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RE: A bone of contention = - 2/20/2004 7:36:36 PM   
Tall Paul



Posts: 4655
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		Like Lou, this stuff brings back memories.... and the old flight test motto... "Fighting back the forces of ignorance and superstition one more day"
Martin Simons* book is invaluable.. I got mine recently in about a week from England thru Amazon. com (shameless plug! )
He lays out most of what has been discussed and cussed here...
figure 1 shows Bernoulli... the change in pressure due to a change in velocity.
Figure 2 shows the air ahead of the wing is influenced by the wing's presence (up to sonic velocities) and thus a "inflow-upwash" is created...
And Figure 3 shows how a propellor does the same thing..
.
*"Model Aircraft Aerodynamics" by Martin Simons, 4th edition..ISBN 1-85486-190-5

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(in reply to banktoturn)
       Post #: 51

RE: A bone of contention = - 2/20/2004 11:09:21 PM   
acropilot_ty



Posts: 122
Joined: 12/3/2002
From: Dana point, CA, USA
Status: offline 		Jon, your post was very clear and I agree with you 100 percent.

Paul, your example of the venturi is great, The total pressure within that venturi is the same at every point, neglecting pressure losses due to skin friction. As the flow accelerates through the venturi the static pressure is reduced, and the dynamic pressure is increased. If you installed a pitot tube at any point in the venturi it would measure the same pressure, total pressure. But if you attached a static port at any point in the venturi it would measure the local static pressure, which will not be constant. Taking the difference between total pressure from the pitot, and static pressure will yield dynamic pressure. This is how airspeed indicators work, which Paul talked about in great detail in previous posts... It requires very careful placement of the static port in order to read atmospheric pressure at that altitude. Mounting static ports in front of the aircrat on the same tube as the pitot port makes the most sense. In cessnas with one static port on left side of the fuselage I can make the airspeed indicator read backward by doing slow flight in a slip with full right rudder... wich indicates that the static pressure on the fuselage is greater than pitot pressure at the pitot tube at high angle of attack and sideslip angle in a 152 or 172. In flight test a trailing static system is often used which uses some kind of calibrated cone that is doesn't care what angle of attack or sideslip the aircraft is in.

Ty

(in reply to Tall Paul)
       Post #: 52

RE: A bone of contention = - 2/21/2004 1:19:32 AM   
Tall Paul



Posts: 4655
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		All Tristars had provisions for a trailing cone. It was used on the first production acceptance flights to calibrate the airspeed system.
A conduit from the aft pressure bulkhead in the cabin up inside the leading edge of the to the rear of the fin tip let us install one any time it was needed.
We used two types. The Douglas cone system used a 1/2" nylon tube, which was keep in a water tank when not used to keep the nylon flexible. The sensor was an aluminum pipe with a calibrated series of ports machined into the tube side. Generally it was 150 feet behind the plane in flight.
The Boeing system used a larger diameter nylon tube with a 1/8" steel cable running all the way thru it, from a location at the cabin end to the cone itself. Both types of systems had the cone about 15' behind the sensor.
We would deploy the cone after takeoff, but before the plane got to cruise speed. Once it was out, it remained out until the plane slowed for landing. On the few times a cruise speed retrieval was attempted, it was found to be impossible to pull it in!
The trailing cone system was developed as the earlier system using a streamlined lead block for weight was found to be unacceptable when doing manuvers such as stalls. It would frequently fly -around- the airplane, or thru it!
The nylon tube and f.g. cone was less capable of damaging the airplane.
I spent many hours watching these things... we frequently had a window with an extension outside the skin of the plane with a mirror so we could see what was going on behind the plane.
During two types of tests, the cone system could be expected to be damaged/destroyed/lost. Stalls. all by themselves weren't bad, but if the pilot hit the rudders... a giant wave from the fin tip vortex would travel down the hose, and usually snap the cone off.
Vmca tests, minimum speed for control in the air, were generally done low over the ocean, abt 1500', with one engine dead. Not idling, it was turned OFF!! not running! The pilot would then see if he could get the plane to fly as slowly as theory predicted! Knowing it required something on the order of 5,000 feet to recover from a symmetrical stall... I always wondered about these test conditions.
With the constant rudder required to maintain heading, the cone would be beat to death. THe cable in the Boeing system wouldn't let anything fly away, but we'd always bring back a useless mess of snapped snarled tubing from these tests.
The Boeing system had another "feature".. during electrical activity lightning could enter the cabin down the cable to the tie-off point!
Sometimes the steel cable would vaporize, and the whole thing depart out the fin!
We scattered them all over So. Cal and the ocean.
Despite a reward notice on the cone itself, we never got one back.
First picture... The Tristar is close to the stall break point, the cone has dropped behind the airplane, with the hose showing no tension on the hose..
2nd picture... the wave from a rudder pulse going down the hose... a real "E-ticket" ride coming! The cone will be VIOLENTLY jerked to the right...
3rd picture... a couple of waves in the hose due to rudder pulses..
4th picture.. when it's being used properly

