ASI

[skippydiesel]

Yep ! its me again - As new aircraft all "lectricary" I think I would also like to have a back up air speed indicator. To fit diminutive size of aircraft and panel, would like to go for about a 50mm (or smaller). Again I would like the benefit of your learned experience &  observations.

 

Make

 

Model

 

Supplier

 

Sonex claimed airspeed range 35 knots to Vne 170 knots - I have always understood instruments of this kind are most accurate in their mid indicating range - is this peoples experience/understanding ? If so, I guess I am looking at an ASI that indicates from about 20 knots - 200 knots - your comments?

 

Mendelssohn ASI range from $500-$2200.

[skippydiesel]

I am about to purchase a back up ASI - could really do with some advice on "scale" in knots, suitable for Sonex air speeds.

 

It seems that I could go for a 50-57 mm (2-2 1/45 inch) ; 

• 20-160 knot
• 30-160 knot

In a get home situation,  circuit/landing speeds are likely to be the most helpful, so I havent gone for anything starting at 40 knots.

 

Your advice will be very much appreciated.

[skippydiesel]

Wow! never imagined that this would be such a difficult question.

[turboplanner]

37 minutes ago, skippydiesel said:

Wow! never imagined that this would be such a difficult question.

Well you have attitude+rpm in level flight which will put you at cruise speed.

Your have 1 stage flap+ rpm in level flight which will fit you into a circuit in the second half of downwind leg.

and so on.

If you are a methodical flyer you will notice that all these stages result in a known speed.

I accept that an incorrectly operating ASI may create enough stress to put these correlations out of your mind, but you still have that last stage of wallowing and buffeting that precedes a stall, and in some aircraft the normal tightness of controls starts to go.

 

[Thruster88]

54 minutes ago, skippydiesel said:

Wow! never imagined that this would be such a difficult question.

I will vote for the 20-160 given your final approach speed will be around 43. 

[kgwilson]

My ASI is 30-160 knots. I always thought it read low at low speeds and used a water manometer to test its accuracy. When I am landing the indicator is virtually showing nothing but I am not looking at it anyway. The test was difficult as well due to the difficulty in measuring the small variance at low indicated speeds. The results are as below up to 150 knots. The ASI read quite low up to about 50 knots then got better and was best between 100 & 130 knots but still low by 4 knots.

 

ASI Reading	Manometer setting	Variance
Knots	Knots
18	30	12
28	40	12
40	50	10
54	60	6
65	70	5
75	80	5
85	90	5
96	100	4
106	110	4
116	120	4
126	130	4
135	140	5
145	150	5

[Blueadventures]

15 hours ago, kgwilson said:

My ASI is 30-160 knots. I always thought it read low at low speeds and used a water manometer to test its accuracy. When I am landing the indicator is virtually showing nothing but I am not looking at it anyway. The test was difficult as well due to the difficulty in measuring the small variance at low indicated speeds. The results are as below up to 150 knots. The ASI read quite low up to about 50 knots then got better and was best between 100 & 130 knots but still low by 4 knots.

 

ASI Reading	Manometer setting	Variance
Knots	Knots
18	30	12
28	40	12
40	50	10
54	60	6
65	70	5
75	80	5
85	90	5
96	100	4
106	110	4
116	120	4
126	130	4
135	140	5
145	150	5

Hi KG I'd check your ASI by visual and its components and the scale marking; distances, connect and check some other aircraft asi's.  If it is reading correct then I'd be replacing your ASI unit; best to be within + / - 2 kts at most.  Can you post an image of the manometer and closeup of its graduated marks.  Even pm a size of the tubes and the distance of the markings (eg 0 - 20, 20 - 30, 30 - 40. etc)  Cheers

Edited December 22, 2021 by Blueadventures

[skippydiesel]

I would suggest that; The air speed, as indicated, is not as important as a consistent reading. If you have recorded the stall, take off, climb, econamy cruise, high speed cruise and your ASI is consistent, then just fly to those numbers.. The only time an accurate air speed is really required is when you are required to fly at that speed by convention/or direction (ATC) - how often is this required in RAA class aircraft?

