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Faro
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Air Pressure Adjacent to Fuselage Side and Tail Cone

Mon Dec 24, 2018 8:17 pm

The dynamic air pressure on airframe surfaces with headlong exposure to the airstream like the nose and wing leading edges is strongly positive.

What about fuselage sides, supposing these experience zero-incidence airflow over their surfaces. The pressure would be lower than the static air pressure, would it not, because of the high velocity airstream giving rise to a low pressure area immediately adjacent to the fuselage side (at zero airstream incidence)? This would exert a outward pull on the fuselage sides.

And what about the tail cone area in the lee of the airflow around the airframe? Would it also experience lower air pressure like the fuselage sides or would the magnitude of the lower pressure be even greater in this area?


Faro
The chalice not my son
 
747Whale
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Mon Dec 24, 2018 9:50 pm

Faro, that's a really complicated subject, because not only does the pressure vary from aircraft type to aircraft type, but varies at each point on the airframe. Move a few feet away and the pressure will change. Vary the speed and the pressure will change. Alter the airflow over the airframe in any way, by approaching compressibility and transonic flight or the angle of attack, and the pressure distribution will change.

Some airframes take advantage of increased pressure and pressure distribution to lower drag (eg, coke bottle/area rule), or provide positive pressure toward the rear of the airframe.

Pressure distribution is complex and variable. Generally it's minimal along the sides of the fuselage, but that really depends on the fuselage station.

Areas that you might expect to be high pressure may be quite different than expected. For example, I once had a windshield blow out in a Cessna 421, which is a light twin piston engine airplane. There was virtually no airflow into the cockpit, or cabin, however, and I could put my hand through the open space without very little apparently airflow across my hand. material that was sticking outside the airplane and extending far enough above the fuselage, however, would catch the slipstream violently if I altered airspeed 10 knots and increased angle of attack. The area in front of the windscreen, which by appearance would seem to receive ram air and a high pressure point, was nearly stagnant.

In an Air Tractor, there's an air vent in the cockpit entry doors, which form the wind windows. I can pour water near the vent, and the pressure outside is low enough it will draw the air from the bottle to the window and suck it out. Likewise, in a light. Cherokee, if the door pops open in flight it can be difficult to close, especially at the top latch area; it's a curved portion of the fuselage and low pressure; objects placed there will be held agains the opening or sucked out.

Standing in the doorway of most jump aircraft, there's fairly stagnant air, without a lot of inflow into the cabin area, or out, until stepping outside into the slipstream. At that point, as one steps into the slipstream, the airflow passage beyond the doorway is directed into the cabin, momentarily. When flying something like a Caravan, there's a distinct difference at low speeds and max door speeds on the descent. With ramps on aircraft like Skyvans, Casas, and C-130's, there's a notable difference with speeds.

I've done a lot of dropping of retardant from different types of aircraft, from single engine through large four engine airplanes, and the airflow distribution is evident by the retardant that stays on the airframe. Sometimes it's quite evident the conditions in which it was dropped, or the speed, based on where it ended up on the drop, and often one can tell how coordinated the aircraft was. This also describes the airflow and pressure distribution. It's interesting to see where all that material goes.

Pressure distribution as the aircraft approaches transonic ranges, with compressibility a factor, changes considerably.
 
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jetmech
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Tue Dec 25, 2018 5:08 am

Faro wrote:
What about fuselage sides, supposing these experience zero-incidence airflow over their surfaces. The pressure would be lower than the static air pressure, would it not, because of the high velocity airstream giving rise to a low pressure area immediately adjacent to the fuselage side (at zero airstream incidence)? This would exert a outward pull on the fuselage sides.Faro

Yes, I'd say the static pressure on the fuselage sides would be less than ambient static pressure in the undisturbed flow-field, but this would probably be less than you might be thinking.

The acceleration of flow required to maintain a constant mass flow rate past an object in a wind tunnel is much greater compared to an aircraft operating in Earth's atmosphere due to the constraint of the wind tunnel walls.

