Understanding "Wing Loading" and "T/W" ratio

[stealthflanker]

Greetings all.

Well as the title said, about "wing loading" As far as i read.. that quantity along with T/W ratio are the "Layman's gate" to understand fighter aircraft agility and performance. Further reading at the L Shaw's Fighter Combat :Tactics and Maneuvering indicated that those two quantities allow four tactical "qualities" for fighter namely superiority in climb and dive (Energy) Superiority in horizontal maneuver (Angle fighter) Superiority in both and inferior in both Energy and maneuvering.

There's however one confusing thing for me, how should one calculate wing loading ? Should it be calculated based on "Wing reference area" OR "total lifting area" Which include some lifting portion of the fuselage (Say LERXes like Su-27) Usual data i saw however typically use wing reference area.

The Shaw's book however showed both approach here :

Which one that could give "closer to reality" value ? The one using reference wing area or the one using "total area that help wing to generate lift such as LERXes or that F-14's "tunnel" between engines" ?


Another thing is the engine thrust to weight ratio. The thing i noticed is that literature give data on the Static Uninstalled SLS thrust, meaning that the thrust value is measured when the engine is at testbench and at sea level altitude. Thus calculated T/W ratio might not be correct. One solution i found was to multiply that SLS thrust with 0.78 to account lossess caused by the inlet system. The source of that 0.78 value was Yefim Gordon's Famous Russian Aircraft book.

Furthermore i expanded it by literature (Raymer) To allow prediction of thrust value at various altitude and speed.

Nonetheless i wonder if my approach is acceptable ?

Hmm that is all of my questions. Thank you very much for any response from the community.

 

[AeroFranz]

Raymer is definitely your friend.
The thrust lapse with altitude should be modeled, for turbojets it can be approximated with multiplying the sealevel max thrust rating by the ratio of the density at the altitude you are considering to the sealevel value. For this you need a standard atmospheric table, easily found online.
So for example, if you want to know how much thrust a turbojet rated for 10,000 lb at sea level will produce at 20,000 feet, you take 10,000 * (Rho_20,000'/Rho_sealevel) = (roughly) 5,000 lbf.
So if you know your current weight, sealevel thrust, and altitude, you can calculate the thrust to weight ratio.
You correctly mentioned the issue of calculating wingloading on a platform that derives large amounts of lift from the body. Unfortunately there is no precise rule, there is even scarce agreement between Europe and the US on whether the portion of the wing that is buried in the fuselage should be counted towards the total area. And this for tube-and-wing configurations, so lerxes and strakes are even harder to account for.

 

[sferrin]

Raymer is definitely your friend.
The thrust lapse with altitude should be modeled, for turbojets it can be approximated with multiplying the sealevel max thrust rating by the ratio of the density at the altitude you are considering to the sealevel value. For this you need a standard atmospheric table, easily found online.
So for example, if you want to know how much thrust a turbojet rated for 10,000 lb at sea level will produce at 20,000 feet, you take 10,000 * (Rho_20,000'/Rho_sealevel) = (roughly) 5,000 lbf.

Does ram-effect come into play? For instance, you might be at 20,000 feet but going 600 mph is going to force more air down the intakes than 200 mph. Also, wouldn't it depend on what altitude your intake is designed for? One intake may spill that extra air overboard at 600 while another might use it all.

 

[AeroFranz]

Absolutely. Besides altitude there is an airspeed dependence. Turbojets can actually see their thrust slightly increase at a given altitude at higher speeds, while turbofans, especially the higher bypass types, see a decrease in thrust.
For subsonic flight, you can assume the inlets, regardless of type, will have decent pressure recovery. For supersonic flight this ceases to be true and different inlets will yield different losses.
So modeling performance (steady, turning, climbing, etc.) can get pretty complicated quickly. You may be able to do a decent job in the simple cases where the approximations hold (like that of thrust variation with density ratio).

 

[Avimimus]

Further reading at the L Shaw's Fighter Combat :Tactics and Maneuvering indicated that those two quantities allow four tactical "qualities" for fighter namely superiority in climb and dive (Energy) Superiority in horizontal maneuver (Angle fighter) Superiority in both and inferior in both Energy and maneuvering.

A word of caution - this way of breaking things down is fairly recent. If you are trying to analyse WWI as well - climb rates and acceleration in level flight are just as important. A major reason is that the sustained turn rates of most fighters were very low as a result of a low trust to drag ratio - so fighters could only use their full maneuverability if they alternated turns with periods of energy recovery (i.e. climbing or accelerating).


Another note: Absolute turn rate and instantaneous turn rate are two very different things. Aircraft can be slow to enter into a turn but still quite maneuverable etc.

 

[Bill Walker]

Concerning wing loading, I don't think there is a "correct" way. If you are using the value to compare two aircraft types, it is more important that the same calculation method be used for both types.

