Just a couple

[dynamite dean]

I have done searches on my other questions I have just a couple any takers welcomed...

1.On a tandem bogies which brake gets the hottest - I haven't a clue!

2. What changes the AoA? - is this a trick question I have picked up?

3.Do contra rotating props have a critical engine? and finally do counter rotating props have a critical engine? - :confused: :confused:

[Tinstaafl]

Not sure about the the bogies. Two opposing factors at work:

1. Which brake set can apply the most pressure without skidding. Possibly the front set as the CG rotates around the gear. Probably only applicable with anti-skid working.

2. Which brake set can dissipate its heat the fastest. Also probably the front set since it's in clean airflow & not shielded by an axle set.

Without further data I tend towards rear brakes getting hotter.


AoA is normally controlled using the elevators to change pitch attitude with a resulting effect on AoA.

Other devices can also affect AoA eg flaps. AoA is defined as the angle between relative airflow & chord. Since the chord line is a straight line connecting leading & trailing edges, a change in the relative position of these gives a different AoA.

This ignores any further effects as the equilibrium of forces is regained.

Counter rotating props don't have a critical engine. That's why they're used.

I wouldn't expect contra rotating props to have a critical engine UNLESS there's some interaction between front & rear propellors that skews the thrust line.

[jrs2-benson]

Just read your post Tinstaafl.

I agree with all you answers appart from the one regarding AoA. I'll stand corrected but I thought that speed controlled AoA. All of the other variables either effect the performance (Power + Attitude = Performance) or the relative position of the chord line to the pitch attitude.

Hope that is correct.

[ChrisVJ]

I have seen this one do the rounds of at least five or six forums and no two answers appear to be the same. Surprising for such a fundamental question. What is also surprising is that a number of the answers were so obviously wrong you had to wonder if the posters had ever done a principles of flight lesson at all. Perhaps they had just forgotten more than I ever knew.

My answer would be.

If you are asking what are the factors that can affect the AoA, then at least these, (but not in order of importance.)

Weight
Weight distribution
Power setting
Speed
Bank
Elevator position
Flaps
Gear up/down

The test I use is: If you take a plane in stable level flight and alter a parameter will it result in a change of AoA directly or indirectly even if temporary? If the parameter change causes a deviation from equilibrium then it almost inevitably causes a change in the AoA.

Speed, in itself, is not independently tied to AoA. (If a plane goes from level flight into a banked turn at the same speed the AoA rises. )

[inbalance]

"Speed, in itself, is not independently tied to AoA. (If a plane goes from level flight into a banked turn at the same speed the AoA rises. "

I don´t agree with that.
Take a real Airplane, not a computerised Atari thing , trim it to levelflight and then bank it. Leave the Hands of the Yoke and watch it. The AOA will stay the same.

[john_tullamarine]

inbalance,

Might I ask how you are measuring angle of attack ? .. presumably there is a measure involved, else how can you assert that the value remains the same when you bank ...... ChrisVJ's comment that the alpha increases in (steady) level banked turning flight (compared to straight and level flight) seems reasonable as the lift vector must increase to provide for an horizontal centripetal force and mass-balancing vertical force ..... that seems to suggests an increase in alpha .......

At its simplest, alpha is a matter of whence cometh the wind, viewed from the aeroplane's axes, in elevation, in respect to the wing section ... if the direction changes then the alpha changes ... anything which can cause such a change .... is of relevance to the original question I would have thought ?

[ChrisVJ]

John Thank you.

Add to my list:

Change in air density.
Wind sheer.
Polluting conditions, eg. ice.

Imbalance.

From steady flight to banked turn at same speed AoA increases. This is a fundamental and well documented fact.

A banked plane in a steady turn must use the same wing area to provide both the lift, ( same as in level flight if the plane of the same mass is not to go up or down) and the acceleration toward the centre (required to make the turn.) This amounts to more than just the lift alone. If the plane is flying at the same speed the extra lift can only come from an increased angle of attack.

A wing stalls at a particular angle of attack, regardless of speed. Thus a plane which stalls at 45 Knots in level flight may well stall at 60 knots in a steep turn or a pull out from a dive. This is the significance of the AoA not being independently linked to speed.

Take (for instance) a Chippie up to 5,000 ft. Fly str & level at cruising speed ( It is 40 yrs and I have forgotten exactly what that is,) crank on 60 deg of bank and haul the stick back into a steady turn so the plane nose neither skids out or bores in. In order to maintain speed you will have to increase power otherwise she will slow down because the wing is doing more work and causing more drag. The wing is operating at a higher AoA than it was in str & level and so causing more drag. That is why you need the extra power to maintain speed. The speed has not changed, the AoA has.

