Va, Vso... VVVVVVVV

[Guest Cralis]

Hiya,

 

I am reading the RA-Aus site about V speeds, and got stuck on the first one! :(

 

Va – design manoeuvring speed. This is sometimes referred to as the 'speed for maximum control deflection'.

 

Is this the maximum air speed you at which you can go full rudder, or full elevator, before the manufacturer things the plane is about to break?

 

Later, it states: "Of course, the aircraft designer may specify a Va speed that is greater than that minimum requirement."

 

That's the bit that threw me. If they say the Va for my plane is 100, then why would they state one that's > than the speed they think the plane will snap at?

 

Please note: It's my understanding that I am questioning. I'm obviously missing something.

 

The text above is from Airspeed, at the part labelled Critical limiting speeds.

 

 

[Guest brentc]

It looks like it's meant to be in the reverse sense. ie., greater than in this case, means that they have put a higher (greater) margin than what is realistically required - thus a lower speed.

 

 

[Guest Michael Coates]

Note Michael Coates has been banned from this site

 

 

[Guest Michael Coates]

Note Michael Coates has been banned from this site

 

 

[Guest airsick]

How bold is too bold? I guess only Michael can answer this!

 

 

Michael touched on the fact that a normal category aircraft has a load limit of 3.8g. This means that, by the design standards, it must have a Va of at least 3.8^0.5 x stall speed. There is nothing to say that Va cannot be greater than this if the designer sees fit.

 

There is a great book on aerodynamics by Jan Roskam et al. He details lots of stuff but shows the simple relationship between stall speed, maneuvering speed and cruise speed.

 

Vs x sqrt(load limit) <= Va <= Vc

 

So, Va can be anywhere between the square root of the load limit time the stall speed and cruise speed.

 

Later, it states: "Of course, the aircraft designer may specify a Va speed that is greater than that minimum requirement."That's the bit that threw me. If they say the Va for my plane is 100, then why would they state one that's > than the speed they think the plane will snap at?

The point is that the designer thinks it will take a lot more than the minimum design requirement to snap the aircraft by pulling back.

 

Further to what Michael was on about with airspeed there is some good reading on this on the Vans aircraft website. http://vansaircraft.com/pdf/hp_limts.pdf

 

 

[Guest Michael Coates]

Note Michael Coates has been banned from this site

 

 

[Guest pelorus32]

So Cralis,

 

just to make sure you are really confused remember that Va for any given aircraft varies with MTOW because stall speed varies with MTOW.

 

Please take note of what Michael says about Vne. There has been an interesting series of safety articles in the RAA mag and one or more of those articles was about the dangers of high speed. It's real. A friend of mine wrote me a very sad email. He had been standing on the airstrip when a good friend of his - the Italian CT distributor - made a high speed pass at about 150 metres and the aircraft disintegrated.

 

Your family don't want to know about an accident like that. Do not fly near or above Vne.

 

Regards

 

Mike

 

 

[Guest Cralis]

Thanks guys.

 

A quick question on the IAS, CAS and TAS... after reading the Human Factor post above. I'm slightly confused on the CAS and IAS. Surely the IAS is the CAS? 'Indicated Airspeed' is shown on the ASI, right? It's 'Indicated' to the pilot? If that's so, then .. surely it shows the speed with calibration? Therefore, the CAS?

 

Or is this conversion done by the pilot?

 

CAS: "No pitot tube (device to measure pressuse used to indicate airspeed) is positioned exactly parallel to the airflow, therefore the input speed – IAS – must be corrected to obtain proper airspeed readings."

 

If so, why don't they just show the CAS on the ASI?

 

 

[Guest pelorus32]

Indicated Air Speed is just that - the indication given by your ASI.

 

CAS is IAS corrected for positional and instrument errors. So by definition the ASI will always display IAS. In many cases the difference is not particularly important. In addition IAS and "real" airspeed often diverge at higher angles of attack where, by definition, the pitot tube will be diverging from the relative airflow - because the pitot tube is fixed.

 

TAS is a function of CAS, Temperature and Pressure so it changes with changes in those parameters.

 

At ISA sea level CAS should equal TAS but nowhere else.

 

Think I got all that right - just a quick note between meetings!!

 

Regards

 

Mike

 

 

[djpacro]

Minor additions:

 

IAS is Indicated Airspeed as given by an instrument with zero error. So what you read on your actual instrument is Airspeed Indicator Reading (ASIR). If you have a calibration of the instrument handy for your particular aeroplane you can then determine the IAS - some tests will require this to be done but for 99.9% of flying don't worry about it as the error should only be 1 or 2 kts.

 

The difference between IAC and CAS is the pressure error correction which normally arises from the static source (rather than pitot) and occurs throught the speed range. Some manufacturers are particularly good at getting the ASI to under-read at low speeds and over-read at high speeds.

 

Google should provide lots of info on this subject. (back to my meetings now)

 

 

[K-man]

Pilots flew their machines at reasonably high altitudes but at very high speeds. One of them deployed airbrakes (spoilers) at the speed of 285 km/h (155 kts), the other was flying at 3000 m

 

(10.000 ft) at 270 km/h (145 kts) IAS.

 

They both encountered severe vibrations caused by flutter.

 

What was the reason for the flutter causing both accidents?

 

Both pilots greatly exceeded speed which should never be exceeded, the VNE.

 

With the IAS to TAS correction factor taken into consideration, they were both flying faster than 315 km/h (170 kts)!

I know this thread began with Va but Michael's post raises an interesting question regarding Vne. The problem outlined shows that in those aircraft inadvertantly exceeding Vne caused catastrophic failure. IAS was below Vne, TAS was higher.

 

We have an EFIS in our aircraft and I have always kept an eye on the TAS. It does get up close to Vne on occasions. After reading the post I tried to find further reference on the internet but apart from a reference to the RV aircraft and the above mentioned Pipistrel problems I did not find reference to other aircraft manufacturers warning of the problem, although some aircraft have a 'barber's pole' which gives different Vne at different heights. Mostly these heights are over 10,000'.

 

What I did find was this. Vne is a speed that is 90% of the demonstrated divespeed of that particular aircraft. Unless someone deliberately sets out to test an aircraft to destruction this speed is likely to be conservative. In that case, if an aircraft was exceeding Vne (TAS), but the IAS was less than Vne, the pilot would be blissfully unaware of any problem, and in fact may be well within the safety envelope of that aircraft. Obviously the Pipistral aircraft had an accurate Vne and the higher TAS was critical.

 

As most RA aircraft probably don't have instruments that show TAS, my question for discussion is this. Is this a problem in RA aircraft and if so what do we do about it?

 

Required reading Don't fly real fast

 

 

[Guest pelorus32]

I know this thread began with Va but Michael's post raises an interesting question regarding Vne. The problem outlined shows that in those aircraft inadvertantly exceeding Vne caused catastrophic failure. IAS was below Vne, TAS was higher. We have an EFIS in our aircraft and I have always kept an eye on the TAS. It does get up close to Vne on occasions. After reading the post I tried to find further reference on the internet but apart from a reference to the RV aircraft and the above mentioned Pipistrel problems I did not find reference to other aircraft manufacturers warning of the problem, although some aircraft have a 'barber's pole' which gives different Vne at different heights. Mostly these heights are over 10,000'.

 

What I did find was this. Vne is a speed that is 90% of the demonstrated divespeed of that particular aircraft. Unless someone deliberately sets out to test an aircraft to destruction this speed is likely to be conservative. In that case, if an aircraft was exceeding Vne (TAS), but the IAS was less than Vne, the pilot would be blissfully unaware of any problem, and in fact may be well within the safety envelope of that aircraft. Obviously the Pipistral aircraft had an accurate Vne and the higher TAS was critical.

 

As most RA aircraft probably don't have instruments that show TAS, my question for discussion is this. Is this a problem in RA aircraft and if so what do we do about it?

 

Required reading Don't fly real fast

This is a complex area which I had tried to avoid complexifying more than necessary. However now that you ask:

 

Vne is set at either

 

1. a safe speed beyond which flutter is likely/known/predicted to occur; or
    
    
   
2. 90% of Vd (as you point out).
    
    
   

 

 

Here's where the complexity comes in! If the Vne of your aircraft is set using parameter 1 above then Vne is a function of TAS, because it's a function of the speed of the airflow, not the dynamic loading on the aircraft.

 

If the Vne of your aircraft is set using parameter 2 above then it can usually be assumed to be a function of CAS - effectively IAS.

 

Here's the problem: Most of the time you don't know how the Vne for your a/c was set. Therefore you must be conservative and use TAS when calculating your speed for Vne.

 

You don't need a Dynon to do that for you. As a rule of thumb you can simply increase the IAS by 2% per thousand feet and use that reasonably safely as an estimation of TAS. I know that that ignores temperature but most of the time it will be accurate enough.

 

If you have a Vne of 140 knots and you are indicating 130 knots at sea level then you have a 10 knot buffer over Vne - not much but maybe OK. If you are indicating 130 knots at 8000' then you have a TAS of about 151 knots and you are in serious danger territory.

 

The bottom line is BE CONSERVATIVE when dealing with Vne - use TAS.

 

Just as an aside, at one time (long time ago) I did a lot of hours in a Hughes 269c helicopter (crew not pilot). On the panel was a little plaque which set out the Vne for different altitudes, clearly indicating that in that a/c at least Vne was a function of TAS not IAS.

 

Regards

 

Mike

 

 

[Yenn]

Just thinking about this, I was doing 120kts today while on descent from 5000', shade temp on the ground was about 32 C. My Vne is 138 kts, so I may have been getting a bit close. I don't usually go over 110kts even at sea level, so maybe it is time I looked at the numbers and maybe got a thermometer to give OAT and then use the wizz wheel to be sure.

 

 

[Guest pelorus32]

At about 70% power setting straight and level with the Rotax 80hp a known aircraft will scream through VNE, so it has nothing to do with divespeed ??

FAR23 - and most manufacturers use it regardless of certification and/or whether it makes sense:

 

Sec. 23.1505 Airspeed limitations.(a) The never-exceed speed VNE must be established so that it is--

(1) Not less than 0.9 times the minimum value of VD allowed under Sec. 23.335; and

 

(2) Not more than the lesser of--

 

(i) 0.9 VD established under Sec. 23.335; or

 

(ii) 0.9 times the maximum speed shown under Sec. 23.251.

RegardsMike

 

 

[youngmic]

Maybe this is the solution.

 

FAR 23.251

 

There must be no vibration or buffeting severe enough to result in structural damage, and each part of the airplane must be free from

excessive vibration, under any appropriate speed and power

 

conditions up to VD/MD. In addition, there must be no buffeting in

 

any normal flight condition severe enough to interfere with the

 

satisfactory control of the airplane or cause excessive fatigue to

 

the flight crew. Stall warning buffeting within these limits is

 

allowable.

VD/MD are IAS values, VD/MD can be no greater than 0.9 VNE

 

FAR 23.1545

 

c) If VNE or VNO vary with altitude, there must be means to

indicate to the pilot the appropriate limitations throughout the

 

operating altitude range.

 

(d) Paragraphs (b)(1) through (b)(3) and paragraph © of this

 

section do not apply to aircraft for which a maximum operating speed

 

VMO/MMO is established under §23.1505©. For those aircraft there

 

must either be a maximum allowable airspeed indication showing the

 

variation of VMO/MMO with altitude or compressibility limitations

 

(as appropriate), or a radial red line marking for VMO/MMO must be

 

made at lowest value of VMO/MMO established for any altitude up to

 

the maximum operating altitude for the airplane.

[K-man]

Now I'm getting confused. Just as I was being convinced Vne refers to TAS the pendulum seems to be swinging back towards IAS. :confused: In this case would it be SAFE to suppose that, unless there is a 'Barber's Pole' or a table supplied with an aircraft such as the Pipistrel and that under the new rules we are not likely to be flying above 10,000', we could use IAS to avoid passing Vne? :thumb_up: Maybe yes, maybe no.