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(in reply to acropilot_ty)
       Post #: 53

RE: A bone of contention = - 2/21/2004 1:39:10 AM   
Tall Paul



Posts: 4655
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		" Lets say your at an EAA flyin and you see a homebuilt with a pitot tube mounted on top of the wing right at the high point of the airfoil... and along side of that pitot tube there is a static port also on the high point of the airfoil. What do you think the airspeed indicator and altimeter would read? What would the pressure be at the pitot, what would the pressure be at the static port... what is the total pressure at that position on the wing? That is what I am talking about".
I would have hoped such a thing had been trucked in, not flown in!
With the static port at the peak of the airfoil. the pilot -might- notice his altitude is changing rapidly when taking off, while the wheels haven't left the ground.The instrument readings would be gibberish.
That something like this could be done isn't surprising.
One of our C-130 mods necessitated an APU for the extensive electronics suite be placed in the left main mount fairing.
On the first takeoff after all the mods had been done, the airspeed and altitude went berserk! Fortunately the plane had been equipped with a flight test airspeed system using a trailing cone for the static pressure and a seperate total pressure probe for airspeed. Switching immediately to the flight test system fixed the instrument problem for that flight.
The cause was the production static port for the C-130 was located in the left main mount fairing, not far from the NACA duct for the APU. A little bit of airspeed allowed the APU inlet to seriously disturb the air at the static port.
Finding THE spot for the static port is quite important.
I guess your C-150 has it wrong.

(in reply to Tall Paul)
       Post #: 54

RE: A bone of contention = - 2/21/2004 3:27:07 AM   
LouW



Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline 		At Lockheed Georgia flight test, on the C141, (and as well as I recall on the C5A as well) we used a boom mounted far ahead of the nose in undisturbed air. There were vanes on the end of the boom with pivots that kept the probes aligned with the relative wind so that they would read true static and dynamic pressures. Early in the program, we flew with a chase plane that had a calibrated airspeed system to further substantiate the readings in normal flight.

(in reply to Tall Paul)
       Post #: 55

RE: A bone of contention = - 2/21/2004 3:39:41 AM   
LouW



Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline 		acropilot_ty, let me see if I understand what you have been trying to say. The static pressure at any given point on a surface moving through the air may vary considerably from the actual static pressure of the air mass remote from the passage of that surface. And the associated dynamic pressure adjacent to that point will vary also according to the relationship developed by Mr Bernoulli. You have not been addressing the measurement of overall airspeed, as Tall Paul seems to assume, but are simply noting the effect of local changes of velocity and static pressure at a particular point on the surface.

(in reply to acropilot_ty)
       Post #: 56

Newton - 2/21/2004 5:04:17 AM   
Ben Lanterman



Posts: 1284
Joined: 10/27/2002
From: St. Charles, MO, USA
Status: offline 		Hi Lou,

You have a long memory :-)


>Oh, I see now. And the thrust of a rocket is only due to the
>pressure within the combustion chamber and has nothing whatever
>to do with the mass ejected from the nozzle, and the kick of a rifle is
>only due to the pressure in the chamber and is not related at all
>to the mass of the bullet fired. That clears up a lot of physical
>effects that are erroneously attributed to Newton, but are in
>fact just pressure problems.

Actually it is a misapplication isn't it. Shooting the bullet doesn't make the gun kick back. Both things are sharing the impulse given by the the explosion. The energy on each is through pressure over the areas involved.

If I am in space setting on an asteroid and toss off a hunk of rock with my arm and hand then I have imparted energy to the system. It is divided into the rock and the asteroid according to masses and inertias. It can be measured by the acceleration of the rock, the rotation and acceleration of the asteroid (with me) or a pressure transducer on my glove measuring the impulse given to both the rock and glove. The fact that I can measure the mass and acceleration of the rock does not mean that it is moving the asteroid. What moved the asteroid is the impulse as measured into the glove - force and time. It doesn't matter if it is a rock or a sping mechanism as long as the impulse is the same.

The same with the rocket and bullet. If you want to measure the thrust of a rocket you put a scale on the nose. Another way would be to measure the pressures acting in and on the rocket and doing a summation of forces. Another way would be to measure the mass and accelerations involved in the exhaust plume. However, if I am measuring the force of the nose of the rocket on the scale it would seem reasonable that measuring the sum of forces on the rocket would be an appropriate thing to measure.

If I want to know what the forces on the wing are measuring the forces (pressures) would be the thing to do.


