Aircraft Losing Velocity At Higher Altitudes

[Moosems]

I created an aircraft in KSP with my limited parts and at low altitudes it gains speed well and flies well for what I have but when I go above 2000m or so it starts losing velocity. I believe it is due to the lack of air at higher altitudes but because I only have the small intake and have reached my parts limit, I'm not sure what to do to get around this. Here is the craft file to the aircraft. If anyone has any suggestions on improvement I'd love to hear them! Images of when the phenomenon starts and my tech tree are below:

https://ibb.co/r28Pm6G
https://ibb.co/411hHCn

[king of nowhere]

without downloading the file, I can assert that it's very normal for early tech planes to have poor performances. just accept that your first low tech plane won't be a great one.

that said, my early tech plane could get to 10 km easily, so it's likely it can be improved. you'll have to wait for someone to download that plane.

i recommend posting screenshots of the plane in flight and in the sph too, with as much relevant data as you can think. few people will want to download a plane and launch it in their game to give you feedback, but a lot more people will be happy to look at the pictures and give feedback on that

[UmbralRaptor]

Honestly, it seems like a reasonable design for what it is. The primary flaw is that it's arguably overweight/has so much fuel, so unless you want to go a very long ways in-atmo, you should drop a tank or two from the design. Other lightening can be done by draining the cockpit of monopropellant, though that doesn't matter much. I'm also partial to disabling the reaction wheels by default to give a more aircraft-like feel (especially with your rather well-defined control surface choices)

 

Note that for completing high altitude contracts, a common trick is to bring some rocket engines along so an aircraft can (briefly) get much higher.