Va (etc) and 760kg

[sfGnome]

I’ve just been reading an AOPA article that gave the rough rule of thumb that for every 2% below max weight, you should reduce your manoeuvre speed by 1%. That got me thinking about the new RAAus 750kg class G. Let’s say that you want the extra load capacity because your favourite travelling companion gets a bit annoyed when you limit them to 10kg luggage in a 600kg plane, so you decide to splash out and go to the bigger class. So far, so good. However…

 

However, most of the time, you’re not carrying a big load. In fact, you’re carrying just yourself and 30L fuel. For the purposes of this discussion, let’s imagine that amounts to 460kg, and that the published Va of the plane is 115kt. For a 600kg plane, that brings your ‘real’ Va down to 101kt, but for a 750kg plane, it drops to 92kt. That makes for pretty slow going in a supposedly fast aircraft. 
 

Now, I know that I’ve made some wild assumptions regarding the similarity of two aircraft designed for different max weights, but taking as an example ICP’s Ventura (newish big sister to the wonderful Sav) which is available in three max weights with similar specs (after taking different engine powers into account), perhaps the assumptions are not so wild after all?

 

The question is, what’s wrong with my analysis, or is moving up to a heavier aircraft a bad idea if you mostly just tootle around?

[Thruster88]

Standard empty weight of a Cessna 150 is 490kg so min flying weight would be about 600kg. Many aircraft have cruise speeds far greater than Va (example all RV's). As for any aircraft the PIC should read the POH and operate the aircraft within its limits. Nothing wrong with flying a group G or even a Cessna 210 by yourself.  

[RFguy]

sfGnome, yes, this is true.

 

The Jab230 is down to 75 kts  max manouvering speed  at minimum weight (415kg) .  (and is 94kts  at 600kg MTOW).

 

it's a consequence of the stall speed.

 

I believe now in the instances I have suffered frightening turbulence, that I had not slowed the plane down enough.

 

I was still flying at 90-95 kts instead of 80-85kts.... only solution is get yourself up to 9999 feet away from the turb

 

Of course at some point you have to come down and get into pattern and circuit, just means slowing right down

 

 

 

 

[facthunter]

10,000 feet won't get you away from the turbulence, either. Later on hot days the Inversion layer will be above FL120 in the centre  (Thermally) places. Nev

[RFguy]

I dont mind the thermal bumps too much. But FL120 hey ? wow. 

 

I dont like turbulence off hills at all.... Does the turb from hills, ranges etc extend beyond 9999 feet  in Australia ?

[facthunter]

Yes in places like Tasmania and near Mt Kosciusko, Mt Canobolus, Barrington tops.   More severe in the Himalayas and European Alps. A Boeing was destroyed in the lee of Mt Fuji. Nev

[sfGnome]

2 hours ago, RFguy said:

I believe now in the instances I have suffered frightening turbulence, that I had not slowed the plane down enough.

What bothers me most is that I must not have been listening - or I wasn’t taught, but I suspect the former - when the topic of variation in Va with weight came up in training. What’s more, I can immediately recollect an instance, one up, stuck under the Canberra steps on a stinking hot day and getting thrown around like a rag doll where I’m pretty sure that I was going too fast for the conditions. One more cup-full out of the bag of luck, and one more into the bag of experience…

[facthunter]

IF you get caught going too fast just fly the attitude loosely and roll a bit with the turbulence as you slow down.(reduce power) Trim carefully to the lower speed when you get there.. Plenty pf planes cruise far above safe manouvering speed. Nev

[kgwilson]

I certainly noticed the big increase in feeling turbulence when migrating from heavier GA aircraft to RA. It was different when hang gliding as it often indicated thermals and lift and my mindset didn't really consider that turbulence at all. The worst I have had in my current RA aircraft was  when at around 8000 feet with a very strong tailwind of about 50 knots I had to slow to 70 knots to even feel partly safe. There was no point trying to fight it. I just went with the gusts and survived.

[RFguy]

sfGnome, I dont recall it anywhere in a text book. Certainly there is an addage "slow down in turbulence" . I find that thermals are very different to hill and ridge turb.  

Stall speed is the square root of aircraft weight ratio, (all other things being equal) , if the aircraft stalls at 50 kts clean   at 600kg, then (all things equal) the stall at 430kg will be sqrt(600/430) or 1.18 times  lower , or  42 kts.  The Va for 600kg might be 90 kts, and will be that ratio lower at lower weights.

 

and Va is related to the stall speed and the maximum airframe G force (and the G force required to push the plane up, change the AoA and stall the wing. 

 

The Va is computed from a manouver, perhaps from a gust or from a control input, that causes the G force to hit the maximum for the catgeory and simultaneously will stall the wing, so there can be no more lift.

