Overly optimistic plane speeds in game decription or ...?

[Paparazzic]

Are the speeds that planes can apparently achieve (according to game description)  way overblown or I just don't know how to fly those planes (totally possible)? What concerns me is that according to game description speed goes up with altitude yet my tests show total opposite, speeds go down. Here's the example I did with 109F4 but you can pick any other plane it seems to be similar problem. I did tests on Moscow autumn map, with auto rads 1.3 and 1.42 ata, 50% fuel. Where's the catch? 

 

alt (m)    B109F4 
             1.3ata       1.42ata        game desc (combat)    
1000    517           538              522
2000    517           534              570
3000    508           525    
4000    496           515    
5000    484           505    
6000    464           480              635

Edited June 25, 2018 by 51IFGmishomor

[[APAF]VR_Spartan85]

I don’t mean to doubt you but is that IAS or TAS?

[EAF19_Marsh]

Probably what Spartan said, Indicated vs True Air Speed. If you are going by the cockpit indicator then you are receiving the measure of air pressure on the pitot tube. Since air pressure declines at altitude, this will be increasingly different from your True Air Speed (how fast you are crossing the ground) which is what the game description lists. Not sure if the HUD gives you TAS.

 

Rule of thumb I was once told, take half your altitude in FL thousands of feet and add it to your IAS to get roughly your TAS. So 6000m is roughly FL200 / 2  = 100 add to 250 (about your 480 km/h in kts) and you get 350 kts or 640 km/h - roughly what the description says.

 

 

Edited June 25, 2018 by EAF19_Marsh

[HandyNasty]

using the following site http://www.newbyte.co.il/calc.html to convert CAS to TAS, I get for example for your 6000m 

 

464 / 480  --> 620 / 641  (compared to the 635 it's says ingame for combat power)

 

Note : not 100% sure the site is perfectly right, I use it as an indicator for CAS to TAS

[Paparazzic]

Ah thank you guys, totally forgot about that :). HUD shows same value as speed indicator so that would be IAS then. 

 

 

[Finkeren]

Also: If you plan on doing tests in the future, always use 100% fuel. The specs in game is for 100% fuel.

[Paparazzic]

1 minute ago, Finkeren said:

Also: If you plan on doing tests in the future, always use 100% fuel. The specs in game is for 100% fuel.

 

Once I recalculated numbers I guessed that 50% probably wasn't used for testing. Thanks Finkeren. 

 

 

[Bilbo_Baggins]

2 hours ago, Finkeren said:

Also: If you plan on doing tests in the future, always use 100% fuel. The specs in game is for 100% fuel.

 

Weight is almost irrelevant for top speed in level flight though. Other factors like atmospheric pressure and trim/stab settings, radiator settings have orders of magnitude greater effect on level top speed. 

 

Take a fighter with full fuel vs 50% and there will practically be no difference in top speed. Weight affects acceleration/climb. Simple physics. Take a fighter with 100% fuel and you'll find it flies the same top level speed- just takes slightly longer to get there.

Edited June 25, 2018 by Mcdaddy

[303_Bies]

It depends on amount of fuel airplane can take. In case of big fuel load angle of attack of loaded plane will be bigger, it means greater drag and lower speed.

[Finkeren]

1 hour ago, Mcdaddy said:

 

Weight is almost irrelevant for top speed in level flight though. Other factors like atmospheric pressure and trim/stab settings, radiator settings have orders of magnitude greater effect on level top speed. 

 

Take a fighter with full fuel vs 50% and there will practically be no difference in top speed. Weight affects acceleration/climb. Simple physics. Take a fighter with 100% fuel and you'll find it flies the same top level speed- just takes slightly longer to get there.

 

This would be true, if there wasn't that small factor of wings having to provide lift. The greater the mass of the plane the greater lift will have to be generated by the wings. With all other things being equal (engine power, atmospheric conditions etc.) the only way for an aircraft to generate more lift at a given airspeed is to increase angle of attack, which - as bies explained - causes increased drag. The same engine power not only has to accelerate a greater mass (which would already lead to a slightly decreased top speed) it also has to overcome greater drag.

 

For obvious reasons, the gains in speed with a reduced fuel load greatly depends on how big a proportion of the plane's total mass is made up of fuel and to a lesser extend how the reduction in fuel load affects the CoG of the plane. For planes like the La-5FN it is hardly worth the trade off of reducing the already very short endurance, but for planes like the Fw 190 and P-40 it can make all the difference in the world.

[ZachariasX]

2 hours ago, Mcdaddy said:

 

Weight is almost irrelevant for top speed in level flight though. Other factors like atmospheric pressure and trim/stab settings, radiator settings have orders of magnitude greater effect on level top speed. 

 

Take a fighter with full fuel vs 50% and there will practically be no difference in top speed. Weight affects acceleration/climb. Simple physics. Take a fighter with 100% fuel and you'll find it flies the same top level speed- just takes slightly longer to get there.

You think lift doesn‘t cause drag? Honestly?