In theory at least, the amount of "room" available for the Earth's atmosphere to adjust around an object within it is "unlimited" and thus the resulting flow acceleration is much less.

Faro wrote:
And what about the tail cone area in the lee of the airflow around the airframe? Would it also experience lower air pressure like the fuselage sides or would the magnitude of the lower pressure be even greater in this area?Faro

I'd say the flow is starting the process of readjusting to its original, undisturbed situation and as such, the flow velocity relative to the fuselage surface towards the tailcone would be slowing down. As such, the magnitude of the lower pressure should be less than that at the wider part of the fuselage.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
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Faro
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Fri Dec 28, 2018 10:54 am

747Whale wrote:
Faro, that's a really complicated subject, because not only does the pressure vary from aircraft type to aircraft type, but varies at each point on the airframe. Move a few feet away and the pressure will change. Vary the speed and the pressure will change. Alter the airflow over the airframe in any way, by approaching compressibility and transonic flight or the angle of attack, and the pressure distribution will change.

Some airframes take advantage of increased pressure and pressure distribution to lower drag (eg, coke bottle/area rule), or provide positive pressure toward the rear of the airframe.

Pressure distribution is complex and variable. Generally it's minimal along the sides of the fuselage, but that really depends on the fuselage station.

Areas that you might expect to be high pressure may be quite different than expected. For example, I once had a windshield blow out in a Cessna 421, which is a light twin piston engine airplane. There was virtually no airflow into the cockpit, or cabin, however, and I could put my hand through the open space without very little apparently airflow across my hand. material that was sticking outside the airplane and extending far enough above the fuselage, however, would catch the slipstream violently if I altered airspeed 10 knots and increased angle of attack. The area in front of the windscreen, which by appearance would seem to receive ram air and a high pressure point, was nearly stagnant.

In an Air Tractor, there's an air vent in the cockpit entry doors, which form the wind windows. I can pour water near the vent, and the pressure outside is low enough it will draw the air from the bottle to the window and suck it out. Likewise, in a light. Cherokee, if the door pops open in flight it can be difficult to close, especially at the top latch area; it's a curved portion of the fuselage and low pressure; objects placed there will be held agains the opening or sucked out.

Standing in the doorway of most jump aircraft, there's fairly stagnant air, without a lot of inflow into the cabin area, or out, until stepping outside into the slipstream. At that point, as one steps into the slipstream, the airflow passage beyond the doorway is directed into the cabin, momentarily. When flying something like a Caravan, there's a distinct difference at low speeds and max door speeds on the descent. With ramps on aircraft like Skyvans, Casas, and C-130's, there's a notable difference with speeds.

I've done a lot of dropping of retardant from different types of aircraft, from single engine through large four engine airplanes, and the airflow distribution is evident by the retardant that stays on the airframe. Sometimes it's quite evident the conditions in which it was dropped, or the speed, based on where it ended up on the drop, and often one can tell how coordinated the aircraft was. This also describes the airflow and pressure distribution. It's interesting to see where all that material goes.

Pressure distribution as the aircraft approaches transonic ranges, with compressibility a factor, changes considerably.



Wow! That is fascinating...aerodynamics is really very very complex...

I wonder, is the area on the fuselage where the positive pressure on the underside (airliners typically cruise with a 2-3 degree nose-up attitude) meets the negative pressure on the sides a significant source of drag or disorderly airflow? Is this area sharply defined or would it typically be smeared out over a broad surface?


Faro
The chalice not my son
 
747Whale
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Fri Dec 28, 2018 6:31 pm

When airflow moves in one direction, it drops in pressure in others. Think of a balloon. Inflated, the balloon has air pressure exerted on the interior surface, in all directions. The air is static. Open the mouth of the balloon, and a narrow passage exists through which the balloon can deflate and the air can escape. The air at this point is not static; while it pushes out on every other part of the balloon, at the neck it has velocity, and as velocity increases in the neck, pressure drops.