A couple of examples - 

 

If you are flying along at 70 kts at 5 deg AoA  straight and level. Straight and level you have 1g on the aircraft. the AoA stall angle is 16 deg, say.

Some turbulent  gust comes along from underneath /forward and causes  the AoA to be 3x higher , generating , for example 3x the lift......  the AoA is  now 3x5 = 15 deg ...either way now you have 3x more lift than you needed for S&L flight.  You have 3Gs..... Or perhaps you pulled back abruptly on the stick and generated that instantaneous high AoA. The wing and the airspeed and the AoA generates a high instantaneous lift from the wing, g = lift/weight, and you have 3x the lift you had dialed in .

Safe. uncomfortable but safe.

***A bigger gust comes along, or you are more hamfisted with the pitch, and and pulled on more than 15 deg AOA-  the wing stalls at 16 deg, you are beyond that so it will generate no more lift. and cannot exceed the 3g limit.

 

But if you were flying at 90 kts S&L, probably 3 deg AoA (because you have more airspeed, the wing AoA can be lower to produce the same amount of lift) , and that big gust comes along increases the AoA to 3x3 = 9deg AOA , now the wing produces maybe 1.5x  more lift at this higher airspeed (because lift produced is airspeed squared) . and that gust now instead of producing 3x the lift in the example above, now the wing is producing 3 x 1.5 = 4.5x the lift required for S&L (1g)  now you have 4.5g and the wings snap off.   and you were only at 9 deg and the wing could have gone to 16 before it stalled, so there was another almost 2x lift available to break something !

 

that's an extreme example, but is how it works.

How did I go everyone at explaining that ? how without a whiteboard for me. 

 

slowing slower, you always have more AoA because the wing needs to work harder for the same airspeed to have lift=weight

and thus there is less 'lift margin' in the wing because if you are flying at 7 deg AoA, then it can only go to 16 deg before it stalls, you can only get 2 and bit times the lift you had for S&L

if flying at 2 deg AoA, you can probably get 7x the lift before it stalls, and 7x the lift is 7g. crunch.

 

 

 

Edited September 2, 2022 by RFguy

[Thruster88]

There is a difference between  maximum structural cruising speed, Vno (start of the yellow arc) and maximum maneuvering speed Va (often not marked on ASI). For the Beech 23 Vno is 136knots and Va is 118 knots IAS, there is no mention of wieght in the POH,  I guess they worked it out on the worst case and kept it simple. For the RV6a Vno is 156knots and Va is 117knots IAS again no mention of wieght. Most thinking pilots will err on the side of caution.

[RFguy]

SEE POH INSTRUCTIONS FOR TURBULENT WEATHER BELOW IN POH : 

 

Beech 23  normal category 2300 lbs max weight  . min weight 1400 lbs

Va : 132 MPH  115 knots Vno 151 MPH 131 knots .

Vs (clean stall)  70 MPH (clean) . (abouT 61 kts). 

 

few assumption in their about quoted weighrs etc (Musk empty 1400, full 2300lbs ) ,based on the muls, i'd say the Muskateer Va is for about half way between max and min weight. 

 

airplane SN M529

 

 

 

 

Edited September 2, 2022 by RFguy

[RFguy]

The funny thing is, a Muskateer wont even fly S&L at Va (115 kts) , it would only do it in a vertical dive 🙂

But, seriously, makes it a very forgiving, potentially safe aircraft. cant get in trouble. 

 

Most RAAUS aircraft are going to have similar Va to the Jabiru 230- related directly to stall. and RAAUS aircraft generally have wildy varying weights from empty to full. 

The J230, a speed machine, cruise at 115-118 kts, you are still going to have to slow down to mid 80s (or slower)  like every other RAAUS plane that has also low stall speeds. you almost want to be able to reflex the ailerons and flaps to get Vstall  up.

Edited September 2, 2022 by RFguy

[facthunter]

RFguy you went good with your explanation.  Controls like the ailerons can impart twisting forces on a wing too.   I've seen damage to a Cessna wing resulting from extending flaps at too high a speed. It distorts the top surface of the wing.  where the flaps are .Nev

[aro]

This was definitely covered in BAK theory for PPL, maybe not for RAA?

 

Manoeuvring speed is based on stall speed. Rough air cruising speed may not be because it is based on a specified vertical gust. If you can hit that gust without exceeding G limits, no problem (for the aircraft... an instant onset of 3.8G might come as a shock to the occupants!)

 

It decreases with decreasing weight, and I'm pretty sure it is calculated at MTOW. Otherwise you don't have a known weight to start your calculations.

[aro]

7 minutes ago, facthunter said:

Controls like the ailerons can impart twisting forces on a wing too.