Edited December 22, 2021 by skippydiesel

[kgwilson]

3 hours ago, Blueadventures said:

Hi KG I'd check your ASI by visual and its components and the scale marking; distances, connect and check some other aircraft asi's.  If it is reading correct then I'd be replacing your ASI unit; best to be within + / - 2 kts at most.  Can you post an image of the manometer and closeup of its graduated marks.  Even pm a size of the tubes and the distance of the markings (eg 0 - 20, 20 - 30, 30 - 40. etc)  Cheers

I borrowed it so don't have photos. I have always planned to spend time to resolve the issues but I need a round tuit & I haven't got one. I am now so used to the errors they only worry other pilots who are passengers when I am coming in to land & at that time I don't even look at the ASI anyway. I can be 30 feet off the ground & there is basically nothing on the ASI so some pilot passengers sort of tighten the pucker & have a noticeable grimace on their face & then I grease it on. 

[Flightrite]

Most speedo’s are inaccurate at the bottom end of the spectrum.

I only ever use them as a guide when driving my toy planes, transport Cat I take far more notice!

it’s a good idea to stooge around on a calm day at say 3K and record the speedo’s indications at varies gas pedal positions and pitch, know your plane without the need for a speedo👍

Edited December 22, 2021 by Flightrite

[skippydiesel]

2 hours ago, Flightrite said:

Most speedo’s are inaccurate at the bottom end of the spectrum.

I only ever use them as a guide when driving my toy planes, transport Cat I take far more notice!

it’s a good idea to stooge around on a calm day at say 3K and record the speedo’s indications at varies gas pedal positions and pitch, know your plane without the need for a speedo👍

Soooo!  You are not using/have no ASI, instruments and are coming in to land on a "challenging" one way strip, with a possible tail wind component (?)- until you are close to the ground, you wont have much idea of ground speed and what you get then is likely to have a poor relationship with air speed (stall?) - how is this going to pan out???

 

The Gods (if there are any) are most likely to punish the pilot who has the arrogance to think he/she can fly like a bird.

 

Humans have a very poor capacity to judge speed, in the air, without instruments. The senses we use on the ground, inner ear (semicircular canals), body pressure and noise, to judge acceleration; Speed - Ears (sound of wind/engine/tyres) Eyes ( peripheral vision picking up relative motion, focus for closing distances), perhaps a little vibration, are for the most part useless above a few hundred feet.

 

The fact that ASI's may be inaccurate, at low air speeds, is not of any great importance,as long as the readings are consistent and you, the pilot, know how they relate to the performance of that particular aircraft.

[Blueadventures]

"The fact that ASI's may be inaccurate, at low air speeds, is not of any great importance,as long as the readings are consistent and you, the pilot, know how they relate to the performance of that particular aircraft. "

 

In this instance I recommend re marking ASI face or replace the ASI.  Consistency and incorrect indicated Air speed is not acceptable to me. 

 

I want to see the 30 to 54 kts range accurate to with 1 or 2 kts.  

[kgwilson]

I flew hang gliders for 20 years & never had an ASI. Know your aircraft & you don't need to rely on instruments. I am never looking at the panel close to the ground. I am looking at the runway in front of me, feeling the behaviour of my aircraft, checking the wind direction on the vegetation & other things that have become instinctive over 40 years or so. If it isn't right I'll go around.

[Blueadventures]

3 minutes ago, kgwilson said:

I flew hang gliders for 20 years & never had an ASI. Know your aircraft & you don't need to rely on instruments. I am never looking at the panel close to the ground. I am looking at the runway in front of me, feeling the behaviour of my aircraft, checking the wind direction on the vegetation & other things that have become instinctive over 40 years or so. If it isn't right I'll go around.

No worries mate, I have checked many ASI's in gliders and RAA aircraft and never seen one any where near out as you describe.  I was taught that abrupt pressure changes applied in the increase and decrease application of air pressure during the calibration tests can damage u/s the instrument.  Maybe yours was damaged at some time in the past.  All my comments are meant to be helpful. Cheers.

[skippydiesel]

9 hours ago, kgwilson said:

I flew hang gliders for 20 years & never had an ASI. Know your aircraft & you don't need to rely on instruments. I am never looking at the panel close to the ground. I am looking at the runway in front of me, feeling the behaviour of my aircraft, checking the wind direction on the vegetation & other things that have become instinctive over 40 years or so. If it isn't right I'll go around.

I agree - When on late late final your concentration should be ahead/outside the aircraft (not on the instruments) but how do you "set up" for final (into a challenging airstrip as described above) without knowing your airspeed.

 

AND

 

A hang glider and I imagine a fully enclosed glider, will give the pilot "feedback" to some degree through wind noise, air pressure on face/body, attitude, etc. A fully enclosed powered aircraft (above a 100ft or so) is unlikely to provide such subtle information to  the pilot .

