3D Printed 64mm MiG 21

[telnar1236]

Have/can you do microcontroller stuff... Arduino, soldering and such? If so and you're not in a hurry (at least March, I'm doing the FTFC23 thing), the cost can certainly be improved. I've worked with these on things like weather stations and altimeters.

• The microprocessor can easily be programmed to process the pressure sensors (and any others you might want) store the data and/or send it to a base station (say a phone, tablet or laptop). This specific module includes WiFi and has pretty good range especially since the plane is above you in clear line of site. I have a cheap FlySky FS-i6X and apparently, this module can be linked to the receiver and send data back to the transmitters and display things like speed and altitude on the transmitter's LCD screen. These cost about $4, weighs 4 grams, and peaks at about 70 mA from your 5V BEC.
• View attachment 234799
• The pressure sensor costs about $1.10, weighs about a 0.5 grams. You need one for the static pressure. And one for each dynamic pressures you want to monitor... say on a pitot tube, etc. The MPU can easily be programmed to take those readings and get the differentials and calculate say... speed and altitude in units of your choice.
• View attachment 234800

The MPU could easily handle all 8 channels you mentioned and a lot more. Like all hobbies, you need supporting stuff like a soldering iron, solder, wire, etc. But the component cost for all 8 channels would be about $15.

Those look like some great alternatives. I'm definitely not rushing to get it put together. There are a couple projects I want to completely wrap up first (this one and my 50mm F-104) and a couple projects that are in line before it (an 80mm EDF F-106 and a design meant to allow a plane to be built out of a number of interchangeable modular fuselage and wing pieces) before I get to putting something like that together.

Do you know how precise that pressure sensor is? One of the biggest challenges I've had has been finding something that can accurately measure small enough differentials. Ideally it would be down to about 5 Pa (0.0007 psi) but I'm thinking that I'll need to settle for something that can measure a differential down to about 10 Pa based on what seems readily available.

 

[Inq]

Do you know how precise that pressure sensor is? One of the biggest challenges I've had has been finding something that can accurately measure small enough differentials. Ideally it would be down to about 5 Pa (0.0007 psi) but I'm thinking that I'll need to settle for something that can measure a differential down to about 10 Pa based on what seems readily available.

Sorry, no. I do remember the resolution is 1 Pa, but it does have quite a bit of "noise" that had to be averaged out. I was using it in a Ventilator design along with PID algorithms. So I didn't really need to know the "real-world" resolution as the PID took care for smoothing anyway. However, it seems like I recall it was able to resolve changes in altitude of 6' feet (me waving it up and down). I definitely imagine when I do get to it... I'll have to incorporate some numerical smoothing for my goals of speed and altitude measurement.

 

[Inq]

I could use some advice...

I've gotten the bug to make my first EDF plane. I have an EDF 70 mm (cheap 6 blade) gathering dust for years. I don't want to do a Heinkel type design. The aesthetics don't do it for me. Although not a U-2 or an F-8, I do want to baseline a fuselage similar to those... long skinny with scale inlets somewhere way up front.

Say... you are doing one of your jets F-104, Mig 21, etc and have some napkin level sizes and weights. I'm not expecting you to check my Math, but I'm supplying the numbers so we're at least winging-it on the same page. The drawing below is just an example. The basic longitudinal dimensions are basically what I'm looking at. The A.C. is also, but the wing will be a lot bigger to get the wing loading down so I have some chance of flying it. Remember, I'm piloting challenged. So I'm looking for "comes to mind" type answers.

1. The main one... how far ahead of the A.C. would you put the C.G. (Novice Pilot) I'm assuming you have a percentage of fuselage length or wing area or... something... that you shoot for as a baseline.
2. In the drawing, the spacing shown for the major weight items are limited by the wires available on them currently. I have it located approximately at the C.G. where it'd have to go to avoid adding artificial weight additions. Would you shoot for this kind of configuration or would you put the engine in the back like most of your models and add significant, heavy gauge wires between the motor and the ESC?

VBR,
Inq

 

[telnar1236]

I could use some advice...

I've gotten the bug to make my first EDF plane. I have an EDF 70 mm (cheap 6 blade) gathering dust for years. I don't want to do a Heinkel type design. The aesthetics don't do it for me. Although not a U-2 or an F-8, I do want to baseline a fuselage similar to those... long skinny with scale inlets somewhere way up front.

Say... you are doing one of your jets F-104, Mig 21, etc and have some napkin level sizes and weights. I'm not expecting you to check my Math, but I'm supplying the numbers so we're at least winging-it on the same page. The drawing below is just an example. The basic longitudinal dimensions are basically what I'm looking at. The A.C. is also, but the wing will be a lot bigger to get the wing loading down so I have some chance of flying it. Remember, I'm piloting challenged. So I'm looking for "comes to mind" type answers.

