3D Printed MiG-21 70mm/64mm research

[GH05T]

Just gonna try and see if I can take telnar1236's MiG-21 project and get it up and running, I've been looking forever for a starting point because my scaling has been horrible. I think I could get their plane to fly by using the cooling ducts that the real plane uses and turning them into air ducts, and maybe by using a very expensive edf that used two paychecks to buy.

 

[GH05T]

Day three on the printing process, I finally got the first part to print on my ender 3 v2, I'm using polylite lw-pla to save weight and also have the same printing efficiency as normal pla. Currently the first part has 36 hours to print and we're at 15 hours in so almost halfway there but I'll keep making updates.

 

[GH05T]

Print has finished, I noticed that the wall thickness isn't strong enough for the lw-pla to handle any loading so I have balsa wood bulkheads being made. Pictures will be added of the progress shortly.

 

[telnar1236]

Print has finished, I noticed that the wall thickness isn't strong enough for the lw-pla to handle any loading so I have balsa wood bulkheads being made. Pictures will be added of the progress shortly.

Looking forward to seeing what you make of this. All the parts were designed for normal PLA so do not expect LW PLA to be strong enough for anything. The balsa is a good idea and might help a lot

 

[GH05T]

Soooo I may have sat something over the two fuselage pieces that I had been working with but I am printing new ones, polymake lw pla works fine for weight but strength is a bit low, the bulk heads are still being made due to me doing them by hand with a wood burning tool and sand paper but I am using my ender 3 pla for the fuselage now

 

[GH05T]

Sooooo my 64mm edf can do a max of 1.78kg or 1780g. I may find a way to fit a 70mm that produces about 2.36kg or 2364g of thrust due to the performance gain and potentially having about twice the thrust to weight. Would that work or am I just trying to use overkill at this point?
Freewing 64mm 4s 3300Kv edf and the flitetest 6s 70mm edf

Also I was wondering if it's a good idea to coat the plane with either enamel or liquid glue, even resin could help strength and smooth flow.

 

[GH05T]

soooooo the 70mm didn't fit in the plane, so I had to increase the design size and eventually I ended up designing my own fuselage, and everything except the front nosecone has bee increased and the wall thickened to support both the 70mm I have and the lw-pla that I really want to use. I have changed wall thickness at the cost of almost no weight gain due to the lw-pla being so lightweight, the previous nose print I did was too thick and yet was super light, after I print all of the fuselage I'll get the weight but at the moment the only change is the thickness and size.

(also just learned to scale and design everything)

 

[GH05T]

Now I am not a professional plane engineer, however by the laws of how planes fly I see a problem with trying to create inlets under planes. As much as it increases airflow to the edf, planes fly because of Bernoulli's equation of pressure (as air flows over the wing it's faster so it has less pressure, the flow under is slower with high pressure) which also applies to the body. These inlets do disrupt that pressure and make it slightly lower, and if they are on the top it prevents that air from sticking to the plane and creating that lift. Either the models are so small that aerodynamics changes due to the surface area ratio actually multiplying due to the downsizing, or I need to study more into the actual math behind adding inlets.

 

[telnar1236]

soooooo the 70mm didn't fit in the plane, so I had to increase the design size and eventually I ended up designing my own fuselage, and everything except the front nosecone has bee increased and the wall thickened to support both the 70mm I have and the lw-pla that I really want to use. I have changed wall thickness at the cost of almost no weight gain due to the lw-pla being so lightweight, the previous nose print I did was too thick and yet was super light, after I print all of the fuselage I'll get the weight but at the moment the only change is the thickness and size.

(also just learned to scale and design everything)

Redesigning everything is definitely the right move. My original design was just bad, and definitely could not have supported a 70mm EDF, even if you could have made it fit somehow. That's the nice thing about LW-PLA, you can add thickness and get more than linear returns on strength for a linear increase in weight.

 

[telnar1236]

Now I am not a professional plane engineer, however by the laws of how planes fly I see a problem with trying to create inlets under planes. As much as it increases airflow to the edf, planes fly because of Bernoulli's equation of pressure (as air flows over the wing it's faster so it has less pressure, the flow under is slower with high pressure) which also applies to the body. These inlets do disrupt that pressure and make it slightly lower, and if they are on the top it prevents that air from sticking to the plane and creating that lift. Either the models are so small that aerodynamics changes due to the surface area ratio actually multiplying due to the downsizing, or I need to study more into the actual math behind adding inlets.

