Va - Manoeuvring speed? ... what manoeuvring speed?

[facthunter]

For the aircraft to change it's trajectory? there has to be a force applied (by the wings) to accelerate the plane upwards. If it doesn't change it's path you won't experience any acceleration because it just carries on as it was. It's almost a question of what comes first. The extra lift or the acceleration. If the plane is very light it will change direction faster.( Describe a smaller radius) and you being part of it, will experience more "G" force. It is similar to a heavy high wing loaded plane flying smoother in turbulence than a light, low wing loaded one which REACTS more to applied forces.

 

 

[motzartmerv]

Thank you Nev. And your post brings the second part of the equation into it. We arent simply talking about control inputs. We are also talking about turbulence etc.

 

ANYTHING that causes the aeroplane to accelerate in ANY direction, if too quickly, will have potential to cause problems. Not just the controls.

 

A heavy object will resist acceleration more then a light object. Very very simple physics. And the reason as I get older (and increase mass) I resist change in both acceleration and 'the world' ...lol

 

 

[Head in the clouds]

A bit circular there HIC. Does the elevator apply a force or not? In one sentence you stated it doesnt and then it does. (not a great instructional technique) I would simply say that yes it certainly does apply a force.I was attempting to keep the problem simple to clarify it. If we are looking for students to understand the issue then the KISS principle should be applied.

How quickly the aeroplane ACTUALLY responds to AOA increase is the exact issue we are trying to understand.

 

The load factor will not increase until the aeroplane describes a new direction, so there for no, the wings will not tear off UNTIL the new direction is attained. Which is the whole point behind understanding inertia..

 

The lighter plane WILL respond to change much faster due to its lack of inertia and there for the LOAD factor WILL increase to critical levels. ..

No, I didn't state it doesn't and then it does at all. You stated that the elevator in one instance doesn't apply enough force to cause a change in direction, I pointed out it never causes a change in direction, it causes a change in ATTITUDE and the WING causes the change in direction. I haven't changed my stance or wording on that, try re-reading what I wrote ... the second time I said the ROTATION caused by the elevator, which is nothing to do with a change in direction. Of course the elevator applies a force, I never said it did or didn't, there wouldn't be much point in having one if it didn't would there? But the force it produces doesn't EVER change the direction of the plane, just the attitude of it.

 

If we can put that to bed and perhaps stop trying to ridicule me, you might get around to the correct answer about why Va gets lower as weight decreases. Hint - contrary to what you stated much earlier, it has a lot to do with the stall ... and your heavily loaded plane example is a really good way to envisage the varying conditions that affect the Va speed.

 

You are right, by the way, that the rate at which the plane reacts is the issue, but its not anything to do with the plane attaining a new direction, if it could attain the new direction the load would have reduced again by that stage so you would have survived the incident. It has to do with the plane TRYING to attain a new direction but not actually changing direction at all, so the wings do then tear off. Which is why the issue is so confusing to so many because, since that is the case, then you would think that a plane with more momentum (call it inertia if you like) would resist the change more than a plane with less momentum (i.e. a lightly loaded one). But - another hint - the heavier plane does resist the 'acceleration' but the wings don't tear off - so - why don't they tear off?

 

Just read FH's post, and yes, what he's said is true i.e. also contrary to what you said earlier the wings don't get ripped off by the acceleration(s), they get ripped off by the force trying to accelerate the craft but the craft actually not being accelerated.

 

But FH's correct post still doesn't explain why the heavy plane which is "battering its way through the turbulence" and providing a smooth ride for the occupants, doesn't have its wings torn off during all that battering, the wings must still be copping all that load mustn't they? Or are they? Under what circumstances could those wings not reach the limit load? (another hint).

