MTOW that mythical number

[turboplanner]

2 hours ago, Bruce Tuncks said:

Oops its 45 knots, which makes the argument even stronger.

The impact damage is also exponential.

[Kyle Communications]

its the same thing.  The spec has been set by the maker and the MTOW is the gross weight it can be flown at

 

[Thruster88]

When I was young and relatively inexperienced with piloting I remember reading this accident  report about weight and balance and how things could change in flight as fuel burned off. Six people died and GA took another hit, it is not surprising that the general public are reluctant to fly in light aircraft. 

 

 https://www.atsb.gov.au/publications/investigation_reports/1992/aair/aair199200014/

[Bruce Tuncks]

Pretty poor siting of the fuel load huh, if the burning off can transform a safe situation into a dangerous one.

It sure was not a Jabiru.

[Bruce Tuncks]

yep, the kinetic energy is also related to v^2, so I agree that the damage would be related to this figure.

[turboplanner]

6 hours ago, Bruce Tuncks said:

Pretty poor siting of the fuel load huh, if the burning off can transform a safe situation into a dangerous one.

It sure was not a Jabiru.

This was a Cruising Aircraft, so long distance requirement, and the PIC has the option to decide to give priority to Range (fuel), total passenger mass, or baggage (freight). He can have all three at max so if he is going for max fuel, he has to cut back on passengers and baggage, and usually the fuel burn will result in different W&B figures, so both the takeoff and destination fuel calcs are done.

 

This pilot did what's been recommended on this thread a few times, and didn't see the problem in taking off with the totals exceeding MTOW. That changed the dynamics of all the W&B components.

 

[facthunter]

The plane must be in balance at all times but the usual points of reference are at take off wt  at landing wt and at zero fuel wt.. Bonanza's had issues with Cof G criticality but the pilot had recency issues with competency in IFR/night also.   Nev

[spacesailor]

One thing l don,t like on the Hummel , is the forward fuel tank. ( pilot sits to the rear to compensate ).

On takeoff the plane is nose heavey (  but in W B ), so longer runway required. 

On landing empty tank means ' tail heavy ', easier to stall on the ' flare ' .

OR 

Am I wrong !.

spacesailor

[turboplanner]

1 hour ago, spacesailor said:

One thing l don,t like on the Hummel , is the forward fuel tank. ( pilot sits to the rear to compensate ).

On takeoff the plane is nose heavey (  but in W B ), so longer runway required. 

On landing empty tank means ' tail heavy ', easier to stall on the ' flare ' .

OR 

Am I wrong !.

spacesailor

Let's see your calcs?

 

[facthunter]

Spacey your basics are correct. Nose heavy will take more lift to get airborne as the download on the tailfeathers counts as if it's extra weight. Nev

[Bruce Tuncks]

Space, can you move the fuel tank back like the Jabiru has it? Even a smaller tank behind you would help

[spacesailor]

Who would alter the designers & aero engineers,  plus test pilots input !.

I have very little schooling, so no,  I think it,s up to the HummelAviation company to to put out a new mod,

(  they have made that little plane,  wider, taller and sleeker ).

The Hummel Co redesigned it as an FAR part 103 ultralight. ( Ultrcruiser )

Then.

Made it bigger for a full four cylinder motor,  H5 ( 150 MPH VNE )

So if were lucky it Will come with retrofitted wing tanks, & only a small header tank. ( alreadt designed by a builder with a 4 pot two stroke )

spacesailor

Edited March 19, 2022 by spacesailor
Already is misspelled

[Garfly]

Regarding W&B calcs, this is a how-to-video from the BMAA

(British Microlight Aircraft Association)

 

 

[facthunter]

I've always seen the datum chosen forward of everything . The principle is the same however except there are no negative moments. Also the pitch attitude of the plane affects the calcs. IF you have a taildragger you have to lift the tail to the correct position when weighing for the initial basic wt and Index Units which is your start point. Nev

[spacesailor]

Is it the same datum, for three wheelers, to be in level  fight position, 

I almost bought a plane, that was dropped while undergoing it,s  w & b check. ( USA ).

In my HB instructions it has to be leveled across the cockpit combing rails, fore & aft !.

Wing incidences are taken from there too.

spacesailor

[facthunter]

The error is more obvious with taildraggers due to the way they sit on the ground. Some horizontal reference is desirable for consistency and accuracy. Nev

[APenNameAndThatA]

On 17/3/2022 at 7:08 PM, Bruce Tuncks said:

Of course it gives you safe flights...  My question is whether or not a 5 knot reduction in the speed would give you flights just as safe, and indeed what that all means.

I’m pretty sure that if you are overweight, you need to go faster, not slower. You need to go faster to develop the same lift at the same angle of attack. Likewise, manoeuvring speed increases with weight increase (you’d think the opposite would be true) but I’m not sure if this still applies when you are over gross . If your COG was too far forward or backwards, you would also want to go faster, I expect, so the horizontal stabiliser did not stall.    

