Less TWR and more speed

[Niobe]

Hi , 

I'm trying to understand something in the game for a while now  , some of the plane I made and design in the game have less TWR than others and I have the surprise while flying them to see more m/s on them like what ! For eg I have multiple 1.79 TWR aircraft that can reach MACH 1 ( DELTA wings) without using wet mod engine and some aircrafts with 4 or even 5 TWR and they can't even get 350 m/s  in full dry throttle so I'm stoke ! I'm looking the aerodynamic closely but I can't still explain...do you guys have these kind of problems ? 

[Snark]

Moving to Gameplay Questions.

[RocketPilot573]

Hi, pictures of these crafts would be helpful. I suspect your higher TwR craft are also very draggy. Generally larger more complicated craft can be bogged down by a lot of drag. There is a lot of drag around 350 m/s (trans-sonic speeds) what can seemingly create a wall of drag that wont let your craft accelerate through it. 

[bewing]

Also, it depends on what kind of parts you use to build the things. MK1 parts have 25% less drag than the equivalent MK2 parts.

[Kyrt Malthorn]

1.  Check your high power engines are getting enough air intake.  Notice different air intakes are efficient at different air speeds/altitudes.

 

2.  Consider other variables at work.  Do the high TWR planes have more drag, and are they flying at similar altitudes, flying completely horizontal or diving (getting some help from gravity)?  Try to eliminate as many uncontrolled variables between test flights where possible.

3.   Pay attention to the right click menus of your engines.  They may not actually be producing as much thrust because - like air intakes - they work most efficiently at different speeds and altitudes.  The difference between a wheesley and a whiplash is huge - they fly VERY differently.

[dafidge9898]

Lots of parasitic or induced drag? Yeah, pictures would help. 

[Geschosskopf]

44 minutes ago, Niobe said:

I'm trying to understand something in the game for a while now  , some of the plane I made and design in the game have less TWR than others and I have the surprise while flying them to see more m/s on them like what ! For eg I have multiple 1.79 TWR aircraft that can reach MACH 1 ( DELTA wings) without using wet mod engine and some aircrafts with 4 or even 5 TWR and they can't even get 350 m/s  in full dry throttle so I'm stoke ! I'm looking the aerodynamic closely but I can't still explain...do you guys have these kind of problems ? 

Welcome aboard!

Jet engines have what is called a thrust curve.  That means that the thrust they provide varies with altitude, and different jet engines are optimized for different altitudes.  The small, weak engines you get early in career mode are low-altitude things whereas the bigger, stronger engines you get later are optimized for high altitudes and provide much less than their full thrust at lower levels.  In addition to this, of course, the atmosphere is thicker down low and thinner up high, so there is more drag at lower altitudes.  Taken together, these give each engine a "sweet spot" altitude where the combination of the thrust it produces and the air density enable it to reach its highest speed.

So bottom line, it's complicated.  But in general, a plane with the early engines will usually have little problem in reaching a low supersonic at relatively low altitude because that's where its engines work best, while a plane with the later engines might not reach Mach 1 until 10km altitude, but then it might reach Mach 3 or more at higher altitudes once its engines hit their stride.  Thus, it will go much faster than the other plane, but it has to be in the right place to do it.

[mk1980]

some of the engines are just not meant for high speeds. check the details of the engine stats. they should display a mach number at which the engine delivers the best thrust.

for "slow" engines like the wheesley or goliath, that number is very low. i think the goliath has mach 0 as optimal speed, ie. it works best in slow flight and gets worse the faster you go.

the high end spaceplane engines are the opposite - the whiplash or rapier engines provide only a fraction of their full thrust at low speeds and only really spool up to their full potential at high mach speeds (mach 3.2 for whiplash, mach 3.75 for rapier)

the panther is somewhere in between. i think in dry mode it's optimal speed is mach 1.8 and with afterburners (wet mode) it's mach 2.5 or something. so even in dry mode that engine is a lot better for supersonic flight than the wheesley, juno or goliath

[swjr-swis]

A plane can be pushed to Mach 2 even with engines that are nominally not meant for supersonic flight (Juno, Wheesly), and high thrust engines meant for supersonic flight can struggle to get another plane even close to Mach 1. It all depends on the design.

A quick tip though, since the game gives no indication whatsoever that this may be a problem: check that no wing parts, fins or elevons are in the exhaust path of your engines. Having parts in a jet's exhaust path, even when they're not being heated, is sometimes interpreted by the game as 'blocking' the thrust. Despite everything looking like it is/should be working, nothing heating up and engines reporting full power, you can still get a dramatic loss of effective thrust because of this.

Drag, and especially body drag, is a huge factor. For the main body and engines, generally, try to use the smallest stack size possible (I find that generally, 0.675m > 1.25m > Mk2 > 2.5m > 3.5m > Mk3). Use as few 'stacks' of parts as you can get away with, and add the minimal amount of wings/fins/elevons needed to lift and control.

The more your main body can stay pointed exactly at prograde in flight, the less total drag and the easier it'll be to go supersonic. Adding a few degrees of angle of attack to the wings so you can point directly into prograde can mean the difference between being stuck subsonic or breaking the sound barrier right after lift off. One tick of the rotate gizmo in snap mode while holding shift is 5 degrees, which tends to be a bit more than optimal, but is easier to experiment with, and you'll notice a lot of improvement.

Edited July 26, 2016 by swjr-swis

[pincushionman]

Another suggestion for making high-speed aircraft:

Use FAR.

The drag model in that mod is far more advanced that what is available in stock, and clipping and occlusion are far better represented. If you're running on a PC, that is.