Smuggle an aircraft it's a fantasy. An aircraft needs a lot of support. Even more if you have to assemble it on site. I'm saying this because of experience.
Where are you going to operate from?. An airbase with it's runways destroyed by ballistic missiles attack?.
The only way to operate an aircraft In a clandestine way is like the swedish air force, from a road. But those in Sweden are build taking into account the operation of an aircraft and besides that the swedish desing their aircraft to operate them from roads with minimum support. The MIG29 is not design to operate on such manner. I don't believe there is a road with supporting structures to conceil aircrafts and equipment capable of supporting this type of operation in Ukraine. I believe the Ukrainian forces needs drones, like the Turkish ones not migs29. If they try to pull something like that it's just a matter of time before de RuAF bomb the operation.
But let's assume for a second they were able to assemble 20 Mig29. What they can do against a contested airspace full of interceptors and extremely capable anti aircraft weaponry?. Do you believe that they can achieve air supremacy with a clandestine air force based on a couple of obsolete mig29? Why the outcome would be different than before when they had a descent amount of Su27/mig29/Su25/Su24 and AA defenses?
They need to put a stop to this madness. They lost. They have to come to terms with reality. I know it's not easy to accept that a "foreign" Agressor has beaten you, but a good commanding officer knows when to rise a white flag. Specially when you are fighting among civilians. You have to think in the common citizen that despite the war has to keep on living. Has to go to work, plan their futures etc. I hope they realized this before the Russians loose their patience and pull a "Grozny" on Kiev or Kharkov or the entire country. NATO, UK, Biden etc, they don't give a them about the common ukranian citizen. Macron is right, Russia deserves respect. Security in Europe will be achieve with Rusia integrated into Europe. Look at the whole world economy for a second. Look what is going on with commodities prices. They are in their highest historical prices. Even food. And when food increase its value more people starves around the world. Sometimes I miss Trump pragmatism.