Acceleration is an increase in velocity or a change in direction. Often both. Airflow around curves is accelerated and where that occurs, pressure drops. The spot pressure at any point on the fuselage is determined by multiple factors, not the least of which is airflow velocity over that point. Anything which causes turbulence in the flow close to the surface, however, can inject high pressure airflow into an otherwise "static" zone. Uneven paint, etc, can do it, as can any surface contamination which affects the surface boundary layer or alters the airflow in a given location.

https://youtu.be/AbY2Dn2sUTo

This is an example of a tuft test on a light airplane, with tufts attached at varioius points, which show airflow disturbances. You can look up many others, and you'll have a good visual image of why it's so complex.
 
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Sun Dec 30, 2018 6:38 am

Faro wrote:
What about fuselage sides, supposing these experience zero-incidence airflow over their surfaces. The pressure would be lower than the static air pressure, would it not, because of the high velocity airstream giving rise to a low pressure area immediately adjacent to the fuselage side (at zero airstream incidence)? This would exert a outward pull on the fuselage sides.


There is a common misconception that the Bernoulli equation dictates that a faster flow of air must be at a lower pressure than still air. It actually says nothing of the sort.

What it expresses in mathematical terms is that a fluid parcel of air will always accelerate down a pressure gradient. Basically, it's just a three-dimensional restatement of Newton's law F=MA integrated across a volume of fluid. So if the air moving past a point on the fuselage is moving parallel to the fuselage and axis of the plane with no acceleration or deceleration, you can't tell much about the air pressure.
-Doc Lightning-

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dlednicer
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Mon Dec 31, 2018 9:59 pm

As 747Whale said, its very complicated. The fuselage nose shape causes the flow to accelerate, but then it slows down aft of this. Over the wing, the flow is accelerated again, and then slows down again.

Here are CFD pictures of the calculated pressure distribution on a 737-700 at cruise conditions. The geometry of the aircraft was obtained in a laser scan and the flow solution was calculated using the STAR-CCM+ Navier-Stokes analysis. What is shown is the pressure coefficient, which is the difference between local static pressure and ambient static pressure, divided by dynamic pressure. Negative is suction.

Image

Image
 
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jetmech
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Tue Jan 01, 2019 10:08 am

dlednicer wrote:
Here are CFD pictures of the calculated pressure distribution on a 737-700 at cruise conditions.

Such interesting pictures!

Is that a row of vane vortex generators on the upper tail-cone area above the horizontal stabiliser?

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair :shock: .
 
flight007
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Sun Feb 03, 2019 9:54 am

747Whale wrote:
Areas that you might expect to be high pressure may be quite different than expected. For example, I once had a windshield blow out in a Cessna 421, which is a light twin piston engine airplane. There was virtually no airflow into the cockpit, or cabin, however, and I could put my hand through the open space without very little apparently airflow across my hand. material that was sticking outside the airplane and extending far enough above the fuselage, however, would catch the slipstream violently if I altered airspeed 10 knots and increased angle of attack. The area in front of the windscreen, which by appearance would seem to receive ram air and a high pressure point, was nearly stagnant.

I


This is very strange ,when you drive car at 200km/h with open window and put hand outside it is huge force on your hand.

What change direction on airflow if you dont feel wind when you put hand outside?
 
747Whale
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Sun Feb 03, 2019 10:13 am

flight007 wrote:
747Whale wrote:
Areas that you might expect to be high pressure may be quite different than expected. For example, I once had a windshield blow out in a Cessna 421, which is a light twin piston engine airplane. There was virtually no airflow into the cockpit, or cabin, however, and I could put my hand through the open space without very little apparently airflow across my hand. material that was sticking outside the airplane and extending far enough above the fuselage, however, would catch the slipstream violently if I altered airspeed 10 knots and increased angle of attack. The area in front of the windscreen, which by appearance would seem to receive ram air and a high pressure point, was nearly stagnant.

I


This is very strange ,when you drive car at 200km/h with open window and put hand outside it is huge force on your hand.

What change direction on airflow if you dont feel wind when you put hand outside?


The airflow is not the same at every point around the vehicle.