1. The main one... how far ahead of the A.C. would you put the C.G. (Novice Pilot) I'm assuming you have a percentage of fuselage length or wing area or... something... that you shoot for as a baseline.
2. In the drawing, the spacing shown for the major weight items are limited by the wires available on them currently. I have it located approximately at the C.G. where it'd have to go to avoid adding artificial weight additions. Would you shoot for this kind of configuration or would you put the engine in the back like most of your models and add significant, heavy gauge wires between the motor and the ESC?

VBR,
Inq

View attachment 234897

I look forward to seeing how the F-8 turns out! If it flies well, I may need to build one. Adding wing area is always a good idea. I tend to add 10-15% but you can add a good 30% without it being that obvious and more without it looking bad.

1. How did you determine the A.C.? That looks very far back. The aerodynamic center is typically 25% of the mean aerodynamic chord. For the F-8, I would go maybe 15-20% of the MAC. If you have some washout in the wingtips, you could push the CG back closer to 20% where if you have none you might want to go a bit more forward to avoid dangerous stall spins. The less stable the plane, the more forward the CG, so for my F-104 I tend to shoot for 10-15% MAC.

2. That location for the EDF looks totally reasonable. Unless you have it mounted in the nose, you don't need to worry that much about the length of the thrust tube. The most important thing is to have enough inlet area. I might add some extra length in the wires to the battery, just to give you some more room to play with the CG. I would also include the weight of the servos and more importantly the wires between the components. Those add a surprising amount of weight and can change the CG a good bit.

Good luck with the project and with starting out with EDFs.

 

[telnar1236]

Sorry, no. I do remember the resolution is 1 Pa, but it does have quite a bit of "noise" that had to be averaged out. I was using it in a Ventilator design along with PID algorithms. So I didn't really need to know the "real-world" resolution as the PID took care for smoothing anyway. However, it seems like I recall it was able to resolve changes in altitude of 6' feet (me waving it up and down). I definitely imagine when I do get to it... I'll have to incorporate some numerical smoothing for my goals of speed and altitude measurement.

Yeah, my concern with so many of these sensors has been resolution and noise. For just recording data, that would probably work well, but since I'm looking at reading the air pressure multiple times each second to provide input data to a flight computer to keep an unstable design in the air, noise could cause some serious problems and any filters would potentially catch real changes in output I need. I've been thinking about building this thing for at least two years and it keeps getting pushed back because of some of the challenges.

 

[Inq]

Adding wing area is always a good idea. I tend to add 10-15% but you can add a good 30% without it being that obvious and more without it looking bad.

No... I mean real training wheels... like 100% bigger and shaped like a Cessna! I'd swap it out for the F-8 wing when people are around and it's on the ground.

Thanks for the advice... This first iteration, will be foamboard wings. Won't be able to get washout unless you have a trick???
I keep some anhedral in the training wheels.

 

[telnar1236]

No... I mean real training wheels... like 100% bigger and shaped like a Cessna! I'd swap it out for the F-8 wing when people are around and it's on the ground.

Thanks for the advice... This first iteration, will be foamboard wings. Won't be able to get washout unless you have a trick???
I keep some anhedral in the training wheels.

A bush F-8 to fight the bush MiG 21!

The thing Flite Test does with the under cambered wingtips on some of their older planes has a similar effect to washout, but causes more drag. It doesn't sound like you're going for a speed plane, so that is a good option if you want the extra protection from tip stalls.

 

[L Edge]

Attempted to fly the MiG today and it didn't go well. Ran off the runway on my first takeoff attempt resulting in the gear breaking off, so I tried a hand launch with the results below. I think the big issue was the inefficient ducting killing a lot of the thrust.
View attachment 234225
Version 2 will be lighter with more efficient ducting.

Ouch!! I had the same problems on not taking off with props as well EDF's(especially) on the runway.

Did a bunch of test where I raised the nose wheel so there is a positive angle of attack on the wing so airflow gets under the wing. Really shortens the takeoff especially for the EDF's. Just need too make sure your at V1 speed for liftoff.

Also, think about adding +1 degree up thrust(exit) to add to the vector of lift. Especially on takeoff.

 

[L Edge]

I've only just started looking into it myself. This pack including a transducer, pitot tube, and tubing looks promising (Amazon.com: QWinOut PX4 Differential Airspeed Pitot Tube + Pitot Tube Airspeedometer Airspeed Sensor for Pixhawk PX4 Flight Controller : Toys & Games ) but it's too expensive if I'm getting 8, so I'm still looking myself.

Now that I see what you areas your exploring, have you seen a pack that allows a transducer to measure the difference between 2 static pressures and allows a FC to move a servo one way or the other to keep the max differential at all times?

