft don't have to assume a nose down attitude to compensate for the extra lift?

Perhaps it's a very subtle attitude at those speeds, but the drag must be huge, so why don't manufacturers create wings that change their profile to compensate? Or is this what swing wings are for?

Thanks.

First question, yes, it is bad form and uncomfortable to fly without using any rudder. It's already been covered by eharding, but I'll give you the flight instructors babble.

The rudder is there to create or counteract yaw. Imagine if you had a scaffold pole and shoved it vertically through the centre of the aircraft. You could then rotate the aircraft around this scaffold pole. That is yaw.

When you use an aileron you are increasing lift on one wing and destroying lift on the other, so one wing goes up and the other down. That is the primary effect known as Roll. The secondary effect of roll is yaw. When lift is created drag increases. So when you roll left or right you slow down one wing or the other and that is yaw.

Whilst most aircraft now minimise this adverse aileron yaw, it still happens, but to a much lesser degree than in something like a Tigermoth.

So to stop your passengers slopping their gin and tonics over themselves you need to apply rudder when you apply aileron. In the direction of turn.

Altitude is irrelevant, speed is irrelevant, it happens whenever the ailerons work.

Second question.

Lift = 1/2rho x Vsquared x CL x S (rho is air density, V obviously velocity, CL is Lift Coefficient (Directly related to Angle of Attack) S is wing surface area.)

The angle of the body to the airflow matters not a jot (unless it is a lifting body design) it is the angle that the wings make to the relative airflow that control the lift coefficient.

Airliners are designed that when they are at cruising altitude at cruising speed, the deck angle will be as close to zero as possible. However the aircraft maybe at 2 degrees nose up., the wings will probably at closer to 4 degrees angle of attack. This is usually the angle for best lift/drag ratio. It changes with every aerofoil section, but for the sake of argument I'll use 4 degrees as an average.

As an aircraft accelerates the lift needed to stay aloft is the same, but the faster airflow means that if everything stays the same the lift will increase, so what you do is lower the nose as you accelerate to stop the aircraft trying to climb. This reduces the coefficient of lift.

If manufacturers could produce a wing that changed profile, they would. It just isn't possible apart from potentially in secret squirrel military projects.

Swing wings are designed to allow decent low speed characteristics and good highspeed aerodynamics. Long straight wings are good for lower speeds (think glider) but are simply too draggy for higher speeds. Swept wings are good for high speeds as they increase the critical mach number (where the airflow over parts of the wing starts to become supersonic, drag goes through the roof at this point and all sorts of things start to happen that I won't go into here.) however, swept wings are rubbish at low speed and landings etc have to be far too quick.

Edited by IforB on Tuesday 17th February 21:58