Va (etc) and 760kg

[RFguy]

I've never read them  in a POH, but certainly have explored them.

 

The most breathtaking stall in the J230 is a power on, climbing, 30 deg turn,  with full flap.  OMG.... that is a dramatic.....

 

[Bruce Tuncks]

Some of the posts here, and in the original article, appear to suggest that a structure is stressed by the G forces alone ... this is WRONG. The structure is stressed by the product of the G force and the weight.

I have always believed that slowing down in turbulence makes you safe, and flying light makes you safe too. I'll try again to understand the article. ( I failed first time due to being slow and not too bright )

The worst turbulence I have flown in was a frontal zone. It felt like you were a rat in a shoe-box that was being shaken as hard as possible. It was impossible to keep your feet on the rudder pedals.  One passenger was scared stiff....  literally !. He was lifted out ( stiff ) by about 5 guys. Everybody slowed right down to about 60 knots and nothing was damaged, except for the guy outlanding when the lead-acid battery exited from behind him through the canopy. But that was caused by an improper battery installation.

[aro]

1 hour ago, Bruce Tuncks said:

Some of the posts here, and in the original article, appear to suggest that a structure is stressed by the G forces alone ... this is WRONG. The structure is stressed by the product of the G force and the weight.

The wings are stressed by the product of g force and total weight... but there are other parts of the airframe where you assume constant weight so you only need to worry about G.

 

Your battery mount!, seat weight limits, baggage floor limits - all are designed to a particular max G loading, and you don't generally know how much margin they have.

 

I remember reading about a loss of control in a 747 where the aircraft was overstressed in the pull out. The wings stayed on but from memory they broke the landing gear up locks and the APU broke loose. Exceed G limits and the thing that breaks first might not be what you expect.

[facthunter]

Could easily be your seat frame and the seat jamb the controls.  If you're pulling out of a dive and hit a gust  It's cumulative. Fronts can be very intense. Often there's a roll cloud with it. Sometimes you won't even be able to read any instrument.  nev

[RFguy]

Good point Nev. on the SE coast, We all know 'southerly busters' at ground level...... dead calm......... then BANG the window panes go with a sudden 25 kts 'pressure gust' out of nowhere...

 

One thing to mention, the Aerobatic category (+6g) are usually only rated  to Aerobatic a fair way below their MTOW.   If you want a high Va plane, get an Aerobatic machine and fly in that category (IE weight). It might not be comfortable , though.. 

 

The broad range of the J230  Va at min weight and Va at max weight  - If we take the Va being precisely Vs root(g=3.8) for the 600kg for the J230 , caution should be exercised extrapolating to higher weights due to the overall higher absolute  load.   Conversely, the low Va for the J230 at min weight, there is room for more Gs of load at the min weight if we consider absolute loads. I dont know enough about this stuff yet, so wont say too much.  

 

The J430 at 760kg  (which has same wing as the J230 , and heavier weight consequently higher stall speed)

is specified to have  a Va of 88 kts, which is BELOW the J230 Va at 600kg. IF the numbers are correct and specified with rigour (????), then this illustrates the above that you cant go on increasing the weight even if it will carry it.... 

So before everyone overloads their J230 (as usual) , they should consider there is an inflexion point in the Va versus weight  chart. IE 94kts at 600kg, might be 90kts at 640kg (where most J230s take off at full fuel on a camping trip with 2 on board) 

 

 

 

 

 

 

 

 

[Bruce Tuncks]

You guys are right but I still need to lose some weight. I just finished reading Hanna Reitsch's memoirs and thought while reading, that she was safer being so light. She was in real life a small woman.

But batteries are darned heavy things, unless you join the revolution and change to Lithium types ( LiPO4 is not dangerous I hope). 6kg down to 1kg is worthwhile I reckon. 

AND, I reckon the regulations are pretty conservative PLUS, aircraft are built with conservative safety factors. The very next time somebody loses their Jabiru wings, I promise to look real carefully into the matter.

Once, I was worrying about how Libelle gliders had their rough-air reduced from 135 knots to 89 knots. Then I found that "smooth air" can contain a sharp-edged upgust of 15 knots!

In my 40 years of looking, I reckon I found 15 knots maybe once, and it wasn't sharp edged.

we foolishly believed the 135 knots and in the bad old days, we did start-runs at 135 knots and full water in rough air. Nobody ever lost a wing, except for the mid-air over Narromine when the hapless pilot returned minus about 3 meters of wing.

[RFguy]

While regulations  and build margins are plenty,

just remember the bell curve when it comes to gusts. Some of them , the outliers, 4 or 5 x the std dev  will be very strong.

That's where the margins are used up. you cant design for the outliers, its too expensive on spec.

 

I dont think a jabiru has ever lost its wings.

Though it is probably good maintenance to inspect the struct attachment points , bolts, glass etc in great detail . and look for telltails in the empanage

 

Edited September 4, 2022 by RFguy

[skippydiesel]

In relation to strong gusts/windshear/turbulence - the security, I hope, I am getting from flying an aircraft designed for acrobatics, is not unfounded.

[Bruce Tuncks]

My theory is that FRP aircraft are way over-strong because the material is variable. So they choose a low maximum stress figure and the weakest likely structure.

This means that an ordinary structure will be over-strength for 2 separate reasons.

This is how it works in practice....  a Jabiru and a Technam ( Italian all-metal plane) suffered identical falls when a u/c leg collapsed when wheeling out their hangars.

The result?  The Jabiru needed a new bolt and the Technam was out of action till a new wing arrived from Italy.

Metal is very predictable and I reckon the allowable figures are truly important to adhere to.... well more so than fiberglass anyway.