>Guys, what you say is absolutely true. Pressure is the only force acting
>on the wing. However that force wouldn't exist if the entire pressure
>field surounding the wing didn't accelerate a mass of air downward.

I don't know about this. I always went to sleep in the middle of the lectures I didn't understand. I might reason though that the pressure field is caused by the angle of attack and flow rate. What happens to the flow later is unique but not necessary. However again my math skills are not sufficient to work the problem.


_____________________________

Ben Lanterman

(in reply to LouW)
       Post #: 57

RE: Newton - 2/21/2004 2:41:27 PM   
LouW



Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline 		And the earth is flat, and combustion is the escape of plagiston. If it works for you......

(in reply to Ben Lanterman)
       Post #: 58

RE: A bone of contention = - 2/21/2004 6:25:46 PM   
Tall Paul



Posts: 4655
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		We had a special nose made for the Tristar to put pressure probes and vanes out ahead of the aircraft to tests in turbulence and gusts..
ISTR 4 pressure probes pointing off-axis about 45 degrees around the very front, and alpha and beta vanes to measure any off-axis flow.
Testing this led to reports of Tristars flying down ski-slopes in Idaho!
The airspeed system on the F-117 was developed from that. The diamond shape has a total pressure pickup at the very front, with flush pickups on each facet of the probe for alpha and beta which the control system uses.
I don't recall where the static port is on the -117.
We used 16-bit Kollsmann pressure transducers for total and static pressure on the Tristar. These were accurate to less than a foot at 35,000 feet, and less than a knot at .9 Mach.
They were so good the aero guys always questioned the results!
We went as far as flying two Tristars together to get comparison data... which always agreed! (and probably made the world's largest radar blip! )

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(in reply to LouW)
       Post #: 59

RE: A bone of contention = - 2/21/2004 8:53:03 PM   
LouW



Posts: 804
Joined: 1/1/2003
From: Moreland, GA, USA
Status: offline 		The Tristar installation sounds like the same thing we used on the C-5A. Not surprising since it was the same company just on a different coast. Didn't we have fun?

(in reply to Tall Paul)
       Post #: 60

RE: A bone of contention = - 2/21/2004 9:02:08 PM   
Tall Paul



Posts: 4655
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		Actually, Lou, it's in my nature as an engineering pedant to prefer to see terms which are defined as specific things to be used correctly, not modified without notice of the modification.
The Bernoulli equation:
.
H=p + v^2*rho/2
.
H is the total pressure.
It consists of the static pressure, which can not be changed for our purposes, and the dynamic pressure, which does change as a function of velocity, either over the airframe or in the tunnel, and the air density rho.
For level flight, rho is usually considered a constant.
p is the static pressure.It is independent of the existence or effect of the airframe.
v^2*rho/2 is the dynamic pressure. That is what works on the airframe.
It can be fairly stated that static pressure changes on the upper surface of an airfoil due to airflow.. But then it is no longer -the- static pressure, and there is a corresponding change to the pressure on the lower surface, which is also not -the- static pressure once it's changed, but must be considered -with- the pressure change on the upper surface as they act simultaneously.
As JohnG points out, such changes can both be "negative" relative to the ambient static pressure but one can be " more negative" than the other, and the sum of the two, not each seperately is what the airframe responds to.
.
It's the non-rigorous use of specific terms which gets my interest, either when they're used too loosely, or just plain wrong.
When used loosely, they can lead to misperceptions, and when used wrong, they can generate serious problems if the bad definition is used to try something which is in itself not capable of working because the concept it is designed to is wrong.
Some of the "technical" explanations seen here appear to be more a subset of the "infinite number of monkeys".. typing Shakespeare.. with the more limited vocabulary of engineering making it easier to assemble technical terms at random and construct sentences which obey rules of English grammar, but in actuality have little information content, and what content there is can be totally backwards to the real world, and as mentioned can cause real harm if followed.
This is why I continually refer to other sources than just what is presented here. Sometimes what is seen hear is pure fantasy. Other times it's just fine, but there's enough fantasy presented as fact to create a "caveat emptor" attitude... Free advice is worth what you paid for it.

(in reply to LouW)
       Post #: 61

RE: A bone of contention = - 2/21/2004 9:06:06 PM   
Tall Paul



Posts: 4655
Joined: 6/23/2002
From: Palmdale, CA, USA
Status: offline 		Lou the Tristar program had more influence from the guys we picked up from Boeing than Marietta, as I recall.
The Georgia guys did more of the autopilot stuff, while the Renton's did the aero.
Most of them returned to their respective "sources" when the L-1011 program failed.

(in reply to LouW)
       Post #: 62

RE: A bone of contention = - 2/22/2004 1:56:57 PM