For the Aerodynamicists - Why is it that....

[Squealing Pig]

Looking for an aerodynamics expanation,

Why is that if you open the right door on a C150/152 in flight, it will turn left ?

thanks in advance

SP

Yes, I know you shouldnt open the door in flight !

[Genghis the Engineer]

I'd go further and say you shouldn't fly cessna singles either.

However, guessing, I'd say that by opening the right door you are creating drag on the right hand side. This tends to yaw the aircraft right initially, creating sideslip from the left (i.e. nose has gone left, so relative wind has gone right).

The C150 certainly has positive lateral stability, which means that it rolls into sideslip. So, it tends to roll left into the generated sideslip, caused by opening the right door.

But I am only guessing.

G

[fantom]

sure you have those left and rights the correct way round G?

:(

[Tinstaafl]

I've done it a lot times. Fascinating just how much roll authority one has available.

Don't quite follow your sequence, Genghis. Would you mind expanding your explanation re the yaw/roll sequence? I'd have thought an initial yaw right would lead to increased lift on the left wing followed by a roll to the right.

My guess is that there's a slight increase in pressure under the wing of the open door causing a lateral imbalance in lift ie a slight increase in lift on the open door side.

[Keith.Williams.]

Genghis,

If what you have just said about positive lateral stability causing roll into sidslip is true, then everyhing I have ever told my students about this subject is untrue. (This is of course entirely probable, as most of them subsequently passed their JAR ATPL exams).

Do you not mean that positive directional stability causes yaw into sideslip? If the aircraft in question yaws into sideslip to a greater extent than it rolls away from it, this might explain the phenomon described in the original question. (I have avoided the concept of one mode of stability being dominant, because this has caused some controversy in this forum in the past).

[Genghis the Engineer]

I did say I was guessing didn't I, but...

(1) The strict definition of lateral stability is rolling moment due to sideslip.

(2) The strict definition of directional stability is yawing moment due to sideslip.


Just checking the sign of lateral stability, we know that a normal aeroplane can be turned on the rudder, if a little innefficiently. So, put in right boot, rudder deflects to the right. This pulls the tail left, and the nose right, creates sideslip from the left, and we know that the aircraft will therefore roll to the right. Damn, there goes my logic, lets try again - I've just looked it up and it seems there's a mild conflict - what as an FTE I know as positive static lateral stability actually has a negative sign (Mike Cook's FLIGHT DYNAMICS - PRINCIPLES is my reference of the moment, not my favourite text on the subject but its available). I think this goes some way to explaining my incorrect explanation, what to us normal folks is positive lateral stability, to an aerodynamicist (I were one once but grew out of it) is actually negative.


So let's look at this again:-

I am starting from the assumption that SP is telling us the truth, and he really does fly Cessna singles. So, working backwards:-

(i) Aircraft rolls left without aileron input

(ii) Assuming that lateral static stability (Lv or L-sub-Beta depending upon whether you prefer UK or US terminology) is a player, and that it is in the normal sense (+Ve to a FTE, -Ve to an aerodynamicist) (I'm pretty confident on both scores so far) then there must be sideslip from the left, that is a yaw string would be deflected to the right.

(iii) So something has happened which causes the nose to go right (or the tail to go left). We know that the only thing that's been done is the right door has been opened.

(iv) Now the last time I swallowed my pride and flew a Cessna I seem to recall that the door was hinged at the front edge, so opening it will effectively create a bulge on that side.

(v) Now conjecting moderately wildly, let's say this effectively creates the top surface of an aerofoil and pulls the nose to the right when the right door is opened.

(vb) Alternatively, opening the door creates disturbed airflow down the right hand side of the aircraft, thus lower airspeed flow over the right hand side of the tail, causing the tail to tend to pull to the left where the flow is smoother.

(vi) Which completes the proposed mechanism. Pretty much the complete opposite of what I said first time, but what the heck - at least this model seems to fit the data.

I hope somebody follows this, 'cos I'm not particularly convinced that I do.

G

[Mad (Flt) Scientist]

An aircraft with positive dihedral (or lateral)
stability will roll AWAY from the sideslip. Note
that we flight scientists ALSO consider this
positive stability - the DERIVATIVE is negative,
but the STABILITY is positive.

That is, with aircraft nose to the right, sideslip
from the left (negative sideslip), yaw string to
the right, the aircraft will roll RIGHT (positive
roll). This is because of:
(1) the forward going/forward swept wing generating
more lift - in this case the left wing. This is
the important mechanism for a swept wing aircraft.
(2) The fuselage crossflow induces upwash on the
inboard upwind wing, and downwash on the inboard
downwind wing, causing again more lift on the
upwind or forward wing. This is the mechanism for
a high wing aircraft, principally. It also
applies to a low wing aircraft with any
significant dihedral (which is added in order to
create this effect).

