Heated Pitot Installation Issues?

How to keep the Cessna 170 flying and airworthy.

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Ryan Smith
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Heated Pitot Installation Issues?

Post by Ryan Smith »

Hello all,

Recently, I’ve been flying a couple of A model 170s. The first A model I flew was a 1949 that was/is essentially the same as my B model in terms of configuration, meaning pretty stock airplane, stock wing, etc.

The next A model I’ve flown has been a 1951 and has been largely the same as the ‘49 with the exception of a heated pitot tube installation in lieu of the standard pitot. Most recently, my former boss purchased a 1950 A model and asked me to teach him to fly it since he got 3-4 hours with me in my airplane to get his endorsement and then started flying his RV8 and has realized that he’s forgotten about flying the 170. His airplane is by far the heaviest (and by far the best equipped), and has a Horton STOL kit and a heated pitot.

For reference, I’ve flown a B model with a heated pitot/stock wing/home brew Lycoming O-360 install that had a similar issue, but it seems like (particularly the A models) the speed is way off from my B model for a given power setting. I’ve flown probably more different 170s than the average bear, and performance numbers have always been consistent, but the airplanes with the heated pitot have me stumped. It seems that the faster the airplane is moving, the larger the discrepancy is in what I expect for speed. I’m normally around 115 indicated at 2400 RPM in my own airplane, but these two are barely knocking on 95-100. I don’t think the prop is the issue as, without knowing specifics, I can tell aren’t that far off in pitch from mine because static RPM is around 2200 RPM and climb performance isn’t appreciably increased, if at all.

Is there some secret sauce for what I’m seeing? I’d be surprised if the airspeed indicators are the culprit in both cases. I can dissect static system inconsistencies in each airplane, but suffice to say the altimeter and VSI in each airplane is accurate and the boss’ airplane has a fresh IFR cert to go with the snazzy GTN650 in the panel.

I know there is a template for aligning the stock pitot tube properly, and I can’t imagine there being that big of an issue with misalignment of a heated pitot. Each A model has the pitot in a slightly different place; the 1951 has it close to the location of the original pitot, and the 1950 is further inboard and further aft along the chord of the wing.

Any insight? One of these days I’d like to fly my airplane in formation with each of them at the same power setting and observe the indicated speed of each. Until then, it’s hard to give target airspeeds to teach someone how to land the airplanes better when I don’t fully trust the airspeed indicator in each airplane.

Any and all advice is welcome.
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Bruce Fenstermacher
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Re: Heated Pitot Installation Issues?

Post by Bruce Fenstermacher »

Ryan, I had a "heated" pitot on my 49 A model and it always indicated slow. I suspected it was because the aircraft was slow but I always wondered about that after market pitot. To this end I can not recall ever finding a template or guidance how a "heated" pitot should be mounted on a 170 wing as the 170 never came with a "heated" pitot. At least not the familiar chrome L shaped one I'm sure we are talking about. The best one might do is find the install instructions for the earliest 172 that had one and follow it. I recall we have a SL, SI or SB for installation of this on a 172 someplace but again I can't recall seeing any actual installation guidance.

BTW for those reading wondering why I say the 170 never came with a heated pitot when clearly that option is listed. The heated pitot option for the 170 is not the common chrome L shape affair we all know but a straight pitot over an 1" in diameter that extends out about a foot and a half from the leading edges of the wing. Looks more like a small mid air refueling point. In all my years of 170 watching I've only ever seen one on a 170 and that was at the Branson Convention.
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GAHorn
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Re: Heated Pitot Installation Issues?

Post by GAHorn »

Ryan… What COLOR are those slow airplanes..? :lol:

The MX Library has the Cessna instructions for the heated pitot installation for early 172/175 aircraft which should be appropriate for 170A/B models.
viewtopic.php?f=36&t=10463

The instructions for location of the L-shaped pitot head would be inboard of the wing strut and ahead of the main spar on a portion of the leading edge.
See line 5 of the isntruct’s below.
L-head pitot (screen-shot partial instructs)
L-head pitot (screen-shot partial instructs)
Circa 1949 Cessna issued Drawing 0511051 approving the installation of a Kollsman Heated Pitot. This installation was similar to the pitots found on C190/195 aircraft and consisted of a straight-tube that protruded forward from the leading edge with a Kollsman 518B-01 head. I have posted the drawing in the MX Library:

viewtopic.php?f=36&t=15955
Kollsman Pitot (screen-shot partial drawing)
Kollsman Pitot (screen-shot partial drawing)
Edit: This drawing does not appear to indicate the commonly-described “shark fin” shape which has flat sides. This Kollsman 518B-01 appears to be round such as this pic I found online:
F836A3D1-D512-4727-A98C-274BDAD989D6.png
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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Ryan Smith
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Re: Heated Pitot Installation Issues?