[skippydiesel]

9 hours ago, Blueadventures said:

"The fact that ASI's may be inaccurate, at low air speeds, is not of any great importance,as long as the readings are consistent and you, the pilot, know how they relate to the performance of that particular aircraft. "

 

In this instance I recommend re marking ASI face or replace the ASI.  Consistency and incorrect indicated Air speed is not acceptable to me. 

 

I want to see the 30 to 54 kts range accurate to with 1 or 2 kts.  

I would ask you;

• Why the emphasise on ASI accuracy (over consistency)?
• What are you actually doing when you fly with reference to an instrument?

 

I would suggest the answear is;  For a given instrument (ASI) reading , you are expecting a repeatable aircraft performance - the reading (30 knots at stall) is not  nearly as important as the consistency of the aircrafts performance at that reading. Does it matter that the actual air speed is 28 knots or 32 knots? Surely the fact that you have noted that at 30 knots indicated your aircraft falls out of the sky & you can repeat the same manoeuvre at the same ASI reading consistently, thereby knowing that your approach (final) speed to landing should be 45 knots (using that ASI in that aircraft)

[kgwilson]

22 minutes ago, skippydiesel said:

I agree - When on late late final your concentration should be ahead/outside the aircraft (not on the instruments) but how do you "set up" for final (into a challenging airstrip as described above) without knowing your airspeed.

 

AND

 

A hang glider and I imagine a fully enclosed glider, will give the pilot "feedback" to some degree through wind noise, air pressure on face/body, attitude, etc. A fully enclosed powered aircraft (above a 100ft or so) is unlikely to provide such subtle information to  the pilot .

Easy, I know my aircraft and what speeds to fly according to the ASI. It has been the same since I built the aircraft. Like I said I need a round tuit. The instrument is fine as my Dynon which takes the pitot & static agrees with it. There could be a leak but if there is and it is in the wing then that is a mission that is easy to put off.

 

I have flown circuits with no instruments & all that happens is I do my base and final turns a bit faster. This is probably instinctive based on the statistics of all the crashes on the final turn.

[skippydiesel]

3 hours ago, kgwilson said:

Easy, I know my aircraft and what speeds to fly according to the ASI. It has been the same since I built the aircraft. Like I said I need a round tuit. The instrument is fine as my Dynon which takes the pitot & static agrees with it. There could be a leak but if there is and it is in the wing then that is a mission that is easy to put off.

 

I have flown circuits with no instruments & all that happens is I do my base and final turns a bit faster. This is probably instinctive based on the statistics of all the crashes on the final turn.

So you are flying "according to the ASI".

 

AND

 

You are not trying for a no instrument approach to landing at a difficult /unknown strip - a very different proposition to a successful landing at your home field

[Yenn]

The reason the ASI does not give an accurate figure at low speeds is usually because of the angle of the pitot head to airflow. Back to the old AOA story. You could make it more accurate at low sped by tipping the forward end down a bit, but then high speed would be reading slow.

[skippydiesel]

14 minutes ago, Yenn said:

The reason the ASI does not give an accurate figure at low speeds is usually because of the angle of the pitot head to airflow. Back to the old AOA story. You could make it more accurate at low sped by tipping the forward end down a bit, but then high speed would be reading slow.

My point is - doesnt matter (so much) about accuracy, as long as you have consistency in reading. The pitot should register the same for a given attitude/speed. As long as the ASI is responding consistently, to that pressure, you will have a consistent reading. So if you determine  ASI reading at stall, with landing flaps,  and then multiply the stall reading by 1.5, you will have a safe approach speed on final, for a near stall at touch down . ASI readings unique to that aircraft.

[Bruce Tuncks]

I don't agree that 2 ASI's are better than one. Which is correct if they disagree? You need a third one, which is getting ridiculous.

A standard pre-solo glider check is to see how the candidate flies with the ( students)  ASI covered up.

What the instructor wants to see is about 5 to 10 knots faster than before. And a standard yearly check is the water manometer so you know the ASI is correct then.

[onetrack]

Don't the heavies have 3 ASI's? Bruce, something tells me you aren't going to cut it, as a heavy driver. 

 

https://skybrary.aero/articles/air-speed-indicator

 

Edited December 23, 2021 by onetrack

[skippydiesel]

45 minutes ago, Bruce Tuncks said:

I don't agree that 2 ASI's are better than one. Which is correct if they disagree? You need a third one, which is getting ridiculous.