 

 

[facthunter]

The wings have to take the load. IF they are JUST strong enough to begin with as soon as the angle of attack is increased they will break off,( in our hypothetical aeroplane) It's the change of angle that produces the LIFT force that stresses the wing structure. A speed increase will too but more gradually and with other forces added to the equation. there even has to be a consideration of the downforce on the tail to rotate the plane to the new attitude. It's considered to rotate about the Cof G but that's not quite so, either. Nev

 

 

[motzartmerv]

HIC Mate. Im not going to make this personal. If you have something to add then i respectfully ask you get to the point. I can sit here picking holes in your statements too, all day, but its not helping clear the matter up is it? Ive offered a point of view, what exactly is your point of view?

 

No. The wings are not subject to the same loads at all. Without a change there is no increase in load. Again, and im getting blue in the face here, without change there is no increase in load. No change in AOFA no load increase. No change in trajectory (thanx nev) no load increase.

 

Ive never seen a "trying" to attain new direction in any formula.

 

The physics relating to this problem can be summed up in two basic (high school ) formulas.

 

p=mv

 

f=ma

 

The difference between the two, but more so the relationship between the two explains this phenomena very well, and one of them answers your last question.

 

 

[Guest Nobody]

At the beginning of the thread I asked what succinct descriptions people used for the reason for varying Va in layman's terms without graphs and formula but so far we've only had graphs, charts and mention of inertia and accelerations.

Sorry I am an engineer and am happy to think in graphs and formulas, its how I understand things. I will try to explain in words below.

 

How does the inertia (momentum?) affect the airframe and help it to cope with the 'acceleration' at higher weight?

Inertia has no effect on Va or as part of the explanation of why Va increases with increased weight. The FAA definition of Va given in my earlier post is "the maximum speed at which the limit load can be imposed (either by gusts or full deflection of the control surfaces) without causing structural damage". What this definition implies is that when traveling at less than Va something else happens before the structural limit is exceeded. For the vertical tail surfaces that could be that the control surface stop is reached ie full rudder deflection. (see the extract from FAR 23 I posted above). Sizing the elevator so that it reaches the stop at Va before the load limit is reached will result in too little authority to flare during landing. The "something else" that happens before the aircraft reaches its load limit is that the aircraft stalls. Think of stalling a bit like a fuse that limits the load that can be generated.

 

Consider two aircraft equal in every respect except that one is full of fuel and one is getting close to its reserves. They are flying along together. Both Pilots pull abruptly back on the stick until the G meter in the cockpit reads 2g. The wing on the heavier aircraft will have to produce greater lift to generate the 2g acceleration and so will have to be working at a higher angle of attack. This is true no matter if is 1g, 2g or 3g or whatever, A giher weight requires a greater angle or attack. Now consider that they are traveling at the Va speed for the light aircraft and pull back as hard as they can so that the elevator hists the stop. The light aircraft will just reach its load limit and be at the point of stall, its angle of attack is at the stalling limit for the wing. The heavier aircraft will have stalled before it gets to the load limit because for every accelertion applied to both aircraft it has a higher angle of attack. If it were traveling faster it would be able to get to the load limit before stalling.

 

Where inertia does come into the situation is that increased mass (and inertia) improves the ride for the occupants in turbulence. Consider the two identical (except for weight) aircraft again. They are flying along next to one another when they encounter a vertical gust. Because they are the same aerodynamically they generate the same aerodynamic forces on the airframe due to the gust. The lighter aircraft responds with a greater acceleration than the heavier one. The occupants feel a more comfortable ride. Consider now two aircraft of identical weight but one has a smaller wing area. When they fly into a gust the smaller wing results in less aerodynamic load, less load results in less gust acceleration and therefore a more comfortable ride. In general a higher wing loading is more comfortable in turbulance.

