LSA's have a MTOW of 600 kg. My plane flys badly if that weight is exceeded. Can a "virtual weight" be calculated from deviation from standard atmosphere?

[facthunter]

If your plane really struggles to get into the air, it's  going to have trouble staying in the air when out of ground effect and /if you make a turn or encounter a patch of even warmer air. Don't go, big Julie, don't go.  Nev..

[pmccarthy]

5 hours ago, facthunter said:

If your plane really struggles to get into the air, it's  going to have trouble staying in the air when out of ground effect and /if you make a turn or encounter a patch of even warmer air. Don't go, big Julie, don't go.  Nev..

Those of us of a certain age will get the quote but not many I suspect. Stop, or I'll fill you full of bronze.

[old man emu]

2 hours ago, pmccarthy said:

Don't go, big Julie, don't go.

 

[APenNameAndThatA]

On 22/01/2021 at 9:08 AM, FlyBoy1960 said:

for a start, you can fly over the aircraft maximum takeoff weight. It is there for a reason ! DUH

 

secondly, there should be graphs in the POH which show you the different calculations for density altitude and from this you can calculate your ground roll for temperatures above standard atmosphere and the climb rate to be expected.

LSA's have no such graphs. Apparently the reason is that LSA's tend to have such short ground rolls.

[APenNameAndThatA]

On 22/01/2021 at 12:53 PM, FlyBoy1960 said:

all of this should be in the manual for every aircraft. They are easy formulas to calculate but some manufacturers include simple tables like this for people that don't have a calculator

 

 

My question was not about ground roll. My question was about MTOW. My question was if one should reduce the MTOW for air that is much less dense than usual. The obvious answer would be, "No, otherwise it would be in the POH, DUH!" But because my plane does not fly well at around MTOW, and planes can take off at 40 degrees C, and this is a forum for chatting, I thought I would ask the question.

[APenNameAndThatA]

On 22/01/2021 at 4:38 PM, FlyBoy1960 said:

if you overload the wing by going past the designed lift profile you will get laminar flow separation.

 

This is also called laminar bucket and the aircraft will not fly, it will drag its tail around the sky with a high angle of attack and it heavy enough will just sink. Pushing the stick forward does nothing except change the angle of attack slightly but it doesn't produce any more lift.

 

Simple answer is,  don't fly over the maximum takeoff weight of the aircraft.

 

I don't know why we are even talking about this because it requires an approval from CASA each and every time you want to do this and it is only approved for things like ferry flights where you are carrying extra fuel coming over from the states ?

 

I don't really want to sound like Capt obvious that this is such a bad idea. Just don't flly overweight !

I was not suggesting that anyone fly overweight. My question was if, in particular conditions, the MTOW should be revised. And Foxbats don't have laminar flow wings. And if CASA does allow you to fly overweight then, actually, you can do it and it is a legitimate thing to discuss. Except that was not what I was asking about. And, the same airframe of floats has a MTOW of 650 kg. And, no I'm not advocating flying overweight.

[old man emu]

And it's a good question, worth the discussion.

 

Now the question has been phrased in a more practical way. As I see it, the question is, "When it gets hot, should I lighten my aircraft?". The only ways to do that practically are to take out unnecessary junk, or reduce the amount of fuel carried.

 

I'm thinking that we are using the wrong figures for air density. At 20 °C and 101.325 kPa, dry air has a density of 1.2041 kg/m³. As pressure increases, with temperature constant, density increases. Conversely when temperature increases, with pressure constant, density decreases. Air density will decrease by about 1% for a decrease of 10 hPa in pressure or 3 °C increase in temperature.

 

That 1% is one percent of 1.2041 kg/m^3.  It's the kg/m^3 figure that needs to be corrected when used in the Lift formula.

 

[APenNameAndThatA]

On 22/01/2021 at 9:32 PM, Thruster88 said:

Isn't the OP asking what weight would give the same performance at 45°C as he gets at 600kg and 15°C?

Thank you for at least thinking about what I was asking. My question was as follows. If the MTOW is 600 kg on a normal day, should the weight be decreased on a really really hot day because performance will be less. Seems like a perfectly interesting question. One answer is "No, DUH, or it would be in the POH".

