Why do pitots...

...face forwards and not backwards?

OK, to make this RV-related, I've spent too long yesterday reviewing what my pitot options are all so I can drill a hole (or not) in the ?$^%& wing skin! However, this question has bugged me for a while now.

If you look at the equation for dynamic pressure, p = 1/2.rho.v^2, if v is negative, you'll achieve a negative pressure (or vacuum) of equal magnitude which you can use for your signal, plus insect infestation and icing would likely be less of a problem

Does anyone know why it isn't done?

Andrew

I am not an aeronautical engineer, but my assumption would be because the air flow would be very turbulent with an aft-facing pitot resulting in erratic readings.

facing backwards? well that would just suck

Oh man...you forced me to pull out my thermo dynamics book and now my head really hurts! Thanks

I think you could do it to some degree but it would not simply be the - of the dynamic pressure of the forward facing pitot.

Dynamic pressure is just a nice way of describing the stagnation process which are the thermodynamic properties of bringing a flow from some velocity to zero. in a pitot tube (facing fwd) the velocity just at the tip goes to zero since there is no flow through the pitot tube. The V^2 in the equation is the flow velocity far in front of the tube. To expect aft facing pitot tube to function equally the air inside the pitot tube would have to go from zero to ambient. This would suggest the process was reversible and since only isentropic processes (meaning no loses) are reversible and that no real isentropic processes exist it cant work 1 for 1.

however, you could certainly measure the change in pressure with the tube facing aft and if you had the ability to calibrate an electronic A/S indicator you should be able to make it work. I suspect however the changes in pressure over the airspeed of an RV would be very small in this configuration (due to the explanation above) and therefore you'd have to have a stable, very sensitive pressure sensor.

It would be an easy test for someone with an extra A/S indicator. Swap the lines - put the static line on an aft facing PITOT tube and the PITOT line on the static source and see what happens. I'd be interested in the results.

Ken
MS Aero

wow

that makes my simple mind hurt....

Jamie said:

I am not an aeronautical engineer, but my assumption would be because the air flow would be very turbulent with an aft-facing pitot resulting in erratic readings.

Click to expand...

Well, my degree says that I AM an aeronautical engineer (or was when I left school many flights ago....), but I agree with you that the unpredictability of the turbulence behind a body would make it very hard to build a repeatable gauge.

I like Ken's explaination as well, but unpredictable turbulent flow is simpler for my brain to visualize....

yeah, I like that better too. Was thinking any calibration would probably only be good for that particular plane and configuration. Throw the flaps down and and everything goes to He**.

So I'll attempt to add to Ken's explanation without just repeating everything he said or adding confusion:

A pitot tube measures Total Pressure, not just Dynamic Pressure. Total pressure (P_T) is

P_T = 1/2 rho *V^2 + P_S

where P_S is the ambient (static) pressure and 1/2 rho *V^2 is the dynamic pressure.

Imagine a very long cylinder sideways in a fluid and, for the moment assume there's no viscosity, and thus no drag. Imagine putting a pressure gauge on the front face, the side, and the back of the cylinder. The pressure on the front *and rear* face of the cylinder will equal the total pressure. The pressure at the port on the side will be reduced due to the higher flow velocity.

Now if you add viscosity (drag) to the fluid, then the pressure at the back face will be somewhat less than the total pressure, due to the work done to the fluid by drag. Total pressure is analogous to the energy of the fluid. So in order for an aft mounted pitot tube to work, you'd have to carefully calibrate to account for how much pressure has been lost to drag. I'd guess the pressure loss due to drag would change over time as well, due to fouling perhaps.

I don't think the turbulence would be too much of an issue because the frequencies of the turbulence would be much higher than what a pitot static system could respond to. The air in the tubes has a lower resonant frequency. For this reason, pitot tubes typically aren't used to measure turbulence in wind tunnel tests.

Another factor that's already been mentioned is errors due to flaps and angle of attack. Typically, it's best to have the pitot tube mounted as far down and forward as possible. This puts the measurement port in a region where there is less pressure change due to angle of attack. So if you had an aft mounted pitot, you'd still want the measurement aperture in that location.

Pitot tubes

While it is true that we don't have to be experts in Fluid Mechanics to build and fly an RV, it is "intellectually stimulating" to have a discussion involving the basics of the Pitot tube.

Hold on to your hats for this one...

When I was getting ready for my Private Pilot check ride (a long time ago), my instructor wanted me to fly with a CFII buddy of his as kind of a "cross-check" of his instructing ability. I had no problem with this until he didn't like my answer to his question on how the Pitot-static system works. Somehow he had gotten all the way through his CFII rating and thought the air flowed through the Pitot tube to the airspeed indicator and then was "vented" out the static port. He said a lot of Pipers have the words "Static Vent" on the side of the fuselage and that proves the air flows through the air speed indicator.