 

 

[Guest Nobody]

To explain it, imagine an aeroplane that is massively massively heavy. Hundreds of times over its MTOW. Ignore the obvious problems with this and just think about whats going to happen when the stick is suddenly pulled back. If the aeroplane is super super heavy, its inertia will cause it to continue on a straight line ( as defined by newtons first law) until its acted upon by an unbalanced force. So to cause a change in its direction (acceleration) a force proportional to its massive weight needs to be applied to it. Now, the elevator is designed to cause a change to the aeroplane AT A SPECIFIC AND DEFINED MASS. If this aeroplane is 100 times heavier then this defined mass, the elevator will NOT apply enough force to cause a change in its direction (acceleration) and it will continue along the straight line with NO INCREASE TO THE LOAD FACTOR. Its the CHANGE in direction, acceleration, that causes the load increase.

It isn’t the force of the elevator that causes the upwards acceleration. When you pull back on the stick the aircraft accelerates upwards. The force on the horizontal stabilizer is downwards due to the elevator moving up. Something else must be generating an upward force.

 

The downward force at the tail rotates the wind to a higher angle of attack and this generates the upwards net force and the corresponding upwards acceleration.

 

 

[motzartmerv]

Yes. I get that. Thank you. But seeing as the entire problem relates to controls and their use, I was discussing its use in that particular situation.

 

The elevator does provide a turning moment about the lateral axis, if you want to be picky..

 

 

[Head in the clouds]

HIC Mate. Im not going to make this personal. If you have something to add then i respectfully ask you get to the point. I can sit here picking holes in your statements too, all day, but its not helping clear the matter up is it? Ive offered a point of view, what exactly is your point of view?No. The wings are not subject to the same loads at all. Without a change there is no increase in load. Again, and im getting blue in the face here, without change there is no increase in load. No change in AOFA no load increase. No change in trajectory (thanx nev) no load increase.

Ive never seen a "trying" to attain new direction in any formula.

 

The physics relating to this problem can be summed up in two basic (high school ) formulas.

 

p=mv

 

f=ma

 

The difference between the two, but more so the relationship between the two explains this phenomena very well, and one of them answers your last question.

Well it's clear that students aren't provided with any better understanding about Va these days, than we were when I first did my training. If I was a student now and having read this thread I don't think I'd know anything more than it'd be advisable to stay below the yellow arc when manoeuvring or in anything but still air.

 

The following is similar to an explanation I came across some years ago and which doesn't contain any formula or graphs but which did provide me with a means of understanding what the formula defines. I concede that it may not be helpful to everyone though.

 

I'll use something like a SuperCub for our example, since it is similar enough to the type of planes we fly, their design limit load and their speeds. Let's say our Cub is designed for a limit load, like many Cessna models, of +3.8g and -1.5g. Let's also dispel a popular myth before we go too far - most people would say that this plane is twice as strong in positive g as it is in negative g, but from a starting point of level flight at 1g it's about the same either way, given that it's intended to be flown upright. In stable flight it is experiencing 1g so it has a strength reserve of 2.8g until it yields/begins to fail in positive loading, and it has a strength reserve of 1g to get to zero g and then another 1.5g to get to -1.5g, so it has to experience a change of 2.5g until it yields in the negative sense.

 

Anyway - so to demonstrate what's going on with the Va thing let's take our plane up to Va speed and at MTOW. At that speed and weight we will be flying at an AoA of, for discussion's sake, 9*, which means we can increase the AoA by another 6*before we reach the critical angle and stall the wing. Va, the design manoeuvring speed, is established on this basis, its the speed at which, if the wing's AoA is suddenly increased to the critical angle, where its Cl is greatest, it still can't generate enough extra lift to exceed the wing's limit load, and any greater angle of attack will cause the wing to stall.

 

I have just seen Nobody's excellent description, so there's no need for me to go into such depth with this, as I would have otherwise, but I'll just finish the first example of the 'fully loaded' airframe. As we're now aware, the stall does in fact have everything to do with Va, and this part demonstrates it quite well I thought.

