Bf-109 K-4 Air Combat Performance

PM sent SithSpawn.

 

I would be happy to walk you through it with some easy techniques to gauge aircraft performance.

What Crumpp said, establishing the approximate Clmax is a rather simple affair with the proper data available, and thats simple ingame thanks to all the real time data available: TAS, G, Alt etc.

Post it here?

 

How did you measure it?

 

My guess: you rearrange the equation to solve for Cl, then measure the stall speed in level flight (when the AoA is at its maximum). Lift is just the opposite force vector of weight.

Edited November 30, 2015 by gavagai

Yeah thanks, he passed on the formula way back when I asked...

 

Well I didn't really know anyone had answered considering you and Echo commented some 7 days after my post ;)

 

Post it here?

 

Look at Gavagai's post, that's it basically.

Edited November 30, 2015 by Hummingbird

There is some "gotcha's" in the basic formula. Knowing the right units and data to plug into it is the hard part!

 

That is the basic lift formula. It uses True Airspeed in Feet per Second (BGS) and the density found on a standard atmospheric model for the altitude you achieve that speed.

 

You will need to correctly convert from Indicated to True Airspeed for the altitude you record the speed.

 

You can also plug in SI (International Standard...Metric) units.

 

Density (p) is slugs/ft^3 read off the chart.

 

http://www.engineeringtoolbox.com/standard-atmosphere-d_604.html

 

Area is your reference area. Typically it is wing area.

 

 

 

My guess: you rearrange the equation to solve for Cl, then measure the stall speed in level flight (when the AoA is at its maximum). Lift is just the opposite force vector of weight.

 

Lift is NOT always in the opposite direction as weight, only when the A/C is oriented horizontally.

 

In a case such as the aircraft, say, pitching upwards, while the weight vector still points downwards (and will always point downwards), the lift is then out of alignment with the weight.

 

Hopefully I don't seem pedantic, I just want to make sure that's cleared up. This is what I do for a living.

 

Maybe that is why humingbird is getting a low Cl? If you only count the vertical component of lift instead of the whole vector then the Cl will appear smaller.

 

Sent from my SM-G360V using Tapatalk

Maybe that is why humingbird is getting a low Cl? If you only count the vertical component of lift instead of the whole vector then the Cl will appear smaller.

 

Sent from my SM-G360V using Tapatalk

 

Nope, everything taken into account :)

Nope, everything taken into account :)

 

Good to know! I might have to try it myself. But I would have to use the metric system because the only slug I know is a mollusk.

What are the implications of the 109 having too low of a lift coefficient? I thought our 109 already was on the high side of historical climb rates for the given WEP rating, even after the recent reduction. How could the climb rate be higher-than-expected, then, if the lift coefficient is too low? Surely it isn't possible that there's too much thrust and/or not enough drag; if there were, then the top speed would be noticeably off (which I've heard nothing about).

 

I presume that the CLmax was also too low before, when the climb rate was ~6000 ft/min (in which case, again, how can those two problems coexist?); if not, then was it inadvertently lowered during the process of correcting the excessive climb rate? There's so much about this that is puzzling.

Echo keep in mind that CLmax is only reached near the critical AoA, which is not approached when you're in a straight line dash or a steady climb.

 

Also regarding thrust, this depends just as much on propeller design as on available engine HP, thus propeller efficiency at all speed regimes has to be modelled correctly as well. To pick an example the engineers at FW AG designed a prop for the Ta-152 that increased the thrust output over the Dora-9 by approx 50 kgf despite a reduction in max HP of 50.

Edited December 1, 2015 by Hummingbird

I don't know very much about aerodynamics, I'm afraid. Why does an airplane climb better when going faster than the ... oh. I think I get it. There's more lift at that speed / AoA, but also more drag, and so there must be more drag gained from having that high of an alpha, than it gains in lift. Which is why the airplane climbs better (sustained) at a lower AoA (and thus higher speed), even though it has less lift, because it also has much less drag. Did I get it right this time?

I don't know very much about aerodynamics, I'm afraid. Why does an airplane climb better when going faster than the ... oh. I think I get it. There's more lift at that speed / AoA, but also more drag, and so there must be more drag gained from having that high of an alpha, than it gains in lift. Which is why the airplane climbs better (sustained) at a lower AoA (and thus higher speed), even though it has less lift, because it also has much less drag. Did I get it right this time?

Principles of flight tells us climb only depends on excess of power. Lift doesn't make you climb, look at gliders only climbing while in a thermal but indeed while losing altitude, just warm air moves faster upwards than they lose altitude. The 109 has a great climb speed because the stunning excess power it has since the very early versions. Probably related also to the fact that is a quite small airplane compared to usual fighters so drag is also smaller making you have even more excess power.

 

S!

I don't know very much about aerodynamics, I'm afraid. Why does an airplane climb better when going faster than the ... oh. I think I get it. There's more lift at that speed / AoA, but also more drag, and so there must be more drag gained from having that high of an alpha, than it gains in lift. Which is why the airplane climbs better (sustained) at a lower AoA (and thus higher speed), even though it has less lift, because it also has much less drag. Did I get it right this time?

 

Drag has two components, parasite drag (roughly the base drag of the airframe compontents, depending on air resistance) and induced drag (which is largely dependent on the AoA of the airplane).

 

At low speeds and high speeds, these two drag components have different effect, in very basic terms, parasite drag component dominates total drag in high speed flight, induced drag component in low speed flight, such as climb (since at low speed, to maintain enough lift you have to increase the "attitude" of the aircraft, i.e. the angle of attack).

 

The weight, size, the wing area (roughly total lift, but lift coefficient can "cheat" there) - power (thrust) - weight relations (wing loading, power loading, surface/power) can produce different qualities for an aircraft that ultimately decide if its fast in level flight, dive, or climbs well.

 

Greater excess power of course will improve all of these, but usually at the expense of increased weight, which will again take away from it, i.e. likely to increase the AoA needed, that is, induced drag.

 

Speed and climb speed characteristics will mainly depend on how parasite drag and induced drag relate to each other at high/low speed... they add up, parasite drag is always there but induced drag is minimal in high speed flight, but a major component in low speed flight. For example an aircraft with low parasite drag but moderate induced drag will be very fast, but mediocre in climb rate.

 

Back on topic, Willy M essentially aimed for maximum power, minimum parasite drag (small and clean airframe and wing area) and moderate induced drag. The latter was achieved with small wings (increase induced drag) combined with a light airframe (reduce induced drag), also helped out by high lift devices that only add drag when needed, i.e. hard turns and landings (slats and flaps). Yakovlev followed a similar route, albeit he made even cleaner and lighter aircraft, but this was somewhat offset by the lacked powerful engines.

So, if the current 109's CLmax is too low, and it gets corrected, what can we expect to be different in flight? Same sustained climb rate, but a lower stall speed? Tighter turn? Faster turn?

When can we adjust the rounds in our guns? That's the real question. This was a feature that Cliffs of Dover had that I don't think any other sim has ever done. I would really like to be able to adjust my tracers so they alternate :(

I dont think it is, the climb rate was already adjusted, I dont think there is any issue there anymore, least that I have heard from Yo-Yo.

 

I thought you just confirmed Hummingbird's assertion that the CLmax was too low, using Crumpp's formula? I am so confused.

: /

When can we adjust the rounds in our guns? That's the real question. This was a feature that Cliffs of Dover had that I don't think any other sim has ever done. I would really like to be able to adjust my tracers so they alternate :(

 

I think you can already adjust them for single player by editing the weapons file.