Pitot tubes - do they all produce the same dynamic pressure?

Instruments themselves may be standardized, but your aircraft (with its specific pitot tube model) comes with a POH which states IAS targets for approach, climb, and especially rotation speed.
I know the difference may be small in practice, but in principle the fact that instruments are standard is no argument to switch pitot tube models. Your aicraft is certified with its POH and pitot tube, and its IAS figures are in no way comparable to another aircraft’s numbers.

Peter wrote:

unless the CAS-IAS error in the POH is really of that order.

I know of some Cessnas POH’s with an IAS to CAS as much as 14 knots at very slow speeds. During experimentation, I saw 19 MPH IAS to CAS on my C150, let alone yawing it with the single static port!

I test with this, and experimented a lot with it on my 150. It’s airspeed indicator (a second one) reads directly in CAS, as there is no position error with the swiveling pitot head, as long as it’s in air unaffected by the airframe or other factors. It’s presently being installed to a DHC-3 Otter floatplane for stall testing I will do on it in the weeks to come. The modification probably affects the stall speed, so I’ll have to verify it’s still compliant, and account for any change to IAS vs CAS.

Static ports are a big deal too! It is a huge task to correctly locate one on an aircraft. The worst example I have seen was a 1965 Cessna 210G I test flew. It had the static ports behind the main landing gear doors. So I watched as I operated the gear, and sure enough, the airspeed, altimeter, and VSI bounced around a lot during gear transit! There’s no way I could get away approving what Cessna did there!

Note that for most of the smaller 100 series Cessnas, there is a parts catalog choice of pitot tubes – either heated, which is what is commonly seen, or the very simple tube, which Peter showed. For Cessnas with the tube, the maintenance manual gives a template, so the mechanic can confirm it’s bent to the correct angle!

Note on the AS350 helicopter, the pitot tube (up to the right of the nose grill) points way down. I had my swiveling pitot head installed on that helicopter for some additional testing following a modification where I suspected and IAS to CAS error was an unintended result. I got excellent results showing that the IAS to CAS was pretty good (not much affected) during powered flight, but there was a noticeable error during autorotation. The mod had not affected the pitot head, but had affected the static port quite a bit. As a result, the modification was removed from the helicopter, and never reused.

This camera on the MD500 helicopter completely blocked the pitot tube, so an extension was made, and connected to the original with a tube, it worked perfectly well!

That was the C172P, for example.

So, during a stall exercise, or upon landing with full flaps, once you touch down, your ASI reads only 33 knots, whilst your “actual” airspeed is 46 knots.

I test with this, and experimented a lot with it on my 150. It’s airspeed indicator

Now that’s cool. It has both total and static ports I assume? Is it wireless?

Sure. Usually, pitots are calibrated such that the ASI underreads at very low speed and overreads at very high speeds. Like that, the ASI is usually very accurate (plus or minus 1 knot) at the speeds where it matters most, i.e. at typical final approach speeds.

boscomantico wrote:

the ASI is usually very accurate (plus or minus 1 knot) at the speeds where it matters most, i.e. at typical final approach speeds.

Well…. Be cautious with this assumption. If you’re comfortable with the IAS of the plane you fly during final approach, that’s great, keep doing that. However, when you start putting mods on the plane, or fly a plane new to you with mods on it, be more cautious. Look for flight manual supplements which may accompany the mod(s), and them be super cautious is there are multiple mods interacting with each other (like STOL kit/gross weight increase/VG’s/wing extensions/AoA system), it can go off the rails with combinations of mods.

In particular, aftermarket AoA systems. Depending upon how they are set up (I’ve done a few), they base the set speed (sometime blue doughnut) on 1.3Vs. But….. if the 1.3 factor of Vs was Vs in IAS, the .3 factor could be wrong on the unsafe side. The AoA system must be set up with reference to CAS, not IAS. In this case, any mods already on the plane (like a STOL kit) which could affect position error, which are not accompanied with a new Position Error Correction Table in an FMS, make accurate AoA set up impossible – and that is often not warned.

If you’re giving yourself nice margins, none of this need be of great concern. But the pilots who mod their planes, and then fly them to the limits, should understand that the limits aren’t always where they think they are!

When I bought my C 150M in 1987, it had the Horton STOL kit installed already. I found that I could fly it around light with the airspeed indicator reading zero. Between 40 MPH, and zero, standard ASI’s don’t read much. So I installed an ASI from a Bell 206 helicopter, which read accurately down to 20 MPH. I could fly around all day long at 22 MIAS. With the swiveling pitot head on, I found that this was really 42 MPH IAS, so nicely slow, but a 20 MPH position error.

LeSving wrote:

Now that’s cool. It has both total and static ports I assume? Is it wireless?

It has pitot and static ports, and is direct pneumatic to a second ASI.