 

So - our plane is flying along at 9* AoA and we meet up with an inflight refuelling tanker, for discussion's sake, and start filling our huge ferry tanks (which are located on the CG), we keep filling our tanks and flying at the same speed. To maintain height we have to keep increasing the AoA, and to maintain speed we keep increasing our power setting. Eventually our tanks are full of fuel and the aircraft is close to double the permitted MTOW, but she's just flying i.e. the wing is flying right at the critical angle, just before stalling. Due to the weight being twice what it was designed to be, the wing is loaded at the same as it would be if it was pulling 2g at the normal MTOW, so the wing has a margin of just 1.8g until it breaks BUT - if you pull back on the stick or hit a gust or some rising air the critical angle of the wing will instantly be exceeded and the wing will stall. That is why it won't be overloaded and break, nothing to do with inertia or momentum ...

 

At a much lighter load, say 2/3 MTOW but still flying at the Va that should be used for MTOW, the wing will only be flying at about 4* AoA and heavy use of the elevator or a gust could increase the AoA to such an extent that the wing's Cl increases sufficiently to exceed the wing's limit load whilst still not reaching the stall, and hence the wing breaks off without the plane ever having had time to change its flightpath. So an actual reaction to acceleration forces i.e. change of direction, is not necessary to break the wing, just the instantaneous load increase will do it.

 

I hope I've described it clear enough to help a student sometime. And thanks for yours Nobody!

 

 

[motzartmerv]

Thats good HIC. Thanks. I see what your saying, now that you've actually said it. Thats an excellent description.

 

I still maintain that being lighter, the aeroplane will resist change in ALL directions, AND, Attitudes much less then it will at higher weights. That includes increasing AofA. VA is an issue for ALL control inputs, not just the elevator.

 

How can we explain these limitations.?

 

 

[turboplanner]

Guys there is a lot of passion flowing here, and I have great respect for all of you in this difficult issue, so what i would like to see is you continue to hammer it out until you get to an understandable conclusion for students, then make sure it is memorialised and sent to RAA for circulation with all getting the credit.

 

 

[Dafydd Llewellyn]

From FAR 23.335:

 

© Design maneuvering speed VA. For VA, the following applies:

 

 

 

(1) VA may not be less than VS√ n where—

 

 

 

(i) VS is a computed stalling speed with flaps retracted at the design weight, normally based on the maximum airplane normal force coefficients, CNA; and

 

 

 

(ii) n is the limit maneuvering load factor used in design

 

 

 

(2) The value of VA need not exceed the value of VC used in design.

 

(Note: Vc is the speed at which the aircraft is designed to meet the 50 ft/sec gust requirements. Some "simplified" recreational aircraft standards do not use it.

 

and

 

Horizontal Stabilizing and Balancing Surfaces

 

 

 

 

 

§ 23.421 Balancing loads.

 

 

 

 

 

(a) A horizontal surface balancing load is a load necessary to maintain equilibrium in any specified flight condition with no pitching acceleration.

 

 

 

(b) Horizontal balancing surfaces must be designed for the balancing loads occurring at any point on the limit maneuvering envelope and in the flap conditions specified in §23.345.

 

 

 

[Doc. No. 4080, 29 FR 17955, Dec. 18, 1964, as amended by Amdt. 23–7, 34 FR 13089, Aug. 13, 1969; Amdt. 23–42, 56 FR 352, Jan. 3, 1991]

 

 

 

 

 

§ 23.423 Maneuvering loads.

 

 

 

 

 

Each horizontal surface and its supporting structure, and the main wing of a canard or tandem wing configuration, if that surface has pitch control, must be designed for the maneuvering loads imposed by the following conditions:

 

 

 

(a) A sudden movement of the pitching control, at the speed VA, to the maximum aft movement, and the maximum forward movement, as limited by the control stops, or pilot effort, whichever is critical.