This is considered 'positive' stability (i.e. a
good thing) because it means that following a
right rudder input, generating sideslip from the
left, the aircraft will roll right and so turn
right. If that didn't work it would be near
impossible to fly with rudder alone, as the
rudder would turn the aircraft in the 'wrong' direction.

So, to consider the door opening mechanism for
turning the Cessna. In order for the aircraft to
roll left with no other disturbances, the
aircraft must have a rolling moment to the left.
If we discount a direct moment due to the door
(which would necessarily be small) then the most
likely effect is indeed the dihedral effect (roll
due to sideslip). However, for the aircraft to
roll left there must be sideslip from the right,
and the nose must have been forced left. This is
not consistent with the operation of the drag
from the door. And as noted, any effect on the
vertical tail would appear most likely to disturb
flow on the right hand side, which would cause a
side force to the left on the tail, yawing the
aircraft the wrong way.

Both these mechanisms are likely to be of small impact, because of

(1) the door is very close to the aircraft
centreline, in terms of lateral offset. So the
yawing moment will be small unless the drag is
large - likely so large as to either cause a
noticeable speed change (not mentioned) or make
it hard to open the door?

(2) the flow over the fuselage of the aircraft is
already disturbed by the propowash and all the
other untidiness on the aircraft. The downwash
from the wing will also cause any disturbance
from the open door to drop down and so miss the
tail anyway.

I postulate (and this is only a guess, unless I
sneak across the airport and find Cessna's wind
tunnel data for open doors on light singles:)):

The open door generates a sideforce on the
fuselage (acting as a flat plate). A door on the
right side will generate a sideforce to the left.
If acting forward of the c.g. (as I suspect it
is) then this will yaw the aircraft slightly
left. This will create the necessary sideslip
conditions for a left wing down roll, and so a
turn to the left

Perhaps a controlled experiment should be arranged?:)

[jtr]

Force applied foward of the CG (to the left if the R door is opened) has a greater effect than drag created by same control (the door)

If however the door was of adequate strength, and you were of adequate ability and stupidity, and pushed the door fully open (perp to rel air flow) the yawing effect perp to RAF would be less than the drag effect parallel to RAF, and the a/c would yaw toward the open door...

immediately followed by your blessed 150 stalling the right wing, rolling to about 130 degrees
Now is a good time to close said door!

[Genghis the Engineer]

The more I think about it the more the "aerofoil" theory seems sensible to me. My reason is that lift tends to act on a conventional aerofoil around quarter chord, so in rough terms I'd expect the force towards the door to act from nearer the hinge edge than the catch edge.


As an aside, a TP working for me had a door come open last month in a slightly smaller but similarly configured aircraft to the C150, except in this case the hinge was at the top. The door came open and sat about 20° below the wing, this created neutral to divergent DR with yawing oscillations through ±20° before he got the door closed and brought it under control. Interesting in this case that although you'd have expected immediate roll or pitch effects through asymmetric disturbance to flow around the mainplane, the main effect actually became in yaw.

I then took a similar aircraft out myself and put a force gauge on the door, at the bottom catch, at 50 kn (compared to Vd around 95) I got a 6daN force to hold the door in position with it just very slightly open.

G

[mstram]

Ghengis wrote :
I then took a similar aircraft out myself and put a force gauge on the door, at the bottom catch, at 50 kn (compared to Vd around 95) I got a 6daN force to hold the door in position with it just very slightly open.


I don't know what your numbers mean there, but I had the right door unlatch on a 172RG in flight. At ~100 knots, I could not even move it against the airflow !

[Genghis the Engineer]

6 daN = about 13 lb force, reasonably significant and if you scale up to Vd with Q, you get a theoretical force of aver 45 lb (not that I was going to try that, the thing has to be pointed straight at the ground to reach Vd, which is bad for the soul and certainly not the time to be mucking about with doors).

Big difference between what I was flying and a Cessna, which is that I had hinges at the top and the Cessna has hinges at the front so even if the Cessna door comes open, it isn't going to go far.

G

['%MAC']

Has this been said, please forgive if it has, I’ve been having an unusually difficult time following this thread. The door hinge is forward of the CofG as the right hand door is opened it forces the wind to diverge to the right, pushing the nose left (Isaac III). So open right door a bit and plane turns left, open right door a lot and it acts not as a rudder forward of the CG, but as an asymmetrical speed brake, and the plane turns right. You know that sounds familiar, I think someone already said that.... oh, well hope I didn’t plagiarize.

[Budgie69]

Bringing to the discussion to a slightly less learned view.