A standard pre-solo glider check is to see how the candidate flies with the ( students)  ASI covered up.

What the instructor wants to see is about 5 to 10 knots faster than before. And a standard yearly check is the water manometer so you know the ASI is correct then.

I am considering the installation of a small (57mm) mechanical ASI as a back up to my primary flight system,  a Dynon Skyview (as a mature person, I still view electronic panels with some distrust)-  I would hope that the back up will never be used and only checked periodically for reasonable function in the circuit to final speed range. In the normal course of events, it will not be consulted while there is a "live" screen in front of me.

 

I have , in the distant past received training and small experience in landing without reference to gauges, always at my "home" field, where all the familiar visual reference points assisted with the process.

I am not convinced that my aircraft handling skills would achieve the same safe landing, at an unfamiliar airfield, with  a host of possible other factors (tail/ X wind/slope/ width of runway/ surface/surrounding hazards/low aircraft hrs, etc).

Those talented people, who after years of experience/practice,  "wear" their aircraft like a second skin, may be able to adjust for all the variables, without the aid of an ASI, however I do not consider myself so blest

 

 

[Bruce Tuncks]

Yep skippy, I don't think I'm that good either. So go ahead, if you have room on the panel. What harm could it possibly do?

[turboplanner]

7 hours ago, skippydiesel said:

I am considering the installation of a small (57mm) mechanical ASI as a back up to my primary flight system,  a Dynon Skyview (as a mature person, I still view electronic panels with some distrust)-  I would hope that the back up will never be used and only checked periodically for reasonable function in the circuit to final speed range. In the normal course of events, it will not be consulted while there is a "live" screen in front of me.

 

I have , in the distant past received training and small experience in landing without reference to gauges, always at my "home" field, where all the familiar visual reference points assisted with the process.

I am not convinced that my aircraft handling skills would achieve the same safe landing, at an unfamiliar airfield, with  a host of possible other factors (tail/ X wind/slope/ width of runway/ surface/surrounding hazards/low aircraft hrs, etc).

Those talented people, who after years of experience/practice,  "wear" their aircraft like a second skin, may be able to adjust for all the variables, without the aid of an ASI, however I do not consider myself so blest

 

 

There's nothing wrong with putting a second ASI on the panel in front of the right seat, and nothing wrong with a third one. Your ASI is only good until the first wasp starts to build a nest, and all three could be taken out if you happen to fly through a pack of locusts, but you may be overthinking the issue. I realised two RPT pilots lost control of an airliner due to pitot tube blockage but a few years ago I'd hired an instructor to fly a C172 while I took a lot of photos down at 500 feet. We had a very successful hour with no misses and as we were heading back to Moorabbin my mind was on how I was going to set the photos up, so wasn't taking much notice of the flight or radio, so not orientated or watching the instrument panel. The Instructor's sense of humour must have kicked in because we had come right down on Base nearly to the Final turn when he said "Your aircraft". I'd never flown a C172, never looked at the instrument panel, was flying from the right seat, and couldn't see much of the panel with the focus I was seeing straight ahead, and quickly realised I couldn't easily see the ASI so focused on the turn to Final, got some throttle off and flaps down and worked my way to the end of the runway and made a half decent landing without looking at the instruments at all. I'm not suggesting I'm, just that most others could do the same because of the instincts built in.

 

Visual reference points and field shouldn't matter. I was taught to let down over the runway from 1500 feet to 1000 feet which  put me at the correct downwind distance from the runway, do the 90 degree downwind turn, fly past the end of the runway look over my shoulder and (on a Cherokee) when the end of the runway was 45 degrees behind the wingtip make the base turn with a descent which put me at 500 feet for the Final turn and the aircraft would be ready to round out over the end of the runway. Once you got to do that well, you could duplicate it at most airfields. When I was learning to fly Jabirus and instructor was giving me ground points to turn on - a piggery for Crosswind, turn Base at the two story house etc. and after a while I realised this wasn't such a good idea because not every airfield had a two story house and if you were scorching downwind with a very strong wind, Final was quite different, so I reverted back to the old teaching, and worked off a wing strut for setting Base turn.

 

So in summary, I don't think you have to "wear your aircraft like a second skin", I think some people are overthinking this. I certainly wouldn't recommend taping some paper over the ASI to see if you can do it, but I would recommend going up with an instructor with a cardboard divider so only he/she could see the ASI to get your confidence up.