 

(b) A sudden aft movement of the pitching control at speeds above VA, followed by a forward movement of the pitching control resulting in the following combinations of normal and angular acceleration:

 

------------------------------------------------------------------------

 

Normal

 

Condition acceleration Angular acceleration

 

(n) (radian/sec2)

 

------------------------------------------------------------------------

 

Nose-up pitching............... 1.0 +39nm÷Vx(nm-1.5)

 

Nose-down pitching............. nm -39nm÷Vx(nm-1.5)

 

------------------------------------------------------------------------

 

So Va is a number of things: Firstly, it's the highest speed at which the wing can be pulled to stalling incidence, in longitudinal equilibrium (i.e. by steady application of the controls) without exceeding the limit load. Note that the wings will be carrying n times the aircraft weight PLUS whatever download is necessary at the tailplane to hold that flight condition. Secondly, it's the highest speed at which any ONE control surface may be abruptly deflected to its stops (momentarily) without overloading the control surface or its support structure or the control system.

 

However, it is still possible to overload the airframe at Va if the aircraft encounters a gust load at the same time as the manoeuvre load; and also by deflecting more than a single set of controls simultaneously. Further, the aircraft has to be strong enough to withstand this load only ONCE in its life.

 

 

[Head in the clouds]

Guys there is a lot of passion flowing here, and I have great respect for all of you in this difficult issue, so what i would like to see is you continue to hammer it out until you get to an understandable conclusion for students, then make sure it is memorialised and sent to RAA for circulation with all getting the credit.

Thanks turbs, but that's pretty much all there is to it, for me the example allowed me to apply a little thinking and then easily understand and apply the formula which is well enough defined in the article on Va in that dubious fount of all knowledge - Wikipedia. If anyone hasn't read the articles linked to earlier in this thread, they'd do well to do so, they're all very informative.

 

Unfortunately though, even having a good understanding of Va at various weights, and the reasons for it, isn't, on its own, going to save a great many lives. A few perhaps, but the vast majority of inflight structural failures don't come from people flying at Va at lower weights than MTOW and then hauling the controls around or hitting mega wind gusts.

 

Even planes only rated for +3.8 and -1.5 are actually a lot stronger than most of us would want to put to the test. I recall an incident during my GA conversion, I had an ex RAAF instructor who was renowned for being tough as nails. We'd just departed Coolangatta (now Gold Coast International) and there was quite a strong Westerly blowing which is well known for producing very strong rotors off the Beechmont escarpment about 10-12 miles inland to the West. We'd just levelled off and reached cruise speed when we hit the most almighty turbulence I've ever experienced, we both cut both of our legs under the panel and had bloodied heads as well although we had the belts done up very firmly.

 

The instructor started yelling on the radio about severe turbulence and 'Required' (not requested) an immediate return to the airport, a clearance which we promptly received. He checked the plane into the workshops for an inspection but of course nothing amiss with the airframe was found. We both had painful backs from the incident and I was still hobbling so went to my bone cruncher (chiropractor) for an adjustment the next day.

 

On that occasion we would have been way over Va for the weight and had a strong gust that probably did take the AoA all the way to a momentary stall but the airframe still survived undamaged. There is a moderate amount of 'margin' built into all limits.

 

The real problem with inflight structural break-ups doesn't come from instances such as we experienced, statistics indicate it comes from inadvertently exceeding Va in a spiral dive having become spatially disoriented, which would usually be due to loss of horizon reference i.e. inadvertent flight into IMC. An attempted rapid pull-out from the high speed dive and/or not levelling the wings first, seems to be the culprit.

 

But as for how we prevent people inadvertently entering IMC I really don't know. I've done it twice and very nearly paid the price each time. Very sobering stuff I can tell you. I'll write up the incidents if anyone thinks someone else might learn from my errors.

 

 

[DWF]

A very interesting thread and topic.

 

 

 

A couple of comments from a practical point of view....

 

 

 

Although it is usually listed in the aircraft flight manual, to the best of my recollection, Va was not marked on the airspeed indicators or placarded in the cockpit of any aircraft I have flown (and there have been quite a few).

 

The yellow arc on airspeed indicators starts at Vc or Vno - Maximum Structural Cruise Speed.

 

Va is the Manoeuvering Speed - and is usually significantly lower than Vno.

