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Recreational pilot's license flight test

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Recreational pilot's license flight test

Old 12th Jan 2017, 04:58
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Recreational pilot's license flight test

Hi all!

I'm a student pilot training in Sydney, Aus. I have my flight test for my recreational pilot's license (RPL) in the next week or two. I had my prelicense flight last week and passed, however the instructor asked a few questions which I could not answer. I asked some of my friends from other flying schools who hold an RPL for help and they say the questions I was asked are more of PPL level. If anyone could help me to understand and answer these questions before my flight test that would be greatly appreciated:

-Why Vs moves with the movement of CG

-Va, the purpose of it and why it changes with weight

Thanks so much
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Old 12th Jan 2017, 07:13
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Vs moves with change of CoG Tricky one??. I'll have a lash...

In cruise any shift of the freight /pax will change CoG and level flight will only be maintained by a trim change to counteract.
As the weight of the aircraft and load remains the same, ergo.. the stall speed at that given weight should remain the same.
To my mind if the CoG is within the specified tolerances of the CoG range the touch down /stall speed should be the same....trim having countered any weight shifts or change of flight attitude. Or am I missing something here ???

Va. Design limitation maneovering speed. The speed at which any abrupt and full control deflection may cause structural damage or failure.
eg Jumbo that got away from an inattentive crew...exceeded G limits and speeds...some bits broke or tore off but it held together. Luckily for all on board.!

Those out there that do know...correct? those that should know.
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Old 12th Jan 2017, 08:35
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Why Vs moves with the movement of CG

One of the considerations with CG position is the vertical tail load needed to keep the aircraft in balance.

Generally, we see the tail load being a down load. As the CG moves forward, you are going to need more tail down load to prevent the aircraft pitching nose down.

Now, keeping it simple, say straight and level, and considering most of the lift to be associated with the wing and tail, we have, generally, an up lift load on the wing, and a down lift load on the tail. The net lift is the difference between the two.

As CG moves forward and the tail lift down load increases (a bit of nose up trim is input to maintain straight and level) overall net lift will decrease.

To maintain straight and level flight, we need (either to go faster or) to increase CL by pitching up a little to increase incidence or angle of attack. Think in terms of starting at the aft limit .. achieves a particular stall speed. As the CG moves forward, we need more CL to maintain straight and level flight but if we were at the stall speed to start with, this can only be achieved by increasing speed a little and so we find that the stall speed must increase.

It follows that the flight manual (or pilots operating handbook, which includes the flight manual) stall speeds will be figured for forward CG limit. One of the reasons the upper forward limit moves aft on light aircraft is to keep the stall speed a bit sensible for other considerations.

Va, the purpose of it and why it changes with weight

The basic idea of Va is looking at loads in the pitching plane. For this case, Va is the speed at which the limit design load factor occurs at the stall speed for that weight.

Reason is that this gives the pilot a maximum speed target for rough air or manoeuvring flight below which the aeroplane should stall prior to exceeding the limit design load factor, ie protects us against being a bit too hamfisted on the stick/column.

Significance of the limit design load factor is that the wing shouldn't suffer any permanent deformation or have anything break for this loading case.

A problem arises with things other than the wings .. eg a lump of equipment riveted to some structure in the back only knows what g-load is applied to it. If the load exceeds the limit design load factor, then the structural stuff which is holding the lump of equipment in place might break or suffer other damage.

So, the problem is that we need to make sure that Va considers any variations associated, for instance, with weight.

Now, if the aeroplane is lighter, I don't need quite so much lift at a given speed so, if I am at the MTOW Va but at a lower weight, I still have some g capability in reserve (compared to the situation at MTOW) .. ergo, I can pull a bit more g than the limit design load factor before the aeroplane stalls.

That, of itself, won't necessarily hurt the wing as the wing is concerned with loads rather than accelerations (ie load factor or g). However, it certainly can cause some bother to other structure in the aeroplane.

Hence we need to provide guidance to the pilot so that the limit design load factor can be respected. The POH often will provide information relating Va to gross weight and you should see Va reducing a little as gross weight reduces.

Side note. Generally, Va will be limited by pitching (nose up/down) loads. However, for some aircraft, the loads or other considerations associated with control movement in either of the yawing or rolling planes might be more limiting (for instance, acceptable handling). As I recall, the HS125 limit was for rudder and directional stability.

Overall, the limit's application is that the pilot should be able to apply a rapid steady input in any ONE control to the stop ie NOT significant inputs in multiple controls simultaneously. Rapid control input reversals and intentional pilot input oscillations are not included in the design considerations .. ie if you are at Va (or somewhat below) and start pushing and pulling on whichever control is limiting .. you might get an unpleasant (and, potentially, fatal) surprise. A read of the accident report on AA 587 is illuminating.