Post by Ryan Smith »

GAHorn wrote:Ryan… What COLOR are those slow airplanes..? :lol:

The MX Library has the Cessna instructions for the heated pitot installation for early 172/175 aircraft which should be appropriate for 170A/B models.
viewtopic.php?f=36&t=10463

The instructions for location of the L-shaped pitot head would be inboard of the wing strut and ahead of the main spar on a portion of the leading edge.
See line 5 of the isntruct’s below.
5E276060-08CC-49C6-BA8E-5B22A1EC6237.png
Circa 1949 Cessna issued Drawing 0511051 approving the installation of a Kollsman Heated Pitot. This installation was similar to the pitots found on C190/195 aircraft and consisted of a straight-tube that protruded forward from the leading edge with a Kollsman 518B-01 head. I have posted the drawing in the MX Library:

viewtopic.php?f=36&t=15955
1B0323D6-C3A6-48AB-AF98-A35557B5F8E1.png
Edit: This drawing does not appear to indicate the commonly-described “shark fin” shape which has flat sides. This Kollsman 518B-01 appears to be round such as this pic I found online:
F836A3D1-D512-4727-A98C-274BDAD989D6.png

I knew I was forgetting something in the post.

One is white/blue, the other is blue and red vinyl covering orange and green over white.

So, my airplane being red, obviously is the fastest, but I genuinely didn’t think it made THAT much difference!

I’ll look over instructions. Thank you!
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n2582d
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Re: Heated Pitot Installation Issues?

Post by n2582d »

GAHorn wrote: Edit: This drawing does not appear to indicate the commonly-described “shark fin” shape which has flat sides. This Kollsman 518B-01 appears to be round such as this pic I found online:
The attachment F836A3D1-D512-4727-A98C-274BDAD989D6.png is no longer available
The round pitot depicted above is a Kollsman 781-02. Here's a picture of my Kollsman 518 B-01.
Kollsman 518 B-01<br />Click to Enlarge
Kollsman 518 B-01
Click to Enlarge
Gary
hilltop170
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Re: Heated Pitot Installation Issues?

Post by hilltop170 »

On the C-195, the heated "harpoon" pitot has a special calibration correction on the Kollsman a/s indicator that the "standard" heated pitot does not have. It's a pitot thing so the 170 with the "harpoon" might have the same issue. Very few instrument shops know about that.
Richard Pulley
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1951 170A, N1715D, s/n 20158, O-300D
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n2582d
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Re: Heated Pitot Installation Issues?

Post by n2582d »

That’s interesting. This Kollsman is different than the standard Cessna heated pitot tube in that it also has a static line/port incorporated in it. Might that be the reason for the need to have a special calibration correction?
Gary
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GAHorn
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Re: Heated Pitot Installation Issues?

Post by GAHorn »

n2582d wrote:
GAHorn wrote: Edit: This drawing does not appear to indicate the commonly-described “shark fin” shape which has flat sides. This Kollsman 518B-01 appears to be round such as this pic I found online:
F836A3D1-D512-4727-A98C-274BDAD989D6.png
The round pitot depicted above is a Kollsman 781-02. Here's a picture of my Kollsman 518 B-01.
Kollsman 518 B-01.jpg
That clarifies it, thanks. It must be that the “B” indicates the flat-sided version and the 518 without the “B” must be rounded.

I believe the “dash number” signifies the operating voltage…. -01 being 12 Volts and -02 being 24 volts.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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n2582d
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Re: Heated Pitot Installation Issues?

Post by n2582d »

Any idea what the purpose of the “shark fin” is?
Gary
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GAHorn
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Re: Heated Pitot Installation Issues?

Post by GAHorn »

Yes, it’s basically a chamber to prevent any water ingested from travelling thru to the plumbing and instrument.
7724E938-2666-4077-BAC7-F9A30A9ABB0F.png
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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