[rf3flyer]

I have nothing to add about what could be causing the difference between what you see and the previous owner claims to have seen, but I will say this.
The idea that a D117 clean stalls at 30 kts seems very fanciful to me and your 43kts seems a lot more realistic. Also, your 110kt cruise strikes me as not fanciful at all, not even with a C90. There is a D117/C90 not far from me and while I can make 90kts S&L it has to slow down to formate on me. So IMO there is nothing really wrong with your setup, other than that ideally the static should be plumbed in, and that you are beating yourself up over nothing.

[Jodelman]

The original information which I have from Wassmer Aviation for the D120 (which is very much like the D117) shows for The Principal Flying Speeds:-

Stall Straight and Level Power Off - 27 knots
Stall 30 degree Banked Turn - 32 knots
Stall 60 degree Banked Turn - 38 knots

[cockney steve]

If the Pitot-pressure line is OK, then , assuming the instrument is NOT faulty, the resultant high reading is surely due to an obstructed Static bleed from the instrument..

A bent Pitot tube will present an altered orifice to the airflow...Assuming it's circular and square with the tube's centreline, the bent end will present an elliptical opening to the airflow...Is it possible that air impinging this altered orifice, could cause turbulence/ increased pressure??

[thegasguy]

Rf3flyer, I agree that a 2/3 of a metric ton aircraft stalling at 27 knots sounds very very unlikely.
If that's right,why aren't all aircraft using the Jodel wing form?
BUT, I have very similar figures to those provided by Jodelman ( thank you) in my original French 'operating instructions' - in kph of course. Marketing hype?
I do think Cockney Steve is onto something if there is indeed a static bleed orifice, and it's partially blocked.
I don't know if there is a 'flow' through the ASI that can get blocked?
The reading has decreased a bit now the 'flipper' is off so turbulence may affect these things.

[rf3flyer]

It's late, I'm tired, I might have got this muddled, but I'm not sure about cockney steve's logic regarding an obstructed static bleed which I take to mean a partially blocked static.

There is no flow 'through' the ASI. An internal chamber sees pitot (high) pressure and is surrounded by the case which sees static (ambient) pressure. The ASI presents the pressure differential between the two as speed.

If there was an 'obstructed static bleed' the climb would be slightly alarming as the lagging retained static would present a slow, even declining airspeed. Levelled out in the cruise the static would eventually bleed to ambient so there would be no mismatch between your cruise speed and the former owner's cruise speed, all else being equal.

At the stall in level flight, you are slowing so pitot pressure is declining whereas the static is stable so how would an obstructed static bleed give a mismatch between your and the former owner's stall speeds?

Descending and slowing to a stall the pitot pressure is declining but the static, though increasing, is lagging enough due to the obstruction that in such circumstance I think retained static pressure would be lower than ambient and the ASI would over read if anything.

That the ASI shows a proper range of speeds at different phases of flight suggests that the pitot side is OK. If the static side is blocked (high retained pressure) then at all phases of flight the ASI will under read. If the static side is obstructed but leaks eventually to ambient then the ASI will just be slow to respond. Were it just a mismatch at the low end I'd be inclined to think it pitot position error at high AoA but that would not account for the mismatch at cruise.

I have seen figures online that suggest the D117 stalls at an improbably low speed but I'd take that with a large pinch of salt, aka marketing.

[Arclite01]

Thegasguy

You really are quite dangerous aren't you.

You don't appear to understand the basics about how aircraft instruments are plumbed, you don't seem to realize the rationale of a water manomenter check, don't appear to have asked an LAA inspector or CAA engineer to have checked over the installation.

And then bending the Pitot head so it fits your idea of the previous owner's airspeed recollections makes it OK.

Remind me never to buy anything from you - ever !!

Arclite

[thegasguy]

Arclite01
Wow! Where do I start?
Taking your rude and inaccurate comments in order:
This is a technical forum and I am interested in finding an answer to a technical problem. As it happens I do understand the 'plumbing', I'm not so arrogant as to keep saying - yes, I know! What neither I, nor anyone else yet, understands is why the LAA inspector tested components of that plumbing do not respond exactly as per the book.
Regarding the water manometer test, I never expressed an opinion about using one, so I can't see how you formed the idea I " can't seem to realise the rationale", whatever that means.
My forum name gives a clue that I use a very expensive digital one nearly every day of my life and I know precisely what they do - and that water ones went out with the ark.
Look back and you will see my post saying that I have had the whole system tested with a calibrated check box ( manometer) and it reads correctly on the ground, both during my permit renewal inspection - by the LAA- and subsequently.
Also clearly posted is the fact that a LAA inspector re-aligned the pitot head, not me - and where did I say it was to " fit my idea of a previous owners recollections!"
I flew a test flight directly afterwards with the same highly experienced LAA inspector alongside me taking notes and satisfying himself that the aircraft is fully airworthy.
I've only ever said that what I see on the ASI isn't what the previous owner said he saw.
Did you properly read anything I said ? Are you capable of understanding and then remembering it?
To say "you are quite dangerous"on a public forum, based on total fabrications and misquotes is out of order.
There are lots of perfectly safe pilots out there who have no idea at all how all the " technical bits" behind the panel work - and never will do.
I'm a perfectly safe pilot who is trying to delve a little deeper into a ( very small) mechanical conundrum, with the help of this forum.
If you have an answer beyond " get it looked at" ( I have, by people obviously a lot brighter than you) then let's hear it.
As for " not buying anything from me" what's that supposed to mean? I'm not selling anything, just talking on a forum.

[Nomad63]

Have you changed/added/removed anything from the airframe?

[thegasguy]

Russ _H
Yes and no !
I did nothing at all until recently. The identical set up gave very different results for me and the previous owner - hence the head scratching and the post.
However, we ( my LAA inspector and I) removed the flip - up pitot cover recently and the readings are now pretty close to dead on.
Why that resolves it,and why it caused it - is still a mystery.
There is a small error according to SD ground speed, about 5 knots at cruise? but I can live with that.
All I was ever asking is why I saw an erroneous 'increase' in indicated air speed , when all the 'book' problems with ASI's give a fixed or lower air speed on the clock.
No one came up with the actual answer and now I've found the cause by a process of elimination, no one seems to have an explanation. Most odd.

[cockney steve]

Presumably, the "flip-up" pitot-cover would be extremely close to the orifice , in the open position. It will present an obstruction to the airflow and the pitot itself will be fed with turbulent air, which, by it's nature will have pressure variations..... I'd guess that'd why a pitot id a tube projecting forward into a clean airflow, rather than ,say, a faired-in hole in wing leading-edge. Hindsight is a wonderful thing.

[mick w]

I very much doubt the turbulence from the 'Flap' would cause any fluctuations in airspeed readings .

[cockney steve]

splendid photo...... I would expect it to bob about in the airflow...It looks very similar in concept, to the flap on vertical tractor exhausts. Is it manually-opened and held rigidly? Given the size of it, I could envisage a buildup of a "bow-wave" of turbulent airflow spilling over into the pitot inlet. The fact that removal of it cured the erratic reading,would tend to support this theory.

[mick w]

It's a pity I don't have a head on view , but the rear Flap has a large surface area , & the Pitot cover bit is only about 5mm across , so very doubtful it could generate any turbulent flow forward of the Pitot itself , btw it's held open purely by Airflow .

[Charliesixtysix]

I thought it was held open by the drag of the RV6 being towed by it

[mick w]

I thought it was held open by the drag of the RV6 being towed by it

Well that RV6 does look a bit draggy !!