To the OP, best of luck with your forthcoming flight test.
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Old 12th Jan 2017, 08:39
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If CG is forward, the nose is heavier, and the horizon stabiliser has to generate more down force (through up elevator) to balance. This larger down force has to be countered with more lift, and higher speed.
A rearward CG requires less tailplane downforce, and the stall speed reduces. There is also less drag generated.

Va is normally considered as maximum speed that any one control can be fully deflected without overstress (and even then, I believe, just the one time. Not full left to full right rudder…). Va reduces as weight decreases.
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Old 12th Jan 2017, 08:41
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With apologies to John Tullamarine - I'm a slow typer and he snuck in first, and his explanation is much better than mine. However it appears that we agree.
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Old 12th Jan 2017, 09:21
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Yes John: "Overall, the limit's application is that the pilot should be able to

(a) apply a rapid steady input in any ONE control to the stop ie NOT significant inputs in multiple controls simultaneously.

(b) rapid control input reversals and intentional pilot input oscillations are not included in the design considerations .. ie if you are at Va (or somewhat below) and start pushing and pulling on whichever control is limiting .. you might get an unpleasant (and, potentially, fatal) surprise. A read of the accident report on AA 587 is illuminating."

After that accident, the FAA eventually produced Special Airworthiness Information Bulletin CE-11-17 - every pilot must read this:
http://www.safepilots.org/documents/...ring_Speed.pdf

There is an excellent description at The Risks of Maneuvering Speed Myths - AVweb Features Article

One often sees simple statements such as:

“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.”

“Maneuvering Speed is the maximum speed at which you may use abrupt control travel.”

Aerobatic pilots should be familiar with the definition provided by CASA in their CAAP 155-1, Aerobatics, which states:
“Manoeuvring speed (VA) is the speed above which full deflection of the elevator control will exceed aircraft structural limitations. Below VA the aircraft will stall before structural limits can be exceeded.”
Question: an aerobatic aircraft with the usual G limits of +6 and -3, flying straight and level at Va - can the pilot abruptly move the control stick fully forward without exceeding structural limitations?

All are woefully inadequate definitions of VA and it is particularly disappointing to see that CASA’s advice to aerobatic pilots is also inadequate.

For engineers, sorry John but ...
"The basic idea of Va is looking at loads in the pitching plane. For this case, Va is the speed at which the limit design load factor occurs at the stall speed for that weight." Nope.
That SAIB with its statement about that accident was all about the rudder. Aileron application is also very important and, related, rolling Gs.
Don't forget the ifs and buts in various versions of FAR 23. Remember that Va need not exceed Vc and Vc need not exceed 0.9Vh. And the Va in the regs is Vamin - the designer may select a higher Va. i.e. Va may not be that corner speed - newer versions of FAR 23 fixed this I believe.

Last edited by djpil; 12th Jan 2017 at 09:23. Reason: Fixed grammar in one sentence to be clearer - hopefully.
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Old 12th Jan 2017, 09:27
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Indeed, Dave, d'accord.

However, I was endeavouring to provide a reasonably simple explanation for the OP and one relevant to his/her stage of learning. I noted the need to consider all three controls (eg the HS125 Va) but didn't see a necessity to provide too much detail at this stage lest I confused more than assisted.

Had I been talking to engineers, my response would have been somewhat different.

However, hopefully. some of the folks do chase up some of the references you have provided.
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Old 12th Jan 2017, 09:50
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Beyond RPL!

Why Vs moves with CofG.

Weight act in front of lift producing nose down moment, the tailplane counteracts this with a downward force. If the C of G is further aft there is less nose down and therefore less downward force requred from the tailplane. You could say the aircraft is effectively lighter; this will reduce the stalling speed in the same way as a reduction in weight would.

Va, purpose and change of weight.
Structural limitation.
At any given angle of attack the lift generated depends on airspeed, if we suddenly increase angle of attack the force will be greater at a higher airspeed.
A heavier aircraft will take a greater force to move than a lighter one.
Therefore if you take two identical aircraft, one at MTOW and one with Pilot and minimum fuel flying at the same speed and suddenly increase the angle of attack to the critical angle of attack they will both generate the same force, but the lighter one will have a greater vertical acceleration due to its lower mass.
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Old 12th Jan 2017, 10:16
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Of course in principle Va should also vary with CG position (for the same reasons as Vs does), but while I've seen this plot given in some miltary fast-jet ODMs I can't ever recall seeing it in the flight manual of a GA aeroplane.