 

I guess it is more difficult to indicate Va as it varies with aircraft weight.

 

 

 

What about Vb - Turbulance Penetration speed?

 

I would think that Vb should be close to or the same as Va

 

however these are the figures from the flight manual of the Jabiru 120C I use for training:

 

Va - 103kts (only one figure given)

 

Vb - 108kts (listed in the Normal Operation section)

 

Vno - 113kts

 

 

 

DWF

 

 

[facthunter]

Break up is usually the result of loss of control in IMC. It is not recommended to do spirals and recoveries in RAAus aircraft either.. IF you do, recover very quickly, or you will have speeds and "G" to reckon with that the plane wasn't built for.

 

We have probably exhausted the Va thing, in a practical sense. Telling a student that the plane will stall before it breaks up, from full control movement if you keep below a certain speed seems a bit weird. Most students don't like stalling and have only done the hold height stick back, back.. with engine idling caper, which is about as useful as an ash tray on an R 1. Nev

 

 

[facthunter]

DWF there should be a Vne..( Indicated AS) too. Nev

 

 

[DWF]

DWF there should be a Vne..( Indicated AS) too. Nev

There is - Vne 146kts

 

I just didn't include it as it was not relevant to the discussion.

 

DWF

 

 

[Dafydd Llewellyn]

A very interesting thread and topic. 

 

A couple of comments from a practical point of view....

 

 

 

Although it is usually listed in the aircraft flight manual, to the best of my recollection, Va was not marked on the airspeed indicators or placarded in the cockpit of any aircraft I have flown (and there have been quite a few).

 

The yellow arc on airspeed indicators starts at Vc or Vno - Maximum Structural Cruise Speed.

 

Va is the Manoeuvering Speed - and is usually significantly lower than Vno.

 

I guess it is more difficult to indicate Va as it varies with aircraft weight.

 

 

 

What about Vb - Turbulance Penetration speed?

 

I would think that Vb should be close to or the same as Va

 

however these are the figures from the flight manual of the Jabiru 120C I use for training:

 

Va - 103kts (only one figure given)

 

Vb - 108kts (listed in the Normal Operation section)

 

Vno - 113kts

 

 

 

DWF

Va is always calculated for the MTOW - so only one value.

 

 

[Guest Nobody]

Va is always calculated for the MTOW - so only one value.

Extract from Cessna 172s POH

 

 

[DWF]

Va is always calculated for the MTOW - so only one value.

Not always!

From the C177RG POH

 

 

 

[Oscar]

The whole thing of improving knowledge of what Va means and how it exceeding it can be disastrous, is of immense importance. However, can I intrude slightly and ask - does the average pilot understand and take into account in her/his flying, of Va as a serious factor in conducting their flight behaviour?

 

Here is what I am trying to suggest.

 

In the case of a Tecnam P96 - for which the possibility (absolutely unproven as yet) of in-flight structural failure as a consequence of exceeding VA is being considered - from what I have been able to glean the normal cruise speed is around 110 -112 KIAS. From the P96 POH, that is actually about 102 kts CAS. VA for the P96 is 81 KIAS, 79 CAS.

 

So what? So what is: if the pilot suddenly has the need to employ full control deflection, he/she may well need to wash off nearly 30 KIAS speed before doing that, if simply cruising along at the available cruise speed. I suggest that the conditions requiring full control deflection and the 'envelope' of washing off nearly 20% of the cruise speed, (or, from a reaction pov, 30 kts off the clock speed) are likely to be inimical.

 

By comparison, the Jabiru J120 example given above, has a Vno of 113 KIAS and a Va of 103 KIAS. From the J120 POH, allowing for airspeed position error, 113 KIAS is 108 CAS and 103 KIAS is about 98 CAS. In broad general terms, I think most people cruise their J120s at around 2850 rpm, giving an indicated KIAS of 110, or a CAS of around 104.

 

What's the point I am trying to make here? There are a couple, actually.