[Aplana_Moscuu]

The 109 needs the stabilizer trimmed for stable flight to achieve optimum speed. If you are pushing or pulling you are braking the airplane with the elevators. For example the F-4 with tecnochat advised maximum continuous power (about 1.195 ATA [63 %]), the elevator needs to be at -79 % to achieve optimum speed and radiators manually open at about 9 %.

 

The same goes naturally for the 190's and the 202.

Edited June 25, 2018 by Operation_Plumbbob

[dkoor]

2 hours ago, bies said:

It depends on amount of fuel airplane can take. In case of big fuel load angle of attack of loaded plane will be bigger, it means greater drag and lower speed.

This is very true. For some cases (fighters) difference can be couple of ten liters while on some types difference can be stellar one tonne, perhaps even more!

Differences are astounding regarding this, and it really all depends on what was the intended use of the particular airplane.

[EAF19_Marsh]

Quote

You think lift doesn‘t cause drag? Honestly?

 

I think his point was the fuel load is not such a significant impediment to maximum speed versus its effect on other performance areas such as climb. Obviously it will have an effect, but given the fuel fraction of these aircraft, it may not be that much.

 

Off my head, if lift required at max speed empty is 1 and empty to full internal weight (fuel) is 20%, you need to develop 1.09% of lift to offset the additional weight (120% of empty weight), which is roughly 1.09% extra drag (actually probably more) so your maximum level speed will be maybe 10% lower but not as much as your climb and so forth will be affected.

 

Typing this while eating a sandwich, so may have messed up the calculation...

[AndyJWest]

The effect of weight on speed depends on the proportion of induced drag to total drag. At low altitudes, this tends to be relatively small, meaning that weight has less effect. Tests at higher altitude should show increasingly greater effects.

[ZachariasX]

18 hours ago, EAF19_Marsh said:

I think his point was the fuel load is not such a significant impediment to maximum speed versus its effect on other performance areas such as climb. Obviously it will have an effect, but given the fuel fraction of these aircraft, it may not be that much.

 

This is a clear "it depends". As said.

18 hours ago, dkoor said:

Differences are astounding regarding this, and it really all depends on what was the intended use of the particular airplane.

 

How much indiced drag you are producing depends on the wing loading and its shape (and profile). Both of wich are a function of the purpose of the aircraft. This means, if you have a high altitude interceptor, it will have a relatively low wing loading, as opposed to an aircraft that is meant to go fast down low. It is also of note that the wing should work in an optimal way at the projected flight speed.

 

This means that if you have for instance a high wing loading (for a plane that is really meant to go fast, adding weight will induce a higher penalty in induced drag, as there will be a larger AoA increase required than a larger wing. Induced drag is very sensitive to the wing AoA. You can expect there a high sensitivity to added weight.

 

If you have a large wing with a low wing loading (like the Spit) you require less increase of AoA. Plus the wing shape there also halps to reduce induced drag. The price to be paid for such is that at high speeds down low, the AoA of the wing actually is smaller than optimal, resulting in added aerodynamic. It makes the Spit not a great performer on the deck, unless you clip the wings.

 

How much weight differences make, you can always see in airliners. The fuel flow (throttle setting) required to remain at cruise speed is considerably less at the end of a long flight than at the beginning where the tanks are full.

 

The other exreme are sailplanes. They have such a light wing loading that you can actually put in about an added 25% weight WITHOUT any noticeble penalty in induced drag. The lift to drag ratio will hardly change. This extreme example is facilitated by adding flaps over the whole span of the wing that maintain optimal flight attitude at all speeds (at minimum weight the optimal speed for best glide is lower than at higher weight) plus it reduces the lift of the wing profile.

 

On the whole, this might be true

18 hours ago, dkoor said:

For some cases (fighters) difference can be couple of ten liters while on some types difference can be stellar one tonne,

but, it is not only plane dependent, but also situation dependent.

 

 

[dkoor]

7 hours ago, ZachariasX said:

How much indiced drag you are producing depends on the wing loading and its shape (and profile).

...

The other exreme are sailplanes. They have such a light wing loading that you can actually put in about an added 25% weight WITHOUT any noticeble penalty in induced drag. The lift to drag ratio will hardly change. This extreme example is facilitated by adding flaps over the whole span of the wing that maintain optimal flight attitude at all speeds (at minimum weight the optimal speed for best glide is lower than at higher weight) plus it reduces the lift of the wing profile.

Sure mate, but I indicated that the main reason why aircraft have more or less fuel is in its application, or intended use.

Another words, you wont load 2 tons of fuel on attack aircraft that operates from first front line in some 100-200km radius. Appropriately you wont load 400 liters in Mustang and send it across half Europe.

But I think you really misread what I wrote, I haven't had drag on my mind at all.?

 

The other exreme are sailplanes. They have such a light wing loading that you can actually put in about an added 25% weight WITHOUT any noticeble penalty in induced drag. The lift to drag ratio will hardly change. This extreme example is facilitated by adding flaps over the whole span of the wing that maintain optimal flight attitude at all speeds (at minimum weight the optimal speed for best glide is lower than at higher weight) plus it reduces the lift of the wing profile.

 

This is really interesting, I didn't know that.

 

Edited June 26, 2018 by dkoor