Bearing in mind that
a: a Cessna 152 cockpit is very small
b: a fair amount of force/body movement is required to open the door
Could it not be that every time this evolution was practiced a knee inadvertently pushed the control column?

I remember when learning to fly (Auster, 1964) being shown that a perfectly trimmed a/c could be controlled by body weight and by putting hands out of the sliding windows. A hand put out of the left widow induced a very gentle turn to the left, and vice versa. Leaning forwards or backwards in the seats induced the appropriate pitch reaction.

[BigJETS]

sounds to me like the open door is placing a low pressure void on the right side of the vert stab and the tail is drawn into the void.

[Tinstaafl]

Well... I'm glad we've all agreed on the cause! :D :D :D

[Tonic Please]

The topic starter, I presume, does not have too much technical idea about aerodynamics, and may have a PPL to understand it lightly...but the level of physics and aerodynamic understanding required to folow this thread is a little too high for the topic starter. Perhaps a simple explination could be a conclusive post?

And yes, I did follow it :D :D :p

Smooth skies (unless door open) ;)

[Squealing Pig]

Tonic Please, The topic starter (myself) has a good understanding of aerodynamics & has more than a PPL (CPL/IR/FI FATPL).

Thanks for the very informative replies needed to read a few of them a few times over to really get to grips with them. very informative.

Cheers guys.....keep em coming!

SP

[Tonic Please]

Squealing Pig - No disregard intended. I would have thought that someone, now I know, of your level and experience would have had an understanding of the topic in discussion. Clearly I was mistaken.

However, interesting reading :)

Smooth skies

[aztruck]

Door opens, wind hits door, nose goes right, said wind deflected up to bottom of wing causing small positive roll force.
At the same time, open door masks airflow over the vertical stab, so the angle of attack put on the fin will not be as effective in getting rid of the yaw condition.
Dont forget about the prop wash. The door is right in line with it so an open door will have a significant amount of extra high energy air to play with. What flight attitude were you in when you opened the door?
Is the airframe "straight", ie will it fly normally without any turning tendencies when trimmed?
Most small Cessnas require some rudder input even during cruise flight to avoid sideslip.
What is the fixed trim tab on your rudder doing for the aircraft in level flight?
if it is preventing the A/C from yawing to the left in the cruise(to relieve your aching foot), then masking the rudder would alter the effectiveness of the device, possibly leading to a yaw back to the left.
Finally the keel effect of the fuselage being thrust sideways into the wind is probably going to force the tail to the right and the nose left, at which time the door wind deflection force will be additive and help to continue the nose to the left.
The left wing retreats slightly and drops, the nose falls further into the slip ...
By the way, do you end up in a spiral dive if you let it continue?
Cessna 172's had their flap 40 setting placarded years ago. No slips with flap or something similar.
Masking the vertical stb/rudder is not something Mr Cessna wants you to do.
Just a few idle thoughts, and i'm not an aerodynamicist or even a trick cyclist.

[bluskis]

Seems to me a question of how far the door is opened is the critical factor.

Taking the previously mentioned flat plate theory, (as any aerofoil considertions are not relavent as we don't have an aerofoil, only possible disturbances to flow), the Cessna will be influenced to yaw right or left depending on whether the normal force on the door passes fore or aft of the aircraft c of g, and this would depend on the door opening angle.

What ensues next is in the domain of the aerodynamicists.

Seems to me it depends on the angle to which the door is opened.

Following the flat plate theory, as I do not believe the door is an aerofoil, although it might well be a spoiler, if the normal force on the door passes fore of the cof g, the plane will tend to yaw left, if the normal force passes aft of the c of g, it will yaw right.

What the plane subsequently does is the realm of the aerodynamisists.

[acbus1]

Dont see why all the complex descriptions.....its just an open and shut case as far as I'm concerned.

[aztruck]

It's left me unhinged........

[acbus1]

Me too!

Cant handle it!

Knew there would be a catch.

[aztruck]

Dont knock it...especially if you're feeling a lintel under the weather..ouch!!

[northwing]

Try opening the other door and see if it turns the other way. If it doesn't then it's a curly-wurly propwash effect.

Incidentally, the force required to open a door in flight is a fascinating subject. I once spent 4 hours in a witness box being cross examined by a QC about it.

[Tinstaafl]

It works with both doors, rolling away from the open door. The more you push open the door the faster it rolls.

All very controllable. Ignoring spiral stability effects, the roll stops once pressure to open the door is removed. The a/c can be rolled upright by doing the same to the opposite door.

[acbus1]

Wonder if you could control the plane by linking the yoke to each of the doors,with springs perhaps.

But of course youd need to learn how to adapt your flying to suit.........and we could call it.........

"Twosprung door technique" :rolleyes: