Vspeeds, IAS, and TAS

[Ada Elle]

So when we were all very young, we learned that Vs0 is expressed in terms of IAS, as are most Vspeeds, except for Vne which is a TAS.

 

The question arises, is Vfe like Vne, and thus should it be expressed as a TAS?

 

If Vfe is a TAS, and Vs0 is IAS, is there a situation where the 1.3Vs leads to an approach speed greater than Vfe, and so you cannot extend flaps? (hot, high, heavy)

 

In that situation, should you approach at 1.3Vs1 and redo your runway required calculations for that approach speed?

 

 

[dazza 38]

You're not serious are you ?

 

 

[facthunter]

The only concern I have commonly encountered in some U/L's is flap extension speed being low so in turbulence you have to be under your recommended turbulence speed to extend flaps and stay within the limitations. Nev

 

 

[poteroo]

So when we were all very young, we learned that Vs0 is expressed in terms of IAS, as are most Vspeeds, except for Vne which is a TAS.The question arises, is Vfe like Vne, and thus should it be expressed as a TAS?

 

If Vfe is a TAS, and Vs0 is IAS, is there a situation where the 1.3Vs leads to an approach speed greater than Vfe, and so you cannot extend flaps? (hot, high, heavy)

 

In that situation, should you approach at 1.3Vs1 and redo your runway required calculations for that approach speed?

Vfe should be IAS because it is a structural limitation on the airframe. Flaps are intended for lift augmentation and better visibility at lower IAS - not as speed brakes.

 

Most POH contain wing loading limitations. For most LSA and GA aircraft - LF as below:

 

Nil flap + 3.8 to -1.9g

 

Flap full +1.9 to -1.0g

 

In other words, when you extend full flap, your aircraft has its' airframe effectively weakened - resulting in the necessity to fly at a lower IAS in order to keep Va valid.

 

Va = Sq rt LF x Vs, so for nil flap your Va in a typical LSA would be 1.95 x 45 = 90 approx. (some manufacturers have quite unbelievable Va's quoted and clearly haven't used this formulae)

 

but, when you use full flap, with say Vso 0f 40, then your Va becomes much, much less and so you need to keep your IAS much lower than the Va quoted in your POH (in turbulence anyway).

 

BTW, the hot,high and heavy would rarely apply in Australia. Very few locations at more that 4500 amsl. In PNG, with a number of locations above 7000 amsl, and the usual correction of +3000 ft ....you could be dealing with a TAS/IAS difference of from 10-20 kts. Your approach speed in a twin at 85 IAS feels alarmingly fast - so in those cases it is a consideration, (although we always flew on IAS - the PNG 'P' charts took care of the rest).

 

happy days,

 

 

[djpacro]

So when we were all very young ....... except for Vne which is a TAS.

Nope.

[facthunter]

IAS self corrects for a lot of things. Density altitude being one.. Your indicated stall speed is the same,( though there may still be factors like position error happening)

 

Flaps extended= weakened structure so in turbulence you have some possibilities to choose from. You must also consider what your climb performance might be like initially before you can safely retract flap, if you abort the landing. Sometimes without retracting at least some of the flap you aren't going anywhere as far as climb performance is concerned and you may also have a large trim change to cope with.

 

You may choose to approach with LESS flap then full flap for these reasons, so a bit of height and density altitude and less flap will make you think you are going too fast. ( Similar to landing downwind if you haven't done it before.) You ARE actually going faster and will need more runway, but you need the airspeed too.Nev

 

 

[djpacro]

Va = Sq rt LF x Vs, so for nil flap your Va in a typical LSA would be 1.95 x 45 = 90 approx. (some manufacturers have quite unbelievable Va's quoted and clearly haven't used this formulae)

Interesting.I've seen people do a simple calculation like that incorrectly e.g. use figures in IAS rather than EAS.

The airworthiness requirements generally specify that as a minimum value of Va. i.e. the designer is free to choose a higher Va however it then could be difficult to comply with structural loading requirements.

 

I've seen at least one aircraft where it seems to me that the structural loadings required at Vd (a margin above Vne) were ignored.

 

I guess that is what can happen with self-certification especially where those signing the certification themselves do not have any responsibility to or delegation from the airworthiness authority and are being paid by the person wanting the certification. That's OK, the market will sort itself out (eventually) when people see what they are getting or not getting for their money.

 

 

[kaz3g]

The only concern I have commonly encountered in some U/L's is flap extension speed being low so in turbulence you have to be under your recommended turbulence speed to extend flaps and stay within the limitations. Nev

My Vfe in the J5 is 58 knots but, as you know, using the flaps is like opening the barn doors.

 

Kaz

 

 

[dazza 38]

My Vfe in the J5 is 58 knots but, as you know, using the flaps is like opening the barn doors.Kaz

VFE on the Tecnam Golf isnt much higher at 60 knots. It would be better if it was around 70 knots.

 

 

[dutchroll]

The question arises, is Vfe like Vne, and thus should it be expressed as a TAS?

No.

Reason: flap and gear limiting speeds are determined by air loads on those structures. Air loads on the airframe are dynamic air pressure related. which is what IAS is measuring.

 

Vne is a different beast. There are many factors engineers consider when calculating Vne. Air loads is one of them, plus flutter calculations (which are TAS related), and others. Vne is expressed as an IAS (conservatively) simply because IAS is how we fly. Go into bigger planes which fly much higher however and you get more complicated pictures of Vne which result in a max speed "envelope". For small planes < 10,000ft, a single Vne expressed as a conservative IAS is close enough and good enough.

 

 

[facthunter]

RV people have comprehensive information on this topic. I don't have the link but worth reading. Nev

 

 

[ben87r]

https://www.vansaircraft.com/pdf/hp_limts.pdf

[Yenn]

Lost me, what is EAS?

 

 

[dazza 38]

Lost me, what is EAS?

Equivalent airspeed

 

 

[robinsm]

Just look up the pilot op handbook for your aircraft, spells it all out right there..

 

 

[Ada Elle]

Nope.

Having read the RV article, I now know that it's meant to be expressed as an IAS, but is often a TAS (ie derated for density altitude). Pipistrel (who make the aircraft I fly most often) have a table of Vne (as IAS) vs altitude that makes it pretty clear that it's a TAS.

 

My question then becomes: on a plane with full span flaperons, what limits Vfe? The flaperon structure can obviously take full deflection at up to Va, so it shouldn't be air loads on the flaps.

 

Also, why isn't flutter a consideration in Vfe, but it is in Vne?

 

(my understanding is that IAS reflects a pressure, and TAS reflects a kinetic energy. so for energy based calculations such as altitude gained in a zoom climb, TAS gives more accurate results. but I don't know what actually limits Vfe)

 

 

[dazza 38]

Having read the RV article, I now know that it's meant to be expressed as an IAS, but is often a TAS (ie derated for density altitude). Pipistrel (who make the aircraft I fly most often) have a table of Vne (as IAS) vs altitude that makes it pretty clear that it's a TAS.My question then becomes: on a plane with full span flaperons, what limits Vfe? The flaperon structure can obviously take full deflection at up to Va, so it shouldn't be air loads on the flaps.

 

Also, why isn't flutter a consideration in Vfe, but it is in Vne?

 

(my understanding is that IAS reflects a pressure, and TAS reflects a kinetic energy. so for energy based calculations such as altitude gained in a zoom climb, TAS gives more accurate results. but I don't know what actually limits Vfe)

So when you're on base, do does this stuff go through your mind ? If so, I suggest you give flying away and take up knitting.

 

 

[Happyflyer]

Come on guys, no need for pesonal attacks.

[dazza 38]

Fair enough Happy flyer, sorry Ada, if I have said something to upset you. You are obviously very intelligent. Imo some of the questions you ask are a lot more in dept than what is needed in general light aircraft flying wether GA or RAA. There is nothing wrong with knowing the ends and outs of everything.

No need to get too anal about the nitty gritty. But each to their own.

 

Have you ever thought of being a test pilot ? Your thirst for knowledge fits that occupation.

 

Where as me, well I am just a red neck Queenslander with too many guns. Lol

 

 

[dazza 38]

Fair enough Happy flyer, sorry Ada, if I have said something to upset you. You are obviously very intelligent. Imo some of the questions you ask are a lot more in dept than what is needed in general light aircraft flying wether GA or RAA. There is nothing wrong with knowing the ends and outs of everything.No need to get too anal about the nitty gritty. But each to their own.

Have you ever thought of being a test pilot ? Your thirst for knowledge fits that occupation.

 

Where as me, well I am just a red neck Queenslander with too many guns. Lol

And remember, just fly the plane. I cant stress that enough. If/ when things ever get pair shaped, forget radio calls, forget navigation, just fly the plane. So many people are dead because they got flustered and forgot the basic rule of flying the aircraft when in a spot of bother. They made things too complicated.

 

 

[K-man]

Having read the RV article, I now know that it's meant to be expressed as an IAS, but is often a TAS (ie derated for density altitude). Pipistrel (who make the aircraft I fly most often) have a table of Vne (as IAS) vs altitude that makes it pretty clear that it's a TAS.My question then becomes: on a plane with full span flaperons, what limits Vfe? The flaperon structure can obviously take full deflection at up to Va, so it shouldn't be air loads on the flaps.

 

Also, why isn't flutter a consideration in Vfe, but it is in Vne?

 

(my understanding is that IAS reflects a pressure, and TAS reflects a kinetic energy. so for energy based calculations such as altitude gained in a zoom climb, TAS gives more accurate results. but I don't know what actually limits Vfe)

Pipistrel is one of the few aircraft which use TAS with Vne. As a result they have to display the table you refer to. I actually did a lot of research into this topic some years back because, like you, I thought using TAS was logical. In fact some years ago a Pipistrel aircraft came apart in flight due to inadvertently exceeding its VNE hence the placarding.

Most recreational aircraft don't calculate TAS unless they have an EFIS and an outside temperature probe. In the case of my aircraft when I have a TAS of 135 I generally will have an IAS of about 120, very rough approximation as obviously it varies but suffice to say maybe 10% to 12% difference.

 

When I looked up how Vne is calculated I read something to the effect that it is arrived at by either flying at the speed where flutter is experienced and taking 15% off that speed to obtain Vne, or a speed that had been safely demonstrated in flight and taking 15% from that speed. Other articles I've read say '10% or more'.

 

There are many interesting articles available if you do a quick search. Here is one from Australian Flying that explains it nicely. The author here refers to taking off 10%. I can't find the original article I read referring to 15%.

 

http://www.australianflying.com.au/news/vne-and-flutter-explained

 

 

[djpacro]

From memory, generally, Vne is 90% of demonstrated design dive speed. Flutter speed to be proven to be above demonstrated Vd by a further margin - so, with Vd specified in EAS then it has to be shown at high altitude to get the highest TAS. Flutter may not be critical, could be tailplane balancing/manoeuvre or gust loads or wing gust loads etc. Flutter can also appear well below Vd yet have no flutter approaching Vne.

 

Best not to assume what is limiting.

 

I'd trust the numbers and the inherent margins in the flight manual of an airplane certified to FAR 23. One or two others I have seen to be wishful thinking wrt demonstrated margins.

 

 

[dutchroll]

My question then becomes: on a plane with full span flaperons, what limits Vfe?

IAS.

 

The flaperon structure can obviously take full deflection at up to Va, so it shouldn't be air loads on the flaps.

I think you're confusing the effects of the same structure when it's acting and moving in two quite different ways.

Yes it's the same physical piece of metal but it's not being moved in the same way (even the basic control surface deflection is different depending on what you're doing with it) and it's not putting the same critical stresses onto the same area. Asymmetric movement like ailerons puts large torsional stresses on the wing. Just extending them symmetrically into the airflow and leaving them there subjects them to a continuous air load.

 

Also, why isn't flutter a consideration in Vfe, but it is in Vne?

Because flutter is rarely a phenomenon experienced at the lower speeds which flap extension is normally limited by. I say "rarely". There have actually been cases of flaperon flutter being highly significant. The Nomad was one of them.

Don't forget too, that "flutter" when you're talking about Vne is not "flight control surface flutter". It's any flutter. It's an aeroelastic oscillation induced in any part of the aircraft. Could be the entire wing (and often is). Or the fuselage, or anything.

 

 

[facthunter]

Looseness in control mechanisms can cause flutter also, so you will/may get it when the aircraft has seen some service and also if a trim tab loses it's linkage or the mass balance is affected by repair/ repaint. The L-188 Electra lost 3 wings before they realised the changed alignment of the Allison engines due engine mount sag combined with another pre-existing design fault, caused the wing to depart in a fraction of a second, after a few sharp oscillations.. You may not get time to do much, so better to avoid going there.Nev

 

 

[Yenn]

I have found EAS. It is CAS corrected for compressability error. I hadn't realised how fast your modern ultralights are going.

 

No doubt if you want TAS while you go transonic you will need EAS.