 

Another answer might be that, theoretically, from some points of view, the MTOW could reasonably be *higher*. My understanding of Vno is that it is set so that if there is a really big updraft, the aircraft stalls (because of the increased AoA) before the aircraft is subjected to more than the design limits. If the air was not dense at all, and the aircraft was flying at Vno, then the AoA would be higher in level flight, meaning that the wing would stall with a less severe updraft, providing an extra margin of safety and an actually higher Vno.

 

For similar reasons, the actual Va could be higher for very thin air.

 

No I'm not advocating taking off at high weight. This is a true story. I took off one day a very small amount overweight by accident and was surprised how much performance was degraded. It was uncomfortable. So, I said to myself, "How would my plane have flown if it was 40 C instead of 25".

 

At least OME was directing some thought to the question. So, thank you OME.

[skippydiesel]

On 22/01/2021 at 10:08 AM, FlyBoy1960 said:

for a start, you can fly over the aircraft maximum takeoff weight. It is there for a reason ! DUH

 

secondly, there should be graphs in the POH which show you the different calculations for density altitude and from this you can calculate your ground roll for temperatures above standard atmosphere and the climb rate to be expected.

I do not believe that there can be any legally acceptable reason for exceeding the permissible max take off weight. Should you do so and crash your insurance may be null.

 

I have only flown 4 RAA aircraft, of which I would only consider myself experienced in one. Quite a long time ago I experienced a few different Cessna 172's . The C 172's have (by RAA standards) anemic take off performance ,so the effect of density altitude is quite marked. The one RAA aircraft I have experience in (10 years and some 400 hrs) has a TO role of  under 100 m, on short grass and will climb out strongly, 1500 ft/min in almost all weather conditions -The highest strip I have used was 2,140 ft and on rare occasion have experienced 40C take offs  (try and avoided anything above 30C) with little obvious reduction in TO performance. With excess performance like this, it is clear that the aircraft may be able to lift significantly higher loads (above the legal limit) however the airframe, when so loaded, may not handle hard turbulence or other in flight challengesl.

[old man emu]

The very first thing you notice on your first solo flight is how the aircraft goes up like a fart in a bath. Why? Because previous to your first solo you've been hauling around the weight of the instructor. Throw the instructor out and for the same velocity/air density combination as it was ten minutes earlier, the Lift generated is much more.

[aro]

1 hour ago, APenNameAndThatA said:

Thank you for at least thinking about what I was asking. My question was as follows. If the MTOW is 600 kg on a normal day, should the weight be decreased on a really really hot day because performance will be less. Seems like a perfectly interesting question. One answer is "No, DUH, or it would be in the POH".

It is definitely valid to decrease weight for an airport that is hot or high. GA aircraft will normally have performance charts that allow you to figure out whether you can take off or land in the space available at a specific weight. If the answer is no, one solution may be to reduce weight.

 

The same applies to any aircraft, however it is more difficult if there are no performance charts for weight. It is still good to be aware of the problem.

 

You can't use the same logic to increase weight because you don't know the limiting factors for MTOW.

[skippydiesel]

11 hours ago, old man emu said:

.................................................. the Lift generated is much more.

Are you sure? Seems to me the lift (when the instructor leaves the aircraft) is the same - its the weight that has reduced so TO performance is improved

[Thruster88]

25 minutes ago, skippydiesel said:

Are you sure? Seems to me the lift (when the instructor leaves the aircraft) is the same - its the weight that has reduced so TO performance is improved

Because the induced drag is less at the lighter weight the aircraft can fly at a slightly higher angle of attack for a given climb speed so yes a little more lift is produced.

[skippydiesel]

47 minutes ago, Thruster88 said:

Because the induced drag is less at the lighter weight the aircraft can fly at a slightly higher angle of attack for a given climb speed so yes a little more lift is produced.

I see that a reduction in weight can improve TO - yes angle of attack, also acceleration (reduced ground role) however for a given wing, engine, prop, density altitude, the lift must surely stay pretty much the same - it is the weight that the lift must act on that has changed ???????

[old man emu]

2 hours ago, skippydiesel said:

Are you sure?