 

Part of Michael Coates post read:

 

"Suffice to say that flutter relates to true airspeed (TAS) rather than equivalent air-

 

speed (EAS), so aircraft that are operated at or beyond their VNE at altitude - where

 

TAS increases for a given EAS – are more susceptible to flutter..."

 

New Zealand CAA’ Vector Magazine (full passage at page 5 of http://www.caa.govt.nz/fulltext/vector/vec01-4.pdf)

 

"The critical flutter speed depends on TAS, air density, and critical mach number. The air

 

density factor is almost canceled out by the TAS factor; and most of us won’t fly fast

 

enough for mach number to be a factor. So TAS is what a pilot must be aware of!"

 

Bob Cook, Flight Safety International

 

Unfortunately this link did not lead to the article refered to.

Youngmic wrote:

 

"VD/MD are IAS values, VD/MD can be no greater than 0.9 VNE"

I can't find that written anywhere either, apart from the RA-Aus article referred to below. Certainly not in FAR 23.251 which is accurately shown in youngmic's post. :confused:

 

In our aircraft, at cruise, our TAS is often getting up towards Vne. A quick calculation of cruise speed 130kts (at sea level ) plus 2% per 1000' (compounding?) gives 161.64 kts at 10,000'. Vne is 162kts. Now we drop the nose a fraction on descent and we are over Vne. Add some high temperature and turbulence and ??

 

Now having said all this my thoughts are with TAS as the safe way to look at Vne.

 

But, there's more. :big_grin: (time for a coffee break before the best part, still to come) :big_grin:

 

FAR.19 Operating limitations. The Airplane Flight Manual must include the

following limitations--

 

(a) Airspeed limitations. (1) The maximum operating limit speed VMO/MMO and

 

a statement that this speed limit may not be deliberately exceeded in any

 

regime of flight (climb, cruise, or descent) unless a higher speed is

 

authorized for flight test or pilot training;

 

(2) If an airspeed limitation is based upon compressibility effects, a

 

statement to this effect and information as to any symptoms, the probable

 

behavior of the airplane, and the recommended recovery procedures; and

 

(3) The airspeed limits, shown in terms of VMO/MMO instead of VNO and VNE.

and (just the relevent bit):

 

Sec. 23.1505 Airspeed limitations.

© .... VMO/MMO must be established so that it is not greater than the

 

design cruising speed VC/MC and so that it is sufficiently below VD/MD and

 

the maximum speed shown under Sec. 23.251 to make it highly improbable that

 

the latter speeds will be inadvertently exceeded in operations.

Now, because it would be easy to inadvertently exceed Vne based on TAS, especially if you did not have an instrument to show TAS, and if your aircraft manual should show VMO/MMO instead of VNO and VNE if airspeed limitation is based upon compressibility effects and your aircraft manual actually shows Vne, then Vne should be based on IAS!! Youngmic and the RA-AUS article below have it right. :thumb_up:

 

Don't fly real fast

 

Vne is based on IAS unless otherwise stated. I rest my case.

 

 

[turboplanner]

Ian, I used to do what you did, nose down back to the airfield = less hours until I read "Stick and Rudder" by Langewiesche - he more or less covers this subject.

 

 

[K-man]

Ian, I used to do what you did, nose down back to the airfield = less hours until I read "Stick and Rudder" by Langewiesche - he more or less covers this subject.

and says .... ?

 

 

[youngmic]

K-man,

 

Just to clarify, the statement that VD/MD is an IAS value is my statement, but I'll see if I can't find further written support of it. As far as I am aware all the placarded V speeds relate to IAS, exluding the V mach speeds of course.

 

I stated so based on my professional knowledge, but I could be wrong it wouldn't be the first time.

 

VNE certainly is an IAS value that's why it is on the ASI.

 

Where it is established during dive and flutter testing that at altitudes within the aircrafts capability flutter becomes apparent then VNE is set accordingly.

 

If it is the case that this gives an artificially low value for lower levels then the manufacturer has the option of using placard VNE speeds for various altitudes, typically above 10,000'.

 

This is how many sailplanes get around the problem and it is common to see a number of VNEs placarded on the ASI for various altitudes. Or the barber pole method is another way.

 

It was pointed out that carbon fibre structures have a proclivety for flutter rather than a static failure mode.

