Help me define aircraft specifications!

[Mjau]

Hello,

 

Just want to say I'm new on the forum here and so I hope I put this thread in the right place!

 

So, I was sitting on wikipedia comparing different aircraft specifications and what not. Specifically I wanted to compare some different thrust to weight ratios. :book:

 

Now I see that there are a few different types of weights to consider making this tricky for me, since I don't know what they mean!

 

There is empty weight: As I understand the weight of the aircraft as it comes from the factory. No fuel, no oil, no pilot, no nothing. Just air-frame, engine and avionics essentially?

 

Maximum take-off weight: As I understand this is what you divide the maximum thrust at sea level with to get the T/W ratio. So is this with all the fuel and weapons on you can load it up with? Then an aircraft that can carry more weapons, even though it's lighter and has more thrust would get a lower T/W by comparison? Doesn't sound very fair!

 

Another problem is the next figure, loaded weight. Now what exactly does this mean? Or, wait wait! Is this the true fully loaded configuration and the maximum TO weight is just theoretical?

 

THen how do you work that number out? Ugh, as you can tell I am confused!

[Exorcet]

You're pretty much right with empty weight, that is just the plane. Using the empty weight for TWR can make sense if you're not concerned with fuel or payload.

 

Max takeoff weight is weight that the aircraft structure can safely hold on takeoff. It's not necessarily representative of a typical weight the plane will see in use and so isn't usually used for calculating TWR.

 

Loaded weight is the weight with a normal fuel and payload, you might use this to get an idea of TWR of a plane in combat.

 

The truth is that there is no one fixed number, because weight will change over time due to fuel burn. For military aircraft, it's common to use somewhere around 50% fuel load to calculate the TWR as it would be in combat.

 

If using Wikipedia, I'd suggest doing the following:

 

Take empty weight + fuel weight + weapon/payload weight and use that. If you're making a spreadsheet, you may want to make the fuel weight adjustable by fuel fraction as this is a way to even out fuel loads between aircraft of different size and weight.

[Mjau]

Thank you for the answer! :D

 

This to me indicates that the importance of t/w ratio is a bit overrated. Unless you use empty weight T/W since it doesnt tell the whole story about performance considering payload and stuff.

[escaner]

Actually empty weight from the aircraft manual includes all the fluids needed for its operation (oil, hydraulic, coolants, etc.) and non usable fuel, which basically is the fuel that remains in the tanks that will not reach the engines.

[Basher54321]

There can be different definitions of Empty weight also :)

 

You could of course put it on a spread sheet and do the values from empty to max take off weight.

 

Also T/W does not take into account for any drag caused by airframe / stores/pylon/lift.

 

Also the given thrust figures out there are static only - the actual thrust produced by a jet engines varies massively depending on speed and altitude.

 

However what you are doing will give you more of an understanding - so it is a good exercise.

[Mjau]

Yeah, spreadsheets sounds like a more meaningful way to compare since it varies so much. :P Raw numbers can be deceiving!