Pitot Probe Length

[criticalmass]

One for the aerodynamicists:-

In an aircraft with a Vne of less than 80KIAS, a cruising speed of 38KIAS and a stall of 27KIAS, fitted with a flush-mounted pitot opening in the tip of the nose, what is the required length for a pitot probe to avoid indicated airspeed errors resulting from local airflow phenomena immediately in front of the aircraft nose?

What prompts my question is something I have been observing during EFATO (Engine Failure After Take Off) training.

As the student noses the aircraft over to re-establish flying-speed, I see gross errors on the ASI - for example, 60KIAS when in fact the particular wing can't ever get to 60Kts anyway and the airspeed is probably about 32-34Kts as we go over the top. Now, I know this is a false reading, but I'd like to experiment to see if I can eliminate it, or at least reduce it.

I am assuming the erroneous reading is the result of gross changes in local airflow around the nose of the aircraft where the pitot intake is flush-mounted during the nose-over manoeuvre.

I'm thinking of adding an extended pitot-probe to move the pitot entry-point into the relatively undisturbed air before the nose, but how far do I have to extend the probe forwards to avoid the errors I have observed?

I assume even an extended pitot will show some sort of error during this major change of aircraft pitch-angle, but some idea of how far forward to extend the pitot would be helpful.

Any thoughts?

[john_tullamarine]

... quick question .. where is the static source ? The pitot is very likely not the principal cause of the observation ....

[barit1]

I concur it's probably the static source.

FWIW - A US high-wing, strut-braced civil aircraft had the pitot-static probe mounted on the jury strut, about 20% chord length below the wing. When the military ordered the same aircraft, they moved the probe to a boom on the wing LE, 30% in front.

[criticalmass]

Ah, yes, the static source...this is a very open cockpit aircraft and the manufacturer didn't see fit to install a proper static source, so I guess it's just the entire open cockpit area with the static port inputs on intruments requiring it left open and un-plumbed behind the panel.

[john_tullamarine]

"Real" aeroplane certification test programs spend a lot of time on static source position and test verification of calibration errors .... I think you have the answer to your original question in your last post ... and I suspect that you could play all day to your heart's content with pitot mods and achieve absolutely nothing ...

[criticalmass]

J_T,

Excellent point...now, would it be better to design the static ports into the extended pitot-probe, or add a flush-mounted static-port (of the type obtainable from the usual suppliers) somewhere in a clean airflow section of the fuselage?

Appreciating all the assistance very much.

[john_tullamarine]

.. now you're cooking with gas ... why not try a conventional flight test vane.

To avoid endorsing any products, suggest you try a search on "swivel head air data boom" and you should come up with some descriptions ...

[Blacksheep]

I assume from the performance figures you gave and the open cockpit description, that we are perhaps talking about a rigid wing microlight here? An advanced technology solution may not therefore not be appropriate.

I think your answer is a combined pressure head. Trying to find a suitable position for a flush mounted static port would be experimental and getting the mod approved would involve flight testing to check fo significant position error at different angles of attack - though I suspect you would enjoy such experimental work.

I've seen combined pressure heads mounted on the port wing strut on a Thruster and also on an MW8 so I suppose that such a modification would more easily be approved then flush mounted static ports. Of course, you'll then introduce the complication of keeping your newly acquired pitot-static system serviceable - regular leak checks and all that.

[criticalmass]

Blacksheep, you have hit the proverbial nail on its little proverbial head.

Today I turned up a whizzbang flush-mounted static-port fitting...and then began wondering where on the pod I should fit it to get it in clean air during all possible flight regimes. This is not as easy as it looks because flat surfaces on a microlight pod are few and far between, and airflow over them may not be all that clean.

All is not lost, however. I am coming around to the idea of a combined pitot-static probe, and with some judicious machining of aluminium I could make one by modifying my existing 1-metre long pitot-probe. Four holes drilled several inches behind the pitot-head, two aluminium tubes aranged concentrically, with a seal at the rear end so both dynamic and static pressures could be plumbed to the VSI, ASI and altimeter is technically achievable with my machine-tools and skill. Perhaps a 1-metre probe is too long anyway, and reducing the length to half a metre would at least make such a conversion a little easier in terms of sourcing materials.

An alternative is to make a separate static probe and mount it near the pitot-probe...two aluminium tubes sticking out from just under the nose instead of one. This avoids the complexities of modifying the already-fabricated pitot-probe in any way.

You're right about the swivel-head air-data boom being a little too high tech - I did an Internet search and found a website with some lovely units - any of which probably would cost nearly as much as I paid for the entire microlight!

So, I may go with my home-made flush-mount static port, or I may modify my existing long probe, or I may fabricate a static-probe and mount it a short distance away from the pitot-probe. Time - and enthisuasm - will decide.

Chances are as soon as the probe(s) are mounted, someone will fail to notice them and bend them both anyway and I'll just junk them and go back to the existing system, with all its imperfections. Perhaps large red "REMOVE BEFORE FLIGHT" flags attached would reduce (but not eliminate) this possibility.

