View Full Version : Aerodynamic drag on light aircraft open door

Pugachev Cobra
17th Dec 2009, 12:10
Hello folks,

This one surprised me a bit, and I'd like some input on the theory behind this fact.

While flying a C152, we decided to open the doors in flight to observe how the aircraft would react.

We opened the right door, and I thought that with additional drag (and like a rudder) the aircraft would yaw to the right.

Instead, the nose turned to the left, and vice-versa with the left door.

Two possible explanations arised, first being that an air cushion is formed between the door and the fuselage, pushing the nose to the other side. The second is that the door is forward from the cg, therefore acting like an inversed rudder. Of course, the two theories complete each other, but maybe some here could explain it better.

Any explanations are welcome!

17th Dec 2009, 12:21
Maybe the door creates a region of lower pressure behind it on that side of the aircraft, causing the rear section of fuselage and the tail to be drawn in that direction?

17th Dec 2009, 12:30
If you open the right door, that would deflect air out to the right... the opposite reaction would be the aircraft yawing left, and vice-versa. See Newton's 3rd law for further info.

If you were able to get the door open to 90 degrees, then maybe you would get the yawing effect caused by drag towards the open door. You'd have to be fast (and incredibly strong) to avoid the initial "wrong-way" yaw though. For smaller door opening angles, I think my first point is the reason. I think that's along the lines of what you meant by air cushion??

I once tried steering a supercub with one arm out of the window, and one out of the door... didn't really work. :ouch:

17th Dec 2009, 14:19
This one is quite simple, really: in a 152 the door is below the wing, and opening it increases air pressure below the wing; thus increasing the airflow above the wing, increasing the pressure differential on that wing. That causes the airplane to roll to the left, and with the strong yaw-roll coupling of the 152, the airplane also yaws to the left.

Kind of like a spilt flap, then :-)

17th Dec 2009, 17:23
Just to confound things, if you open the door while taxiing, the plane will turn towards the open door.

17th Dec 2009, 18:18
....in which case the nosewheel steering is pretty crappy :)

18th Dec 2009, 14:24
While flying a C152, we decided to open the doors in flight to observe how the aircraft would react.

Why on earth would you think that is a good idea? Does the AFM tell you that opening the door in flight is approved? Or are you just making things up as you go along?

Lightning Mate
18th Dec 2009, 14:46
This is on the wrong thread. There is another one for amateurs.

18th Dec 2009, 16:32
Cessna doors open all the time by them selves. You dont even notice leaning on them and then suddenly you can say bye-bye to any stuff you have been stupid enough to put next to the door. They normally open just an inch ot two and can be closed with out drama or exitement.

But to intentionally open doors inflight and to possibly to a large angle, not so smart... Leave test flying to the pros.

19th Dec 2009, 03:10
Just to defend OP's honour slightly...

It has been well known for several decades that a C152 can be "steered" crudely in the air by opening a door in flight. Now I don't know who was the first to do it, and I'm not in the habit of opening doors in flight unless the flight manual tells me to. But in the case of this particular aircraft, it does not lead to disaster. Really.


19th Dec 2009, 09:29
.. now try an Aztec, single pilot, with a door popping open after takeoff. Interesting...

FCS Explorer
19th Dec 2009, 11:39
oh c'mon! we all wouldn't try opening a door on something that is "more plane" than that flying scooter, but where is the fun in flying if u stop opening doors on a c152?
another thing that works nicely when you are alone up there is switching seats L/R for steering. (no comments on my weight please!:})

20th Dec 2009, 14:59
I am totally for trying out things and experimenting!!! true pioneering spirit
thank you for sharing your experience.
my two cents worth
the door acts like an aileron.which ever aileron is down that wing goes up.
however somethings got to give...
loss of altitude,power or speed....
I wouldn't try it close to the performance boundries
and the flight manual flew out the door long ago...:)

21st Dec 2009, 04:56
Open the door on some aircraft, and you are dead.

