Dynon Pitot Head

[skippydiesel]

My new aircraft (A Sonex with no stall warning system) comes with a flashy Dynon set up (wow! is this ever going to tax the geriatric grey matter). Lots of capability but does not have the Dynon pitot head with angle of attack . As the aircraft is still in the final stages of completion , now is the time to consider enhancements - what does the brains trust think of  fitting the Dynon pitot head with angle of attack sensing, that can be used as a warning system in leu of a stall warning ???

[Franky]

No brainer!

Just do it

Wired into your headset gives you early stall warning by ever increasing frequency of beeps as the stall approaches.

Also displays a graphic on screen showing how close to stall you are.

Wiring in audio to your headset you also get any system warnings annunciated into your headset enhancing your instrument scanning

Franky

[skippydiesel]

Thanks Franky - I agree now, I will have to check to see that the mounting point is compatible/convertible.

[skippydiesel]

I have come across a Sonex builder, using a Dynon EFIS, who came up with a very simple low cost angle of attack installation;

 

On the biases that an  A of A is just a system for registering the difference in air pressure between two air ports (holes) - one the (pitot) pointed directly into cruise air flow - the other at an angle that will give increasing air pressure as wings angle of attack increases. 

 

He drilled a suitable hole in his pitot installation plate, fitted a simple, long, aluminium pull rivet (not pulled with the steel "nail" removed) into the hole, epoxied rivet in place,

 

Connected rivet (tube), to the existing port on the Dynon module, which then registered the difference between the pitot air pressure and the rivet opening at 90 degrees to it.

 

He then conducted a number of tests from stall to high speed cruse to "calibrate" the installation - repeated several times to establish consistent readings.

 

For a few dollars in tubing, a bit of epoxy, plus the sacrifice of one long aluminium rivet, he believes he has a workable A of A  and has calibrated his Dynon system to give him an audio/visual warning when he is 10 knots above stall.

 

What think you? Feasible or BS?

[IBob]

Skippy, here is the study, released by the FAA, detailing a workable A of A indicator that uses differential pressure between a forward facing pitot and one tilted downwards, usually 45deg or thereabouts:

https://www.tc.faa.gov/its/worldpac/techrpt/tc18-7.pdf

There is nothing new in this (most commercially available A of A setups work on this principal) but the study serves to verify that with the pitots correctly positioned under the wing, angle of attack across the range can be read.
I am currently prototyping a standalone unit based on this.

The display will sit on top of the instrument panel, in line of sight.

Edited January 4, 2022 by IBob

[kgwilson]

The blaring horns of the Cessnas & Pipers I used to fly got right up my nose as they start well before the stall, most likely because of a fear of litigation if they actually were close to the stall & someone was reliant on them.

 

I made a conscious decision not to include any stall warning in my aircraft when I built it. I did lots of stall testing up high & know my aircraft well so do not need one. All aircraft are different though. Mine will fly right through the stall and just mush down. It won't drop a wing unless I put some power on & use the rudder. If I do that it goes very fast into an incipient spin which would develop into a full blown spin if I didn't bring it out immediately.

 

Horses for courses I guess.

[facthunter]

I think they are pretty much mandated on GA planes. I recall they )(DCA) wanted one fitted to the DH 82 after said plane was  removed from the list of approved initial trainers. The plane has NO electrical system whatever and I was part of those submitting it had distinct pre stall buffet characteristics that rendered the stall warning unnecessary, in my view and that of several others.

  The up to 9 OTA, knots above stall angle initiation is still useful. The plane won't climb (or fly) better at stall speed and if you use the warning and it's just beeping intermittently you are getting the best angle climb or close to it so whatever your weight  and DA are and can be used even if you HAVE to turn.. To just get it to actuate as you flare on landing is going to guarantee no weight on the nosewheel also. Nev

 

[skippydiesel]

On 05/01/2022 at 8:31 AM, IBob said:

Skippy, here is the study, released by the FAA, detailing a workable A of A indicator that uses differential pressure between a forward facing pitot and one tilted downwards, usually 45deg or thereabouts:

https://www.tc.faa.gov/its/worldpac/techrpt/tc18-7.pdf

There is nothing new in this (most commercially available A of A setups work on this principal) but the study serves to verify that with the pitots correctly positioned under the wing, angle of attack across the range can be read.
I am currently prototyping a standalone unit based on this.

The display will sit on top of the instrument panel, in line of sight.

IBob ! you aint dealing with "the sharpest tool in the shed" here mate - the treatise on A of A,  you gave the link for, had my eyes glazing over by the time I managed to get through the first page.

 

Presumably you have worked your way through the 115 pages of  "Low-Cost Accurate Angle of Attack System" and can now comment on the feasibility of, an even lower cost AofA,  as per my contribution  (with the rivet head) above.

[IBob]

Hi Skippy. Don't be too tough on yourself: I printed the thing out, and digested it by slow and painful degrees!

 

The guts of it are in the conclusions on Page 39, though this takes some interpretation too. My reading of it (in no particular order):
1. Two pitots, one faced forward, the other down at 45deg, were attached to the underside of the wing. The probe they used is shown on Page 7. I would guess the two pitots or ports are approx 100mm below the wing skin.

2. It is recommended that these be mounted between 25% and 60% of the chord of the wing, from the leading edge.

3. The ratio of the pressure readings from these two pitots can be used to calculate an accurate angle of attack.

4. This works at whatever the ambient pressure is: there is no need to correct for density altitude.

5. This works regardless of aircraft speed.

6. The results are relatively linear.

7. Calibration is required for the specific aircraft.

 

There is a bit of work in it, and if what you principally want is a stall warning, there are probably simpler ways to come at it.