 

Firstly, in the case of the Jab, the potential for unintentionally exceeding Va when a situation requiring full control deflection suddenly arises is about 6 - 7 % in a 'normal' flight situation (and Vb is extremely close to Vno). By comparison, in the p96 would be 'normally' cruising at something around 20% above its Va. I have no idea of the Vb for the P96 but conventional wisdom suggests Vb is usually fairly close to Va.

 

In terms of pilot behaviour - or in this case, the approach to selecting cruise speed in conditions that require prudence, the J120 pilot ought to knock off maybe 5 kts. It appears to me that the P96 pilot should knock off around 20 kts - which is a pretty decent chunk of speed to throw away when trying to get across the ground in a country as large as ours. We get, particularly in summer, some pretty serious CAT, 'blue-air' thermals etc.

 

So my question is this: do pilots routinely factor in their aircraft's Va as a factor in selecting a safe cruise speed for the conditions - especially the 'likely' or 'possible' conditions over and above the apparent conditions?

 

 

[facthunter]

They should factor it in when they buy the plane. Nev

 

 

[Head in the clouds]

The whole thing of improving knowledge of what Va means and how it exceeding it can be disastrous, is of immense importance. However, can I intrude slightly and ask - does the average pilot understand and take into account in her/his flying, of Va as a serious factor in conducting their flight behaviour?Here is what I am trying to suggest.

 

In the case of a Tecnam P96 - for which the possibility (absolutely unproven as yet) of in-flight structural failure as a consequence of exceeding VA is being considered - from what I have been able to glean the normal cruise speed is around 110 -112 KIAS. From the P96 POH, that is actually about 102 kts CAS. VA for the P96 is 81 KIAS, 79 CAS.

 

So what? So what is: if the pilot suddenly has the need to employ full control deflection, he/she may well need to wash off nearly 30 KIAS speed before doing that, if simply cruising along at the available cruise speed. I suggest that the conditions requiring full control deflection and the 'envelope' of washing off nearly 20% of the cruise speed, (or, from a reaction pov, 30 kts off the clock speed) are likely to be inimical.

 

By comparison, the Jabiru J120 example given above, has a Vno of 113 KIAS and a Va of 103 KIAS. From the J120 POH, allowing for airspeed position error, 113 KIAS is 108 CAS and 103 KIAS is about 98 CAS. In broad general terms, I think most people cruise their J120s at around 2850 rpm, giving an indicated KIAS of 110, or a CAS of around 104.

 

What's the point I am trying to make here? There are a couple, actually.

 

Firstly, in the case of the Jab, the potential for unintentionally exceeding Va when a situation requiring full control deflection suddenly arises is about 6 - 7 % in a 'normal' flight situation (and Vb is extremely close to Vno). By comparison, in the p96 would be 'normally' cruising at something around 20% above its Va. I have no idea of the Vb for the P96 but conventional wisdom suggests Vb is usually fairly close to Va.

 

In terms of pilot behaviour - or in this case, the approach to selecting cruise speed in conditions that require prudence, the J120 pilot ought to knock off maybe 5 kts. It appears to me that the P96 pilot should knock off around 20 kts - which is a pretty decent chunk of speed to throw away when trying to get across the ground in a country as large as ours. We get, particularly in summer, some pretty serious CAT, 'blue-air' thermals etc.

 

So my question is this: do pilots routinely factor in their aircraft's Va as a factor in selecting a safe cruise speed for the conditions - especially the 'likely' or 'possible' conditions over and above the apparent conditions?

It's a very good point Oscar and I don't think most recreational pilots give it a moment's consideration.

 

I went flying with a friend who has owned his J160 for years. he built it from a kit.

 

He's an exceptionally 'thinking' sort of a pilot, gives a very thorough pax briefing (including all about his EFATO intentions(!)), does every check imaginable, very capable. He's a former QANTAS flight engineer who goes all the way back to the Connie days etc.