If you've ever been on a motorcycle behind a large truck you can appreciate the change in airflow when you move close to the truck, behind it. If you don't release the throttle somewhat, you'll accelerate. Why? The airflow behind the truck is not the same as the airflow next to the truck, or in front of it. Likewise, if you're riding in your lane and a large truck passes you, as it approaches form behind and passes, you'll be pushed to the side by a dam of air or air being displaced by the truck, as it passes. Next you'll be buffeted by the turbulent airflow along side the truck. This air is rough, and spills over the fast air and the snow, stagnant air along side the truck.

If you could open windows every foot along the truck and stick your hand out them, you'd find a different pressure at each location.

Airflow in some places on an airframe is dynamically similar to the speed of the airplane. If you're flying at 200 knots and stick your hand out there, you get 200 knots of force on your hand. In other places, however, there's virtually no pressure and in some places negative pressure. In some places, airflow reverses direction and moves forward. If you were to tape paper tufts to the wing, fuselage, nacelles, and tail, you'd find that in some areas airflow moves the direction you expect, from the front of the airframe to the back, as slipstream. In other places, it doens't move at all; stagnant, or static places. In yet others, it moves in a circular pattern, some forward, toward the front of the airplane.

In the case of the Cessna 421, when the windscreen blew out, a big chunk of the glareshield and some of the interior went with it. The top of the glareshield was sticking out the windscreen foward, a long thin piece of metal sticking forward into the slipstream. At the airspeed we flew when the blowout occurred, the twisted metal was neutral, undisturbed, not blowing around. The airflow forward of the cockpit was surprisingly stagnant at that angle of attack and speed. If I decreased airspeed 5 knots, thus flying a higher angle of attack, the very tip of the glareshield would begin to rotate and buffet a little. The airflow had changed just enough to impact it. If I decreased airspeed 10 knots, the glareshield began to buffet considerably more, and at speeds below that, it began to whip, and cause buffeting in the airframe and in the controls, especially elevators. By lowering landing gear and flap and decreasing speed, that buffeting was less, and so we were able to test various speeds and configurations at altitude before planning the approach and landing, to ensure we knew what to expect and what speeds we could safely expect to fly.

With each speed change and configuration change, we had a change in airflow around the cockpit and forward windscreen area. The glareshield sticking out ahead of us and above, was in a position to give us an early indication of those airflow changes, kind of like a big metal yaw string on a glider (in a sense). If you haven't seen a yaw string, it's little more than a bit of string taped in front of the cockpit or windscreen, that gives a visual indication of what the airflow is doing outside the cockpit. In a way, the pieces of airplane sticking out of the cockpit did something similar, in a way we could use to plan our descent and approach.

The point of all that is to illustrate how location on the airframe affects airflow and pressure. Not all locations are the same. While 200 km in your car might produce the effect of blowing your hand back hard when you stick it out the side window, I wasn't in a car. I was in an airplane, and I wasn't sticking my hand out the side window. Much like real estate, it's all about location, location, location. Every point on the airframe is different, and when the designer and engineers look for a place to mount static ports, they look for a place on the airframe that experiences little or no change in airflow at all speeds and angles, and they look for a place that has neutral pressure. When those occur in the same place, that's where the static port goes.
 
Dalmd88
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Sun Feb 03, 2019 4:00 pm

As far as the tailcone goes on the MD 80/90 series, if the tailcone unlocks in flight it will not fall off. The outside air pressure keeps it in place until after landing roll out. I guess that would be a positive pressure bubble on the tailcone.
 
stratclub
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Re: Air Pressure Adjacent to Fuselage Side and Tail Cone

Mon Feb 04, 2019 12:12 am

Dalmd88 wrote:
As far as the tailcone goes on the MD 80/90 series, if the tailcone unlocks in flight it will not fall off. The outside air pressure keeps it in place until after landing roll out. I guess that would be a positive pressure bubble on the tailcone.

Really? The tailcone is in a non pressurized part of the aircraft and I would assume that the pressure inside and outside of the tailcone would tend to equalize and do nothing to hold the tailcone in place if it was not latched.

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