View Full Version : asymmetric forces question


MalteseJambo
16th Dec 2010, 21:12
Hope someone can help me with this one; used to have a good diagram on this but seem to have lost it,

Following an engine failure, a aircraft is then trimmed to fly straight and level. which statement is correct?

All the resultant asymmetric forces are equal and opposite, the aircraft is in balance with no yaw or side slip. The slip ball will be central

Thrust and drag are equal but not opposite, the rudder side force is unbalanced, which makes the aircraft side slip towards the failed engine

thrust and drag are equal but not opposite, the rudder side force is countered by a combination of the fuselage side force and the side component of thrust.

Thrust and drag are equal but not oppsoite, the rudder side force, is unbalanced which makes the aircraft side slip towards the live engine.



VJW
16th Dec 2010, 22:21
been a while, but pretty sure the first statement is correct at least!

rudder is used to balance yaw otherwise directed to dead engine. slip ball definitely central if trimmed, therefore a/c is in balance.

Lord Spandex Masher
16th Dec 2010, 22:27
Actually, with the ball in the middle, wings level, you'll be tracking towards the dead engine.

Second answer.

vfrrider481
16th Dec 2010, 22:29
As I understand,

If you are in straight and level flight and constant speed then thrust must equal drag.

However, the thrust is in line with the engine, but the drag is from the whole of the airframe and therefore is closer to the centerline. Because of this the aircraft wants to rotate around it's normal axis towards to dead engine.

Rudder is used to stop it rotating, by generating a rotational force towards the live engine. However this force is actually generated by the fin creating a side force. Whilst the rotation is stopped, this side force acts laterally and has nothing to stop it pulling the aircraft sideways through the air.

Thus,

Thrust and drag will be equal but not opposite, the rudder side force is unbalanced which makes the aircraft side slip towards the failed engine.....

I think.

VFR

john_tullamarine
16th Dec 2010, 22:30
Don't have a pic to hand but, if someone else doesn't post one, a chap could sketch the standard thing up quick enough for this discussion ..

Points to recall are

(a) the yawing moment is balanced by rudder input

(b) the rudder input for (a) causes a resulting lateral force imbalance

(c) this force imbalance accelerates the aircraft to the L/R as appropriate (not a huge acceleration but there, nonetheless)

(d) eventually, the aircraft's L/R thrust due to the rudder deflection (causing the velocity component) is balanced by a R/L drag force and we end up back in equilibrium but with two component velocities - ahead and to the side (sidelip velocity)

(e) the two velocity components result in a sideslip angle

(f) to get rid of the unwanted sideslip angle (and reduce the associated drag) one banks slightly into the operating engine side to create a sideslip opposite to that caused by the rudder. Generally, for the main area of critical flight interest (initial OEI climb) the typical bank angle will be in the 2 - 3 range ..

It is for this reason that some jet OEM guidance is not to worry too much about this level of finesse and fly wings level, accepting the minor loss due to slideslip. On the other hand, for a GA twin, the extra performance which one might claw back by banking into the operating engine might just be the difference between going up slightly .. not going anywhere much .. or going down.

Note that, for this discussion, we are talking about performance, not handling - the more commonly known 5 bank OEI relates to Vmc handling and is a maximum to prevent innovatively low Vmc values ending up in the POH. That is, don't fly 5 bank OEI at blue line as you are

(a) wasting your time, and

(b) might just as well fly wings level as the performance for the two situations will be pretty much the same.

a aircraft is then trimmed to fly straight and level

straight infers no sideslip .. level ? ... maybe ... if you a lucky in a light twin ..

All the resultant asymmetric forces are equal and opposite, the aircraft is in balance with no yaw or side slip. The slip ball will be central

Afraid not - if the ball is central ... the wings are level ... and you are in a sideslip and not flying straight. Whether the T/D forces are opposing vectors in the same line is not particularly relevant as that is what the rudder input is about - reject

Thrust and drag are equal but not opposite, the rudder side force is unbalanced, which makes the aircraft side slip towards the failed engine

inconsistent with the original statement (this is OK for the initial transition but not for the final trimmed state) - reject

thrust and drag are equal but not opposite, the rudder side force is countered by a combination of the fuselage side force and the side component of thrust.

that sounds to be not overly unreasonable if we are flying wings level - (but ignores the finesse need for bank to get optimum performance). Reject or accept according to how you want to interpret the question.

Thrust and drag are equal but not opposite, the rudder side force, is unbalanced which makes the aircraft side slip towards the live engine.

nonsense. Once the thing settles down the rudder force will be balanced out by this and that in the way of summed components of force. However, to get a sideslip to the live engine requires insufficient rudder or excessive bank.

Lord Spandex Masher
16th Dec 2010, 22:39
It is a bit of a poorly worded question.

It does say "trimmed" for S&L, (not necessarily flown;))

Trimmed to fly S&L would imply a slight 'nose towards the dead-engine' mis-trim, wings level.:E

john_tullamarine
16th Dec 2010, 23:22
It is a bit of a poorly worded question.

concur

Trimmed to fly S&L would imply a slight 'nose towards the dead-engine' mis-trim, wings level.

don't think so .. straight infers no sideslip .. otherwise you aren't straight ...

BillieBob
17th Dec 2010, 07:23
straight infers no sideslipNo, 'straight' infers a constant direction; 'in balance' infers no sideslip. However, in the asymmetric case, due to the arrangement of forces explained previously and assuming that the wings are level, the aircraft will sideslip towards the failed engine although, with the ball centred, it appears to be in balance.