 

The maximum structural speed is linked to IAS. But light planes, manufactured of carbon reinforced plastics, with long, slick wings are more prone to flutter at high speeds than to structural failure.

 

This is true but probably could use a little more explanation. 

 

High aspect ratio wings are prone to flutter is better.

 

One of the problems with any FRP (fibre reinforced plastic) is whilst they're very strong in tensile strenth, in fact carbon fibre is about 5 x stronger than steel, they're week in torsional strength.

 

This creates a build problem resulting in extra material being used in the layup to accomodate for this weekness, you then end up with wings that are very strong statically but just meet the torsional rigidity requirements. This is where flutter creeps in. In reality carbon is pretty good as it is very stiff weight for weight compared to fibre glass, a long thin fibre glass wing is a quite heavy wing due to torsional requirements. It is not unusual to see a FRP wing with a 2.5 x limit load factor where as other materials (alloy typically) are at 1.5 x limit load. ie. 4g wing wont break till after 6g.

 

The demise of the Beech Starship can be linked to this, the FAA wanted to be sure the thing was going to be strong enough so it was built considerably heavier than it needed to be. These were the earlier days of commercial FRP structures and weight was the penalty and performance was ordinary. They ended up building a wierd looking under achiever.

 

Consider the world of a minimum weight high performance high aspect ratio FRP built recreational aircraft flown by a recrereational pilot with basic training and there is certainly some alignment of the swiss cheese occuring.

 

Before you get to exited about flutter speeds you need to realise that at altitude (9500 for eg.) you are not going to acheive anywhere near your 130 IAS sea level cruise unless turbo'd so a natural buffer sets up. Also the 2% is a handy rule of thumb but your GPS function or whiz wheel is better.

 

OAT @ 10,000 -10c IAS 110 kts = 126 TAS (130 IAS = 149 TAS)

 

OAT @ 10,000 +25c IAS 110 kts = 133 TAS (130 IAS = 158 TAS)

 

At the end of the day yes flutter is related to TAS but this should be dealt with within the VNE, VMO IAS values by the manufacturer.

 

 

[K-man]

Pat on the back for all those who put some great information on this thread. I reckon that this has been a really good discussion. Thanks guys.

 

 

[turboplanner]

K-Man:had a look for the quote, couldn't find it, but how's this:

 

"But when the pilot is already flying at near top speed, where induced drag has almost disappeared and parasite drag is the only important type of drag, and if he then tries to fly a little faster, he does get the full working of the cube law straight in the neck"

 

 

[K-man]

Ouch! :thumb_up:

 

 

[Ewen McPhee]

Great thread

 

I found google has some of Langewiesche's Book Stick and Rudder book on line as a preview

 

Stick and Rudder: An Explanation of ... - Google Book Search

 

 

[turboplanner]

Stick and Rudder

 

Available from Amazon.com: Online Shopping for Electronics, Apparel, Computers, Books, DVDs & more

 

New US$21.56

 

76 Used and New from US$7.60

 

Brilliant read, hard going, but you'll never do low swoops and wingovers at low level after reading it

 

 

[djpacro]

The airspeeds VD, VNE etc per FAR 23 are defined in EAS - pilots are provided info in the manual and placards in IAS (forget CAS & M for this discussion). The designer must demonstrate freedom from flutter to 25% (from memory) above flight demonstrated VD. So, he must demonstrate that taking altitude into account. If the manual/placards do not show a reduction in VNE (in IAS) with altitude then the designer would've demonstrated compliance at that VDF (EAS) at max altitude (typically allowing for max level speed at max altitude then diving to test airspeeds as VDF can only be achieved at an alititude lower than max achievable - worth remembering if horsepower is significantly increased in later models).

 

If you have a well maintained FAR 23 factory built aeroplane certified by a regulatory authority then you can be pretty confident that flutter will not occur within IAS values per the manual. (I'm unable to comment on aeroplanes that have been self-certified by the manufacturer to other than FAR 23)

 

PS - flutter (especially of ailerons) can also occur at low speeds i.e. around cruise speed and lower - several cases of this discovered during initial flight testing of prototypes.