He did like how I flew the airplane though, and I passed my check ride with the Examiner the next week.

We use them, sort of...

In the soaring world, we often use a thing like a backwards-facing Pitot tube on our sailplanes. It is a slot or orifice drilled on the back side of a piece of round tubing a certain distance from the closed end of the tube. We call that tube either a Braunschweig or Nicks probe depending on whether it is a slot or hole. You connect that orifice to the port on a VSI that would otherwise go to the static port.

I don't know the mathematics of it (except, of course, that 1/2 rho V^2 features prominently), but the effect is that the pressure signal from the probe orifice cancels out accelerations and decelerations caused by pilot-induced pitch attitude changes. As a result, the VSI reading actually reflects your gain and loss of total energy (kinetic + potential), and not just your raw vertical speed. That compensated instrument is far more useful when it comes to finding and centering thermals and other lift sources.

Thanks, Bob K.
http://www.hpaircraft.com/hp-24

Nozzle Efficiency

First, your premise that insects get into the pitot because it faces forward is, I believe, incorrect. Most insects get into the pitot system while the plane is sitting on the ramp or in a hangar, so direction does not matter. You may be right on icing.

Referring to pitot tubes we have at work for instrumentation on process equipment, not on airplanes, but same idea...

In theory, yes, you are correct. You can reverse the sign on the velocity term and arrive at a pitot that faces backwards. In reality, this will not work. Every object moving through fluid (or in my case at work, every fluid moving around an object) will leave a wake behind it. You can think of this wake as the result of the incomplete pressure recovery following the passage of the object. And therein lies the crux of the problem. With incomplete pressure recovery, you will have an exceedingly complex problem relating the pressure at this port back to forward airspeed. The forward facing pitot does not suffer this problem, as it is always subject to the stagnation pressure, unless you've done something fun and placed it too close to some part of your airframe.

So what is this?



From an X-15 after a particularly warm flight.

Pitot

I believe the X-15 used something called a "Q-ball" for airspeed indication at high mach numbers. This was mounted at the tip of the nose. Judging by some of the scary flights I have read about, perhaps this backward facing vent was the aviator's relief tube vent?

n5lp said:

From an X-15 after a particularly warm flight.

Click to expand...

Great picture Larry - I did a little research in a fabulous book on the history of the X-15 program that NASA just published and is now available for download at:

http://www.aeronautics.nasa.gov/ebooks/index.htm

(scroll down to "X-15: Extending the Frontiers of Flight" and pick your format)

The picture was obviously of the #2 vehicle after it had been set up for the high speed portion of the program and covered with an ablative coating to survive high-temp re-entries. I found this paragraph which leads me to believe that it is not part of the pitot-static system:

A different pitot-static system was required for the X-15A-2 since the MA-2S ablator would cover the normal static locations. Engineers chose a vented compartment behind the canopy as the static source, and found it to be suitable during flight tests on the X-15-1. The standard dogleg pitot tube ahead of the canopy was replaced by an extendable pitot because the
temperatures expected at Mach 8 would exceed the thermal limits of the standard tube. The retractable tube remained within the fuselage until the aircraft decelerated below Mach 2; the pilot then actuated a release mechanism and the tube extended into the airstream.

I would guess it is either an antenna or total energy probe - I thought I had a detailed drawing of the external configuration somewhere here, but can't find it right now. When you finish reading the book (you know you want to!), let us know if you figure out a better answer...

Ironflight said:

Great picture Larry - I did a little research in a fabulous book on the history of the X-15 program that NASA just published and is now available for download at:

http://www.aeronautics.nasa.gov/ebooks/index.htm

(scroll down to "X-15: Extending the Frontiers of Flight" and pick your format)

...

Click to expand...

You know I have never been able to read anything long on a computer but it just so happens that I should have a nifty reader waiting for me after a little weekend RV trip to deliver a dog harness. I'm looking forward to the read.

The picture posted by n5lp...

...is of one of the two UHF antennas mounted on the exterior of the X-15. Early flights were with one antenna mounted on the dorsal side and one mounted on the ventral side. Later flights were made with both antennas mounted on the ventral surface.

These UHF antennas were of the blade / vane type with a preformed leading edge and sprayed albator coating.

An earlier reference in this thread referred to the "nose" geometry. It was referred to as a Ring-Ball Nose. It was used to accurately compute hypersonic airflow direction, angle of attack, and pitot pressure.

Huh?

There are some really smart people on this forum! Some of us are just happy when we plug two wires together and things light up.

OK, so since this thread is still twitching...

...what's this thingy*, and why does it have a hole drilled up it's clacker?



*apart from obviously being a vane-type AoA sensor.