[Tinstaafl]

I took the AoA question to mean 'How is AoA controlled?', not 'Name all factors that affect AoA?'.

Hope that explains why I answered as I did.

Inbalance, AoA must increase in a level turn. Because the lift vector is tilted, to maintain altitude it must be increased. This is done by pitching the a/c nose up. This increases the AoA.

The resultant effect is to increase the total force the wing produces ie both lift and drag. The drag results in a negligible loss of speed at low angles of bank but is countered by adding more power/thrust at high AoB.

I think you're mistaking pitch attitude and/or no change in trim for a constant AoA.

jrs2,

Apart from devices that modify the wing's LE & TE, or alters the rigger's incidence, the relationship between chord line & pitch attitude is fixed.

What is of concern is the relationship between chord (or indirectly, pitch attitude) and relative airflow.

Using the relationship you mentioned ie Att. + Pwr. = Perf. and a level flight example:

All other factors being equal, at a given speed the a/c will require a certain power setting and a certain AoA (controlled by adjusting pitch attitude with elevator).

Hence the terms in the formula you quoted: Power and Attitude.

If you add power while attempting to maintain altitude you will have no choice but to use the elevator to lower the pitch attitude/AoA.

It's not the speed that is changing the AoA, but your choice to adjust pitch attitude to maintain a constant flight path.

If you don't change the pitch attitude the a/c will eventually stabilise in a climb at the same speed (near as an ASI shows). AoA will be slightly less than it was in level flight at that speed but the a/c is now in a different flight path with a different resolution of forces.

[LOMCEVAK]

If you read what Inbalance says carefully, he is absolutely correct. If we ignore compressibility effects, in pitch you trim an aircraft in straight and level flight to an AoA (not an altitude or IAS). Therefore, if you trim straight and level, roll into a turn and RELEASE THE CONTROLS, the AoA will be the trim value so long as there is no significant pitch rate.

A lot of the discussion above is dependent on perspective (chicken and egg syndrome). In a stabilised manoeuvre there will be a given elevator angle which is directly a function of AoA and pitch rate. However, this elevator angle will, for a given AoA and pitch rate, be a function of mainly EAS, c.g. position and Mach number. Pitch rate is a function of TAS, normal acceleration and attitude. It is then a juggling act of these parameters from your required perspective. The above list may not be totally complete, and I will edit this post after a bit of thought if I come up with any more.

[Tcas climb]

Critical engine on counter rotating prop would be the upwind one in a crosswind.

[dynamite dean]

Thanks fellers that'll do nicely!

[inbalance]

Of course the AoA will increase in a leveled turn. But the Airplane will never hold the Altitude in a turn by itself.
As I told before, trim it to levelflight and then bank it. Leave the hands of the controls and the AoA will stay the same.

The aircraft will lose some altitude of course.

So not the bank will increase the AoA but your input to the elevator to hold the altitude will.

[jrs2-benson]

When answering the question, "What controls AoA", I think it is important to separate the dependent and independent vaiables.

I my opinion all of these are tied together by airspeed which is a dependent variable. It relates to weight, power, ect which are independent variables. I think that I may be right in saying that all equations with AoA in them also contain Airspeed or Dynamic Pressure.

Sorry if thats wrong!

JRSB

[ChrisVJ]

As I said. Surprising there are as many answers, almost, as there are pilots.

However Tinstaafl may have a point, if the question in a test is "What CONTROLS AoA?" I'd have to plump for "Elevator."

[john_tullamarine]

inbalance and LOMCEVAK,

I am now duly briefed regarding the various qualifications to the previous commentaries ......

[Tinstaafl]

I missed your point about not trying to maintain altitude, Lomcevak.

Entering the turn I think will still lead to a change in AoA for at least one wing. I always understood inner & outer wings to have different AoAs.

From S&L where both wings have the same AoA, to a condition where each wing is different must involve a change somewhere.

A hairsplit, I know... :rolleyes:

[Oktas8]

In a constant angle of bank climbing turn, outer wing has greater AoA. In a constant AoB descending turn, the inner wing has greater AoA.

You'd have to have both an unusually high angle of climb and a small radius of turn to get a difference of more than a small fraction of a degree I think.

Even then each wing's AoA would change dynamically if the pilot allowed the aircraft to set its own angle of bank - rolling in or out etc. The different airspeed experienced by each wing would tend to override the different alpha. But you're right Tinstaafl - this is really splitting hairs!

O8