However, one must try to remain optimistic. Again, many thanks to all who have replied.

[john_tullamarine]

I wasn't suggesting that you BUY a swivel head boom - the typical all-singing, all-dancing unit does all sorts of things which you don't need as I interpret your comments.

They are not rocket science so it would be pretty simple to fabricate one ... most of your other installation problems will go away.

My recollection is that Kim White made one up for his machine quite some years ago and it worked real beaut .. sure you could get some pointers from the many folk around the SYD basin who know about such things ...

[DFC]

I think that a 1m probe is a bit over the top. - think where that will go if you hit something!.

One reason for what you experience could be the mechanism of the instrument moving in response to the force applied in pitching the nose down. Yes I know it should not move like that but we are talking about a microlight here and many I came across have damaged instruments due to no pannel mounting suspension and rather solid undercarriage combned with rough fields and/ or hard landings.

Do you ensure that there is no slugs of water in the pitot system.

The reason why I am looking at those posibilities is that in order to cause an indicated airspeed of 60Kt you require a certain dynamic pressure (pitot-static).

Try some figures to see how much the static would have to drop by at constant pitot pressure or how much the pitot pressure would have to increase by at constant static pressure to cause a +30Kt indicated error. at such a low speed.

i.e. is there a 60Kt airflow entering the pitot head when the aircraft is only doing 30 odd knots - that would require quite a local acceleration of the airflow.

Regards,

DFC

[Seat1APlease]

I know this doesn't answer your question but on jets our general training principle was to go for attitude first then correct swing and airspeed next, chasing airspeed as a first priority was discouraged, and usually if the correct pitch is established smoothly then the speed won't be too far out. I know efato on a single engine is different but couldn't the same technique be adapted.

The three are of course interlinked, but would it not be an option to go for an initial attitude then correct the speed and heading when stabilised. I suspect that to fit instrumentation to give accurate readings whilst in a manoeuvre is likey to be heavy and costly, in the context of a microlight.

[criticalmass]

DFC,

The only reason for making a 1m probe was that was the most convenient length of aluminim tubing I could purchase locally, and I figured the air 1m in front of the nose of the microlight would probably not be suffering any effects from the proximity of the airframe (such as it is) at the speeds normally encountered.

I take the point about where the probe will go if I hit anything - to which I hasten to add the only thing I intend to hit is the air, and only then in the normal attitude of flight.

Your point about instrument-panels in microlights not having adequate shock-mountings is also cogent although the undercarriage suspension on this craft is actually quite forgiving. I don't allow students to land it hard, and I absolutely forbid myself to land it any harder than the impact of a butterfly alighting on still water, but it also has to be said that the majority of the panel are Chinese-sourced instruments built to a price, and therefore possibly of questionable accuracy in certain flight regimes.

The existing pitot-tubing is transparent, allowing visual detection of foreign objects (insect, water or otherwise).

All your points are noted and they are all valuable so I'll keep them in mind throughout, many thanks.

Seat1A please,

The microlight has no weight, no energy and absolutely no momentum, and they climb at a steep angle but a relatively low airspeed.

In the event of EFATO, the number one priority is to re-establish safe flying-speed, which requires a radical alteration in pitch-angle of the pod, from approximately thirty degrees up to about thirty degrees down in a matter of as few seconds as possible.

This is a consequence of pulling the control-bar firmly back to prevent the wing from stalling and to re-establish flying-speed. Indeed, it is so radical that the even most cocky student emerges somewhat quieter and rather reflective after the manoeuvre has been demonstrated to them.

The error in indicated airspeed was actually pointed out to me by a very alert student ( I am in the habit of looking at the runway which tends to come up rather rapidly in this manoeuvre rather than at the clocks and dials) who was naturally curious as to what was happening.

Since the pod of the microlight isn't rigidly connected to the wing (the only bit that is actually flying), nose-attitude of the pod is rather meaningless because it tells you absolutely nothing about angle-of-attack of the wing, several feet above your head. Microlights have unique characteristics which three-axis pilots (I have been one myself) find somewhat disconcerting at first, and nose-attitude being essentially a very poor guide of anything except the amount of engine-power being applied is one of them.

As a general comment, using aluminium tubing, sourced locally at a large retail hardware store, keeps both weight and costs down. The machine-tool time is my own, and the other materials used came mainly from scrap etc.

It is more my academic interest in this phenomenon which prompted my question and drives my endeavours, rather than a need to have the ASI read absolutely accurately during EFATO. Any student who calmly reads the ASI as he goes "over the top" on an EFATO exercise probably shouldn't be flying microlights anyway - something in the fast-jet category might better suit such a cool-headed individual.

All assistance given is most gratefully acknowledged; it's all adding to my little store of knowledge.

[barit1]

It occurs to me that alpha (angle of attack) might be more useful than airspeed. Take a coathanger, bend it to attach to the wing LE, and tie a string streamer on it so you can see the airflow relative to the LE radius. There are all sorts of layouts for this, one with multiple strings above and below the wing centerline.