 

I'm finding it an interesting project.

I have put in a fair bit of work to filter the readings from the two ports in order to give a clear (non-jittery) readout, but with minimal time lag.

It's been on the back burner with extended family visits here, but I hope to get back into it soon.

Here is a pic of my Mk1 probe, lashed to the Volvo for ground testing. The neighbours took to twitching the curtains as the wife and I made numerous runs up and down the street at various speeds and various pitot angle settings...........)

 

[IBob]

PS: I believe Dynon offer a 2 port pitot head which then enables their A of A option.
They make good gear, so I would expect it to work well. However, the readout is (I think) down with the rest of the glass cockpit stuff, which is not where I would want my eyeballs on finals.

The audio system mentioned above is a possible workaround for that. It will be interesting to see how well that works in practise.

[skippydiesel]

IBob; I have the aircraft (yet to fly) fitted with  Dynon Skyview system, that has the capability to deliver both audio & visual warnings of an immanent stall  (calibrated and set by pilot) if fitted with an A of A "pitot" head. I can just go and purchase the head but the idea of doing something more in line with a home built (the un pulled rivet concept) has intrigued me.

[IBob]

Skippy, if I understand you correctly, you are proposing to implement the Dynon imminent stall option, but with a DIY second pitot setup, rather than with the Dynon head?
If that is the case, I would be looking to emulate the Dynon head in some way: that is, I would have the second downward tilted pitot close to the forward facing pitot, and at something like the same angle as the Dynon: there are plenty of pics of their head, which show the position and angle. This should be easy to arrange with a short length of pipe, or similar.
And in my view it would be most likely to give a good match to whatever filtering and calculation is built into the Dynon system.

Edited January 6, 2022 by IBob

[skippydiesel]

4 minutes ago, IBob said:

Skippy, if I understand you correctly, you are proposing to implement the Dynon imminent stall option, but with a DIY second pitot setup, rather than with the Dynon head?
If that is the case, I would be looking to emulate the Dynon head in some way: that is, I would have the second downward tilted pitot close to the forward facing pitot, and at something like the same angle as the Dynon: there are plenty of pics of their head, which show the position and angle. This should be easy to arrange with a short length of pipe, or similar.
And in my view it would be most likely to give a good match to whatever filtering and calculation is built into the Dynon system.

Not so much proposing, as intrigue by another's supposed successful installation for the cost of a repurposed rivet, dab of epoxy and a length of tubing. 

 

Your observations are very logical & well put - you make sense to me.

 

 Badly articulated by me but what I was after was a critique of the above concept (not mine) - May be tiny holes drilled at precise angles in a solid bar of aluminium, installed at a specified angle & position, is not really required, IF the Dynon computer can consistently give a read out from any two "ports" that can register the changes in pressure as a wing changes its angle of attack from cruise to stall. After all, if its a safety (rather than measuring) device it's about having a consistent warning rather than an accurate read out.

[IBob]

Skippy, as I see it:

1. How the pitot ports are formed.......solid bar/tube/drilled rivet........is immaterial, provided that they are in clean air and not affected by surrounding structures.

2. After that, you are looking to maximise the change of pressure as the angle of attack shifts from approx 0deg to 20deg: the greater the change, the better the accuracy and resolution of the reading will be.

3.Various manufacturers appear to set the second port at between 45 and 60degrees. I believe they do this to optimise the accuracy of their equipment. Setting the second port outside this range (or mounting it elsewhere) may result in a less accurate reading.

[skippydiesel]

All good stuff IBob - you may also have noised that some have the two ported bar at about 45 degrees also. Not so Dynon, who just have a small hole below the pitot hole

[IBob]

Skippy, so far as I can make out on the two ported bar that is swung forward at approx 45deg:
The upper port is in the upper sloping face but is drilled back horizontally (not perpendicular to that face).

The lower port is in the in the bar end, which is angled down at approx 45deg , and is drilled perpendicular to that end.

 

As for the Dynon, the pitot port is at the front tip of the probe in the usual manner, while the other port is part way under the rounded tip, so set in a surface that is angled down.

 

There must be many ways to manufacture a pair of pitots ports, one pointing forward, the other angled downward, which is the aim of the exercise. I have chosen to use a bar for ease of fabrication and mounting, and not to have it swung forward, as I am wanting to use an existing hatch for mounting, while maintaining the port position back under the wing as recommended.

Edited January 7, 2022 by IBob

[skippydiesel]

8 hours ago, IBob said:

Skippy, as I see it:

1. How the pitot ports are formed.......solid bar/tube/drilled rivet........is immaterial, provided that they are in clean air and not affected by surrounding structures.

2. After that, you are looking to maximise the change of pressure as the angle of attack shifts from approx 0deg to 20deg: the greater the change, the better the accuracy and resolution of the reading will be.

3.Various manufacturers appear to set the second port at between 45 and 60degrees. I believe they do this to optimise the accuracy of their equipment. Setting the second port outside this range (or mounting it elsewhere) may result in a less accurate reading.

Ref 1. The non pulled rivet head would be in the boundary layer  and close to the pitot tube  - not so good I guess.

 

Ref 2. Is the boundary lair sufficiently effected by the wings change in A of A to be used as part of the sensing system ? - possibly not.

 

Ref 3. The non pulled rivet head (port) would be at approximately 90 degrees to the pitot port - I would imagine this large orientation difference would result in minimal readings at any A of A much above the stall.

[skippydiesel]

Forgotten the name of the person who PM'd me regarding a Dynon A of A/pitot head - may be you didn't get my response in the affirmative - try [email protected]