In terms of cheater inlets, there is a very small loss in lift if they are below the wing, but almost all of the lift comes from the decrease in pressure over the wings as opposed to the increase in pressure under them. For a delta like the MiG 21, this is even more true, especially when slow/at high angles of attack. Putting inlets above the wing, will have a minimal effect in terms of hurting lift, so long as they aren't on the wing itself, but will starve the EDF of air and dramatically reduce thrust when the plane is slow. Either above or below the wing, inlets will disrupt flow minimally except for the area right by the inlet, so inlet position is almost entirely dictated by EDF performance. Essentially the EDF just doesn't move enough air to be a problem aerodynamics-wise. Much more important than inlet position is inlet geometry. Ideally you want about 110% FSA since you get nearly the max static thrust with minimal losses in terms of thrust at speed.

 

[GH05T]

I'm back on the project again, I had some personal problems as well as a very high power bill to deal with but now I'm just designing and getting the prints done. I can however say that there was a very sketchy was I could find the measurement of every piece of a mig-21 and that allowed some changes to be made, however now it's just the days of printing that I have to wait for.

 

[GH05T]

Also I learned today that it's so lightweight that I can use the same design for a 64mm edf build, however I see an issue with the power my 64mm has. The motor on it is more powerful than my 70mm motor yet the super small blade diameter limits it, but I can say that it is the most powerful 64mm I have seen. (Even if it uses a super small 4s with small bullet connectors)

 

[GH05T]

Back again with an update, I have finally modeled the wings completely. I made them symmetrical so it can fly upside down and the leading edge has slots for some planned spars, I do know that the real mig 21 has these bits at the tips, at least the bis variant does, and that means the tip is actually thicker than the root, however my wing is the opposite due to how I want the aerodynamics to work. Next up is the stabilator that I plan to make, also I have been using selfCAD for the whole project due to the fact that I learned how to design anything using it

 

[GH05T]

Also I have a question for anyone who may have thought about ducting for edf powered planes, if I ran a highspeed fan ahead of the edf to suck in air for slow speed flight, would that increase the overall performance or hurt the highspeed performance?

 

[telnar1236]

Also I have a question for anyone who may have thought about ducting for edf powered planes, if I ran a highspeed fan ahead of the edf to suck in air for slow speed flight, would that increase the overall performance or hurt the highspeed performance?

Broadly speaking, you would be putting two EDFs in series if you did that. You might get a bit more performance, but you would double the weight and power consumption of the power system without even close to doubling thrust. You're almost always far, far, better off just increasing the diameter of one EDF. EDFs get a bad reputation for slow flight, but with proper ducting, a strong enough EDF unit, and cheater inlets, they don't actually do that badly.

 

[GH05T]

*status update* So here's a screenshot of the final iteration of the second and third fuselage pieces redesigned to 70mm edf scale, the thrust tube for the edf is integrated in the third section 175mm inside of the 200mm section, there is an angled duct on the bottom in front the tube, the angle was based off of my calculation for slow flight aerodynamics. The second section houses the lightweight receiver, the nose gear housing, and the esc. After two months of hard work I have the second and third sections, the symmetrical wing sections and components, and diagrams with accurate measurements of each section, however I recently have come across some more freetime which can help me move this project along faster. *status update*

 

[GH05T]

Ok, so I have some Diagrams for the MiG-21 that I'm using for a partial reference, however I have them for the ducting and the scale of the build.

 

[GH05T]

Almost done with the build, all those weeks of hard work finally came together to make my dream plane. Currently I am waiting for a set of servoless landing gear to come in so I can get measurements for my cad software, it'll help me actually implement them into the design without having to cut into the build. I also may have snuck in my own variable nozzle in the last section, which is going to be within the nozzle cover section I'm designing after the one on the actual mig 21. I've finally got wing design perfect after spending hours finding programs and tutorials, I've come up with using onshape to get a length, and then using a program somebody graciously made for users to easily make airfoils from the NACA library. TSAGI 12% happens to be the MiG-21 airfoil design, (you will not believe the difficulties in finding this information online), which happens to have more efficiency at lower-to-mid speeds which is why the actual MiG-21 was better in a low energy state of flight.

 

[telnar1236]

Almost done with the build, all those weeks of hard work finally came together to make my dream plane. Currently I am waiting for a set of servoless landing gear to come in so I can get measurements for my cad software, it'll help me actually implement them into the design without having to cut into the build. I also may have snuck in my own variable nozzle in the last section, which is going to be within the nozzle cover section I'm designing after the one on the actual mig 21. I've finally got wing design perfect after spending hours finding programs and tutorials, I've come up with using onshape to get a length, and then using a program somebody graciously made for users to easily make airfoils from the NACA library. TSAGI 12% happens to be the MiG-21 airfoil design, (you will not believe the difficulties in finding this information online), which happens to have more efficiency at lower-to-mid speeds which is why the actual MiG-21 was better in a low energy state of flight.

It's starting to look like a MiG!

 

[GH05T]

Wait until you see the new engine ducting and adapter, I fitted it to hold up to 80mm of edf space and it will seal the ducting and create suction to pull air to the edf efficiently. So even a 64mm edf could fit with a seal and still be efficient!