 

In fairness, I note that RA-Aus planes with floats are generally 50kg heavier and structurally the same. Or are they? 

[Bruce Tuncks]

My Jabiru 230 POH says the airframe is stressed for 700kg but it is limited to 600 kg by regulations.

My understanding is that without the  rule of 50 kg more , float planes would be effectively excluded from RAAus

Your aerodynamics are correct apename, but regulations are imposed on top. For example, if we were to turn back the clock and lower the max permitted stall speed from 45 to 40 knots, then a lot of safety stuff ( like extra fuel ) would be thrown out but the crashes would have less energy.

Personally, I doubt that overall safety would be improved, and the crash statistics support this.

[Bruce Tuncks]

Yep, if you look up the accident reports, you will find that "loss of control" is the most common fatal event. The most common reported event is "near miss" .

[Bruce Tuncks]

The "rough airspeed" figure is a regulation which, if lowered by 5 knots, would enable a lot more weight to be carried by the same structure.

This was the tactic used to enable top motors to be carried on gliders.

The rough airspeed is where  the wing stalls when hitting an upgust. The stresses caused by this are in proportion to V^2, so a 5 knot reduction will lower the stresses a lot. Or enable more weight to be carried at the same stress levels.

Stall speed is different, as has been said, this always increases with weight, well for the same lift coefficient it does.

As I said earlier, the upgust is 40 knots by regulation.

Now there are limits to how low you can make a rough airspeed. It is possible for an unusual attitude to allow inadvertent overspeeding. So there is a formula the designers need to apply.

[turboplanner]

4 hours ago, Bruce Tuncks said:

The "rough airspeed" figure is a regulation which, if lowered by 5 knots, would enable a lot more weight to be carried by the same structure.

This was the tactic used to enable top motors to be carried on gliders.

The rough airspeed is where  the wing stalls when hitting an upgust. The stresses caused by this are in proportion to V^2, so a 5 knot reduction will lower the stresses a lot. Or enable more weight to be carried at the same stress levels.

Stall speed is different, as has been said, this always increases with weight, well for the same lift coefficient it does.

As I said earlier, the upgust is 40 knots by regulation.

Now there are limits to how low you can make a rough airspeed. It is possible for an unusual attitude to allow inadvertent overspeeding. So there is a formula the designers need to apply.

Before you marry your theory, take another look at some of the scenarios we've discussed here and the hundreds of accident reports where the pilot was unable to control the aircraft as it was taking off or landing; you don't need rough air in the equation for those cases.

[Bruce Tuncks]

I have a mate who was sucked into a cloud in his glider and who subsequently oversped the whole thing.  He had no effective control with no visual horizon, and he was quickly over the speed at which he could deploy the airbrakes.

I told him that since his glider was made from carbon fibre, there would likely be no hidden damage to the structure as the first sign of a carbon fibre failure would be an explosive breakup.

[Bruce Tuncks]

sometimes I wonder if my advice was correct...

[djpacro]

Quote

The "rough airspeed" figure is a regulation ...

Not a regulation applicable to small aeroplanes.

Quote

The rough airspeed is where  the wing stalls when hitting an upgust. The stresses caused by this are in proportion to V^2 ....

Nope. Nope - gust loads are a function of V not V^2.

 

Quote

As I said earlier, the upgust is 40 knots by regulation

Nope.

[APenNameAndThatA]

On 21/3/2022 at 7:57 AM, Bruce Tuncks said:

The "rough airspeed" figure is a regulation which, if lowered by 5 knots, would enable a lot more weight to be carried by the same structure.

This was the tactic used to enable top motors to be carried on gliders.

The rough airspeed is where  the wing stalls when hitting an upgust. The stresses caused by this are in proportion to V^2, so a 5 knot reduction will lower the stresses a lot. Or enable more weight to be carried at the same stress levels.

Stall speed is different, as has been said, this always increases with weight, well for the same lift coefficient it does.

As I said earlier, the upgust is 40 knots by regulation.

Now there are limits to how low you can make a rough airspeed. It is possible for an unusual attitude to allow inadvertent overspeeding. So there is a formula the designers need to apply.

If Va increases with increased weight, then so will Vno, I expect. Both are counter intuitive. I suppose that the reason is that the engine mounts are designed to break before the wing spar. If the aircraft is heavier, then it will accelerate slower if it is heavier, hence, the increase in Va with heavier weight. 

 

I agree that having a 600 kg MTOW when the plane is designed for 700 kg MTOW is an example of bureaucracy affecting MTOW. On the other hand, that does not mean that it was/is wrong. On the other other hand, I can see an argument for 750 kg MTOW with the stall speed and PAX the same, to increase strength of planes while maintaining energy on impact and low number of deceased people.