You are correct. It was getting late and I was tired when I wrote that. 

 

The total Lift created will be the same, but the excess of created Lift over required Lift will be greater because the required Lift has been decreased by the weight of the instructor.

[Jabiru7252]

What does one mean when they say "Foxbats don't have laminar flow wings"? Isn't all airflow over a wing 'laminar' (during normal flight) or am I having a senior's moment?

[old man emu]

Back to the Algebra, but this time I've done the arithmetic for you.

 

We'll start with old mate, Lift formula:

Lift = {1 x (density x TAS)^2 x 1} /2

 

The amount of Lift we are going to get is dependant on two variables - the density of the air in which the aircraft is operating and the True Airspeed in air in which the aircraft is operating. Let's look at TAS.

Where 

V = Indicated Airspeed

r(ISA) = air density in ISA (103.2 1013.2 mb and 15C) = 1.22 kg/m^3

r (Actual) is air density in which the aircraft is operating.

 

From this equation, the following table can be constructed:

The next step is to calculate the difference in Lift between that generated at ISA to that generated at higher temperatures.

 

Where:

r = Actual air density

TAS = True Airspeed

k = (Coefficient of Lift x Wing Area) all the things we must acknowledge but do not vary, so we give them a value of one (1)

 

 

Can I have a rest now?

 

 

[Jabiru7252]

No, you cannot have a rest. Go fix your typos. You typed 103.2mb instead of 1013.2mb. 🙂

 

[old man emu]

2 minutes ago, Jabiru7252 said:

Go fix your typos.

I have. Now can I go, please Sir? It's lunch time and my mates want to play marbles, but I lost mine.

[Jabiru7252]

Permission granted. P.S. I love maths.

[facthunter]

Formula's are great if you apply the right one. The universal gas equation has 3 components to it, all equalling a constant.

  Also if you reduce the weight you can climb slower at the  same ANGLE OF ATTACK. Angle of attack relates to stall speeds and also best climb speed. You don't increase lift to climb except a tiny bit to initiate it. Lift stays close to weight unless you have lot's of power and do a steep climb and THEN you need LESS lift.  Nev

[Yenn]

Laminar or non laminar depends upon the design of the wing. Some wings are designed as laminar flow and they work well, but bugs on the wing will reduce lift by breaking up the laminar flow and ice will do a really good job of it.

MTOW is a figure produced by the designer of the plane, but we have to comply with the legal MTOW as set by CASA. It would be possible to have a plane with a 600kg MTOW, but for it to be only flown at 544kg max.

[APenNameAndThatA]

8 hours ago, Jabiru7252 said:

What does one mean when they say "Foxbats don't have laminar flow wings"? Isn't all airflow over a wing 'laminar' (during normal flight) or am I having a senior's moment?

I think that the smooth lines representing the air flow around pictures of wings causes confusion. Most air around a wing, at the surface, is turbulent. That is not the same as airflow separating from the wing, which is what happens in a stall. 
. 

Some mathematician or other proved that it is impossible for *any* wing to have only laminar flow.

. 

I went looking for something on the internet about laminar/turbulent/stalled airflow and didn’t find anything about that. But, apparently, the WW2 Mustang wing was designed to be laminar but wasn’t because it wasn’t smooth enough, even though they tried to make it smooth enough! 
.

I am not sure if non laminar includes a tiny bit at the leading edge. 

.
I imagine that vortex generators, which stop separation by energising the airflow, are very not laminar! So, they decrease stall by increasing turbulence. 
 

 

[APenNameAndThatA]

6 hours ago, facthunter said:

Formula's are great if you apply the right one. The universal gas equation has 3 components to it, all equalling a constant.

  Also if you reduce the weight you can climb slower at the  same ANGLE OF ATTACK. Angle of attack relates to stall speeds and also best climb speed. You don't increase lift to climb except a tiny bit to initiate it. Lift stays close to weight unless you have lot's of power and do a steep climb and THEN you need LESS lift.  Nev

That is counter intuitive, but after you have initiated the climb, the aircraft is no longer accelerating. But more work is being done against gravity. I’m confused. Thanks. 😆

[facthunter]

Excess power enables climb. Nev