 

We took off and departed on a fairly blustery day and as soon as we were at 2000ft, he levelled off and pushed it up to max permissible revs to show me how comfortably it would cruise at 130kts ... I mentioned Va and he said he wasn't planning on doing any tight manoeuvring at that speed, just gentle turns. I mentioned turbulence and he said no problem the Jabs are built strong!

 

As far as my method is concerned, if I'm going somewhere and want to get there reasonably quickly I stay at Va until I'm in cruise and also above the inversion layer so I can be pretty well assured of smooth air - I described my heavy turbulence experience a few posts back so I tend to be a little once bitten, twice shy these days.

 

Unless I really have to get somewhere fast I have a bit of a benefit regarding staying below Va because I just don't have this need for speed thing that so many seem to have. I tend to get to top of climb and then often even slow down and enjoy just boodling along and looking around, I'm rarely in a hurry. Too many years flying helicopters I guess.

 

 

[Oscar]

They should factor it in when they buy the plane. Nev

Yes - but do they? I mean that seriously. Here, on the one hand, you have a pretty plain-Jane little Jab, it's fairly obviously just a somewhat tarted-up version of the original LSA55 with what I suspect most pilots would consider to be pretty much the 'baseline' of performance for RAA aircraft, somewhat on the 'cheap and cheerful' side. You look at the performance figures quoted on the Jab. web-site, and there are plenty of other aircraft claiming better figures, with nicer looks and finish etc. (and let's be honest here, everybody keeps a bit of 'reserve' about Jab. engine reliability in mind, with good reason).

 

If you have paid (or are prepared to pay) quite a considerable chunk of extra $$ for something that is quite a bit more 'attractive' than a J120, are you really going to consider that, when the going gets a bit 'rough', you will routinely pull back your speed (and that little Jab will, albeit not remarkably quickly, waddle off into the distance if on the same flight path?)

 

It seems to me that this discussion of understanding and applying knowledge of Va is very important.

 

 

[djpacro]

.... However, can I intrude slightly and ask - does the average pilot understand and take into account in her/his flying, of Va as a serious factor in conducting their flight behaviour?

My experience in flying with many pilots is that they do not know what VA is for the aeroplanes they normally fly, do not know that there is usually a placard adjacent to the ASI and do not know what Va means.

 

So my question is this: do pilots routinely factor in their aircraft's Va as a factor in selecting a safe cruise speed for the conditions - especially the 'likely' or 'possible' conditions over and above the apparent conditions?

I do indeed.One aeroplane that I normally fly has a VA of 93 kts (just a single value defined regardless of weight) in normal category and it cruises at 110+ kts IAS. In aerobatic category at a reduced weight and nil baggage the Va is 115 kts, reduced by 6 kts if 100 kg lighter (e.g. solo rather than two up).

 

Another aeroplane that I normally fly has a VA of 134 kts, just a single value declared, in aerobatic category. Refer the attached diagram for its flight envelope.

 

 

No reduced value declared at any reduced weight in aerobatic category. At a higher weight in normal category it is still the same VA despite the reduced load factor.

 

So, in normal category at 134 kts I am permitted to quickly move the stick full back. Regardless of a higher inertia (mass plus pitch moment of inertia) I can do it easily - especially as the CG is further aft so it is much more responsive to elevator - lower stick force per G - and far exceed the limit load factor of 3.8 defined for normal category.

 

Likewise, aerobatic category flying solo instead of two up so significantly lower weight than maximum for aerobatics - I am permitted to suddenly apply full back stick - and it will readily respond with a much higher load factor than the 6.0 limit for aerobatic category.

 

Per Dafydd's extract from FAR 23, the stalling speed is based on the "maximum airplane normal force coefficients" so lower stall speed than in the "book" - aft cg rather than forward cg per the performance section of the flight manual.

 

As for gusts - the attached flight envelope shows the standard gust load factors per FAR 23. VB is generally not defined in many airworthiness regulations for small aeroplanes.