To delibrately open a door in flight "to see what happens" is reckless at best.

ECAM Actions.

21st Dec 2009, 19:25
Opening door in flight was demonstrated as well as spinning when I took my PPL on a C150

My instructor played the part of "stupid passenger"
Surprising thing was, that you had to open the window when closing the door again - very educating.

If opening the door in flight is hazardous - sure it would be mentioned in the POH - and the door locking mechanism a little more secure.

21st Dec 2009, 22:56

Let me know when you are going flying, and I will make sure I am in another FIR.

22nd Dec 2009, 07:20
C'mon guys, this is a technical forum, not a flight school. Just imagine the OP said "...and a door came open in flight and had this effect..." and reserve the righteous indignation for something more worthy.

Compared with what people normally do with light aircraft like this, e.g. fly over gross/out of cg, set off uphill on long, wet grass towards trees, go IMC without any training, etc. I'd say it was pretty mild and done in the true sprit of inquiry. If things had started going pear-shaped, they could have just shut the door. :rolleyes:

Anyway, it turned left due to the inverse Coriolis Effect, coupled with a spanwise flow moment caused by boundary layer re-attachment at high Reynolds number. So there.

Pugachev Cobra
5th Feb 2010, 12:13
I know it's a little old, but I didn't see the last comments as being reckless and hazardous.

It was a training flight with a qualified instructor, and he wanted to demonstrate what would happen.

I figured he knew what he was doing, and it was better to do it controllably, instead of years later it happening unexpected without knowing the outcome.

Later he simulated a "yoke stuck failure" (no elevator and no aileron), suggesting that if I had difficulty in turning, I could've tried opening the door.

Interesting what JEP said, it also simulates a "stupid passenger".

Either way, I learned a lot that day.

5th Feb 2010, 12:51
What a load of hot air.

My FIC buddy and I did this during our Instructor Course several years ago. No probs with any light aircraft with that type of door, as far as I have ever experienced. You may need to yaw to shelter the door to close it on some (BE-76 twin and it takes two of you - popped on take off) or slow down a bit.
Note - the IAS when you do it on the left door drops to about 20KIAS as you open the door further. The C150 Aerobat can be flown with the doors off in the summer if you wish (no FOD or loose articles). As for falling out - please... if you have on your harness or lap belt, where is the difference between that and a weight-shift microlight.

I would also open my door during a RTO exercise go give a sudden noise and rush of air, as well as EFATO at 150' to touch and go (Brief being "I will stop for loud bangs, control problems, engine fire or failure, blocked runway or smoke in the cockpit putting any burning components downwind turning L/R onto a target heading of XXX). How many PPL's have an action plan to get out little Jonny from the back and forget that if they don't turn downwind they will burn their legs during evacuation???
Better practice these things with a good QFI than experience them the day you panic and it kills you and all the POB.
Why not ask to carry out a few extra things on your next check - you may just learn something rather that preach what you have little experience of.

Anyway back to the tech bit -
It would be interesting to stick on with tape 6 or so 4" bits of string to see what happened to the flow. My guess is that (with the fact that I saw the IAS bleed off) the slipstearm is deflected causing a High P - or stagnation area. This will in turn:
1) increase the High / Low ratio between the wings making the opened door wing have a higher pressure differential and thus more lift on the opened door side.
2) speed up the airflow over the wing from the deflected airflow and increase the Low P and lift.
Anyone add anything??

5th Feb 2010, 16:14
I once took off in an old Cessna 140 - on a long runway - when the right side of the cowl popped open at 50'.

For about three seconds I contemplated making a circuit - with the engine likely overheating - and I decided not to risk that. So I cut power, slipped left for a second or two to kill some energy, and easily landed on the remaining runway.

No harm done, except to my ego for accepting the plane with badly worn cowl latches.

5th Feb 2010, 20:45
Text book - you did the right thing and the best way to learn a lesson. I can say that because I did the same thing an a PA38 about 18years ago after rushing my pre-flights. Never again....