PDR
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Old 12th Jan 2017, 10:51
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JT, the two links that I provided are reasonably simple explanations of Va for pilots of small airplanes - both contain the full and correct definition of Va. I recommend that pilots read at least one of them.
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Old 12th Jan 2017, 11:04
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Folks, djpil is a technical expert in these matters. Suggest all do have a read of the links.
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Old 12th Jan 2017, 11:30
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... and at every Avalon Airshow I ply JT with red wine to convince him to spread that story ....
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Old 12th Jan 2017, 12:25
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Of course Runaway Gun has it pretty right and PDR1 gets onto the issue of the real stall speed with g loading.

Esp with high powered prop airplanes the stall speed at full throttle is lower than power off per the published stall speeds. I had some accurate data recording equipment in a Pitts some years ago - achieved limit load factor of 6 at a speed significantly less than Va by abrupt aft stick movement.
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Old 12th Jan 2017, 12:26
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Originally Posted by Avalonaviatrix
Hi all!

I'm a student pilot training in Sydney, Aus. I have my flight test for my recreational pilot's license (RPL) in the next week or two. I had my prelicense flight last week and passed, however the instructor asked a few questions which I could not answer. I asked some of my friends from other flying schools who hold an RPL for help and they say the questions I was asked are more of PPL level. If anyone could help me to understand and answer these questions before my flight test that would be greatly appreciated:

-Why Vs moves with the movement of CG

-Va, the purpose of it and why it changes with weight

Thanks so much
If you don't feel ready for the flight test talk to your instructor. Below is a link to the RPL Flight test report form, maybe go though it with your instructor and have them ask you questions about each of the ground items. Also review the in flight items to ensure you have been trained to safely perform all manoeuvres.
Good luck with the flight test.

Here's a copy of the RPL flight test form:
https://www.casa.gov.au/file/159281/download?token=70JeWftD
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Old 12th Jan 2017, 12:51
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achieved limit load factor of 6 at a speed significantly less than Va by abrupt aft stick movement

For interest, what sort of pitch rate did the Pitts achieve for that point ? I would imagine reasonably high ?
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Old 12th Jan 2017, 13:13
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You can get into interesting questions like "what is the stalling speed of a tiger moth at the top of a +1G loop?". Personally it was the continual posing and discussing of questions like this by my instructor (when I was 18 and doing the full-time Flying Scholarship that led to my PPL) that allowed me to actually understand the concepts rather than just parroting the answers from a standard text. Once I had a decent grasp my instructor would then finish off the topic with a "and if you are asked that question by an examiner the stock answer they want is '...'. "

In this way he both educated me and trained me to pass the test (which are often not the same thing!). Another point which is often not appreciated is that stalling speed is not only reduced in a climb (where part of the weight is carried by the vertical component of thrust), but also in a dive (where part of the weight is carried by the vertical component of drag)...

PDR
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Old 12th Jan 2017, 14:58
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As we get into thread drift

Part of the thing here is aerodynamics as per theory is based on essentially stable conditions ie Vs demonstrated by 1kt/sec decleration to the stall. The aero boys (as above) know different but AFAIK don't have it fully have it worked out.

Each airframe/thrust machine is different but some poor guy doing RPL has problems with limited theory- and RPL used to mean PPL aerodynamics (BAK? used to teach that)

Last edited by Deaf; 12th Jan 2017 at 15:02. Reason: BAK?
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Old 12th Jan 2017, 19:58
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Well, what is the stalling speed of a Tiger Moth at the top of a one G loop? isn't it same as straight and level (1 G environment)? The wing doesn't know it's upside down. Or, are we talking in terms of zero G at the top of the loop, in which case how can it ever stall?
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Old 12th Jan 2017, 21:26
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Originally Posted by Sunfish
Or, are we talking in terms of zero G at the top of the loop, in which case how can it ever stall?
Exactly that - a picture that fixes in the mind the important features of stalling (or at least it did for me).

PDR
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Old 12th Jan 2017, 21:43
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that allowed me to actually understand the concepts rather than just parroting the answers from a standard text

Which is why we encourage a range of discussion, generally while avoiding too much complexity. Links to the more complex are valuable so that those who are interested can refer. Certainly, it is a little difficult with questions from early beginner training pilots as too much too fast can often be counter-productive.

Generally, the pilot doesn't need the whole nine yards of the story but, certainly, needs enough presented in a reasonable manner, to facilitate the relevant learning process.
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