However, the effect of wing loading on maneuvarability and agility is sometimes indirect. Given that extra lifting areas, like LEXs, may have different efficiencies than other lifting surfaces, I suspect simple wing loading calculations become less meaningful when comparing two types of aircraft with different extra lifting surfaces. Your best comparison is measured maneuvering performance, like instantaneous and sustained turn rates. For theoretical aircraft types, a more detailed calculation of maneuvering performance, including wing loading, t/w, drag, installed thrust, g limits (aerodynamic and structural), control power, etc., would be more meaningful.

 

[riggerrob]

Finally, consider that not all lifting surfaces are lifting during all modes of flight. for example, twisted wings may mean that wing tips are not lifting during cruise and only lift at steep angles of attack (e.g. landing).
Similarly, LERX may be set at such shallow angles of attack that they provide no lift during cruise and only start lifting during steep pitch-ups.

 

[Sundog]

Raymer is definitely your friend.
The thrust lapse with altitude should be modeled, for turbojets it can be approximated with multiplying the sealevel max thrust rating by the ratio of the density at the altitude you are considering to the sealevel value. For this you need a standard atmospheric table, easily found online.
So for example, if you want to know how much thrust a turbojet rated for 10,000 lb at sea level will produce at 20,000 feet, you take 10,000 * (Rho_20,000'/Rho_sealevel) = (roughly) 5,000 lbf.

Does ram-effect come into play? For instance, you might be at 20,000 feet but going 600 mph is going to force more air down the intakes than 200 mph. Also, wouldn't it depend on what altitude your intake is designed for? One intake may spill that extra air overboard at 600 while another might use it all.

Also, to add to what Aerofranz noted, in terms of your inlet design it depends on the maximum speed and whether or not is a variable geometry inlet. Obviously the VG inlet will operate more efficiently across the speed range, but you pay the penalty of higher weight and complexity. If it's a fixed inlet it is usually optimized for one design point while trying to minimize the losses throughout the rest of the range. If you look at any of the flow equations, altitude is taken into account by air density (Greek symbol rho). Also, where you can see the affect of ram air is in planes that have auxiliary inlets. Now note, you see these on aircraft with fixed inlets, since they can't vary their area especially when they are on the ground. They need the extra inlets to make up for the lack of the ram air affect to develop full thrust at low speeds. You'll see these on the F-5 (sides near the engine face), the B-2 (top of the nacelles), F-22 (top of the fuselage near engine face), etc. Now the F-16 and F-18 don't have these, so I assume they are OK with the reduced thrust until they get rolling fast enough or the inlets are sized to be larger than they need to be at high speed. It's always design choices and compromises.

 

[Sundog]

Greetings all.

Well as the title said, about "wing loading" As far as i read.. that quantity along with T/W ratio are the "Layman's gate" to understand fighter aircraft agility and performance. Further reading at the L Shaw's Fighter Combat :Tactics and Maneuvering indicated that those two quantities allow four tactical "qualities" for fighter namely superiority in climb and dive (Energy) Superiority in horizontal maneuver (Angle fighter) Superiority in both and inferior in both Energy and maneuvering.

There's however one confusing thing for me, how should one calculate wing loading ? Should it be calculated based on "Wing reference area" OR "total lifting area" Which include some lifting portion of the fuselage (Say LERXes like Su-27) Usual data i saw however typically use wing reference area.

The Shaw's book however showed both approach here :

Which one that could give "closer to reality" value ? The one using reference wing area or the one using "total area that help wing to generate lift such as LERXes or that F-14's "tunnel" between engines" ?


Another thing is the engine thrust to weight ratio. The thing i noticed is that literature give data on the Static Uninstalled SLS thrust, meaning that the thrust value is measured when the engine is at testbench and at sea level altitude. Thus calculated T/W ratio might not be correct. One solution i found was to multiply that SLS thrust with 0.78 to account lossess caused by the inlet system. The source of that 0.78 value was Yefim Gordon's Famous Russian Aircraft book.

Furthermore i expanded it by literature (Raymer) To allow prediction of thrust value at various altitude and speed.

Nonetheless i wonder if my approach is acceptable ?

Hmm that is all of my questions. Thank you very much for any response from the community.

If I was calculating the wing area for the F-14, I would use the area shown on the left for low speeds. The fuselage glove area and area near the tails aren't going to add that much to the strong flow field from the wing with them extended. Where I would add the fuselage area in is with the wings fully swept, in which case I would calculate it like a delta wing. As for how much to add in between those states is what wind tunnels and CFD are for. I'm referencing level flight. As noted up thread, once you start going to higher alpha you get vortices from gloves/strakes/fences and that is usually highly non-linear and very unique to the flow field of each design.