Very true. G limits are based on a straight pull. If you are rolling at the same time (rolling G) the limits might be much lower. So in a spiral dive etc. level the wings then pull out of the dive - don't do both at once.

[RFguy]

Hi Aro. Jab POH has minimum and max weight Va, which track the stall speed at weight.  

Yeah smooth and rough air cruising Vno, not quite sure what it is actually based on. perhaps it is precautionary so you are not too far about Va ?

 

NEV

I always thought with flaps  extended beyond Vfe, that  the flap actuators and rods and joints would bend, not the wing. interesting. But flaps out of course lowers stall speed further, , thus lowering Va. which really means your approach with all the flaps out doesnt want to be too fast such that it's beyond Va for full flaps. 

-glen

 

 

 

Edited September 2, 2022 by RFguy

[facthunter]

You don't want actuators etc to bend or you'll get ASSYMETRIC flap. The Cessna flap tracks distort the wing if speed exceeded. On some planes the Vfe is too close to the recommended turbulence penetration speed. Nev

[djpacro]

5 hours ago, RFguy said:

…..based on the muls, i'd say the Muskateer Va is for about half way between max and min weight. 

I’d say it would be exactly as required by the design airworthiness regulations as explained succinctly by FAA AC 23-19. Vs below is at maximum weight.

[RFguy]

Excellent stuff DJP. thanks for clarifying

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_23-19A.pdf

 

page 23 sets out the framework

Some notes :

 

The definition of Va is BOUNDED by this circular, and ultimately  mfr guide, it DOES NOT assure the operator that the wings wont fall off . (section 48-b). 

 

The definition of Vo  was introduced in 2007.

 

Vno is the maximum speed at the aircraft can withstand a  50 fps(3000fpm) or 30kts gust load without overloading the structure - but it isn't unbounded- and  it does NOT require combinations, eg for a T tail, you are not required to apply the gust for both vertical and horizontal tail surfaces simultaneously.  

 

Other interesting ones- cargo in cabin compartment- should be relevant to J230--- section 23.561- as the rear seating area of a J430 becomes cargo  (yes, it is named 'bags') in a J230. There are high requirements for the load restraints.

 

 

[facthunter]

Also having a stall reduce the airframe load still gives you another problem, doesn't it? Stalling might be a designer's answer to airframe loads but it's then the pilot's problem. Keeping control of the aeroplane is paramount. Nev

[djpacro]

4 hours ago, RFguy said:

Excellent stuff DJP. thanks for clarifying

 

 

I get a steady stream of curious flight instructors asking about it when they can't get numbers to tie up when they do the  arithmetic. Of course, they must use CAS - are those stall speeds in CAS or IAS? When the manufacturer does the calculations do they use the stall speed at the same CG published in the POH or not - some manufacturers publish stall speeds for both fore and aft CG positions.

 

There are several reasons why a manufacturer may state a higher Va than Vs.sqrt(n). One is that they are allowing for future growth of that model so just do the certification once at the anticipated higher weight - as you saw, Va is only used to get control surface and tail loads, nothing to do with stresses in the wing. When an engineer first specifies Va it is before the prototype has flown so the stall speed is only estimated and perhaps the engineer builds in a buffer. Perhaps the actual stall speed is less than originally estimated. The structural engineering certification paperwork is largely done, they're not going to revise Va and delay the FAA's review if they don't have to.

[Yenn]

FRguy stated'

 

Stall speed is the square root of aircraft weight ratio, (all other things being equal) , if the aircraft stalls at 50 kts clean   at 600kg, then (all things equal) the stall at 430kg will be sqrt(600/430) or 1.18 times  lower , or  42 kts.  The Va for 600kg might be 90 kts, and will be that ratio lower at lower weights.

I was always under the impression that higher weights resulted in higher stall speeds, so I reckon there is something wrong here, which leads me to suspect the whole argument delivered.

If stall speed varies at the sqrt of weight changes, by my reckoning stall at 430kg would be 59kts.

[RFguy]

I think you missed a reciprocal in there Yenn  50 / 1.18   . not 50 x 1.18 ? . I did say "stall speed varies"/.. How about "proportional to the reciprocal of the squareroot of the weight ratio"  ?

DJP - Interesting.  I find that most certified aircraft CAS / IAS tracks pretty closely, at least over the sort of aircraft i might fly up.. .  Many of the LSA machines IAS and CAS can diverge considerably. The J230D indicates 120 kts  when CAS is 115. 

if doing the math, certainly requires looking up the cals, especially between Vs (dirty) and Vfe region .

 

Jabiru publish Vs at  forward limit of CG.  
 

[facthunter]

With Max weight and power off too That's the usual condition specified for Vs.  (worst case). Nev