 

[Inq]

I look forward to seeing how the F-8 turns out! If it flies well

This is almost comical! How in the world would you be able to tell if it flies well? It goes up, it comes down, it goes into lawn dart mode. The odds are great even if its the best flying machine of all time.

Kidding aside...

How did you determine the A.C.? That looks very far back. The aerodynamic center is typically 25% of the mean aerodynamic chord.

I didn't use any aerodynamic procedure. I just used first principles of my understanding. I first did an area * centroid summation relative to the wing root. Simplistically broke it up into the red and blue regions. Dividing by the wing's area gives the balance line shown at (1). I am aware that the tip contributes less lift (per area) because of tip losses and I know the saw tooth supposedly make the root section even more effective (if I wasn't using a foamboard wing) but I don't know of a way to quantify that. Step (2)... the A.C. is placed 25% of the chord along this line. Is there a more / better method of calculating a wing's A.C.?

I might add some extra length in the wires

I'm glad I took your advice and just assumed I was going to add wires. Although little things like servos and wire rods were small compared to the motor/ESC/battery, the entire plastic nose area is 32 grams while the tail cone alone is 27 and that is before the structure and pieces required for the flying horizontal stabilizer, and the huge F-8 vertical stabilizer. I can see, I'm going to drive that battery way up front to balance the C.G.

 

[telnar1236]

Ouch!! I had the same problems on not taking off with props as well EDF's(especially) on the runway.

Did a bunch of test where I raised the nose wheel so there is a positive angle of attack on the wing so airflow gets under the wing. Really shortens the takeoff especially for the EDF's. Just need too make sure your at V1 speed for liftoff.

Also, think about adding +1 degree up thrust(exit) to add to the vector of lift. Especially on takeoff.

Yeah, the positive incidence thing is a good idea. It actually had a few degrees in CAD but the spring rate on the nose gear ended up being too low, so it was actually a bit nose down on the runway. I'll definitely be using it in the version 2. As for the thrust angle, I tend to try and avoid that in anything where there is any risk of the center of lift shifting forward at high angles of attack. I've lost a few planes because I couldn't get out of a stall without killing the throttle. But overall, I think the biggest reason for the crash was a lack of thrust due to inefficient ducting and a bad hand launch on my part.

 

[telnar1236]

Now that I see what you areas your exploring, have you seen a pack that allows a transducer to measure the difference between 2 static pressures and allows a FC to move a servo one way or the other to keep the max differential at all times?

I haven't seen something like that available commercially. I think that for anything more complex than a simple pitot static probe you would end up needing to just build it yourself. It shouldn't be too hard, but you would need to figure out a way to tell which way the servo would need to go and therefore might need multiple sensors or more complex control logic. Inq had some great recommendations on pressure sensors.

I'm curious now. What kind of application are you thinking of?

 

[telnar1236]

This is almost comical! How in the world would you be able to tell if it flies well? It goes up, it comes down, it goes into lawn dart mode. The odds are great even if its the best flying machine of all time.

It won't fly with that attitude... But seriously, I didn't realize how big a plane you were talking about. The wing loading should be very low from what you're describing which should make it pretty manageable. And it sounds like you're taking a reasonable approach, starting with a larger wing and a light airplane. I'm expecting you to be pleasantly surprised with how easy that plane will be to fly.

I didn't use any aerodynamic procedure. I just used first principles of my understanding. I first did an area * centroid summation relative to the wing root. Simplistically broke it up into the red and blue regions. Dividing by the wing's area gives the balance line shown at (1). I am aware that the tip contributes less lift (per area) because of tip losses and I know the saw tooth supposedly make the root section even more effective (if I wasn't using a foamboard wing) but I don't know of a way to quantify that. Step (2)... the A.C. is placed 25% of the chord along this line. Is there a more / better method of calculating a wing's A.C.?

It looks like your AC is actually pretty correct. Intuitively it looked too far back, and to be honest, I'm not sure I'm 100% following your approach, but it seem like the AC is in about the right place by my own calculations so it looks like your approach works. Guess I need to remember to trust the math.

I'm glad I took your advice and just assumed I was going to add wires. Although little things like servos and wire rods were small compared to the motor/ESC/battery, the entire plastic nose area is 32 grams while the tail cone alone is 27 and that is before the structure and pieces required for the flying horizontal stabilizer, and the huge F-8 vertical stabilizer. I can see, I'm going to drive that battery way up front to balance the C.G.

Glad the advice came in helpful. I learned it the hard way with three or four planes that needed a ton of weight in the nose or tail before I just started building in room for the battery to move.[/QUOTE]

 

[Inq]

It won't fly with that attitude... But seriously, I didn't realize how big a plane you were talking about. The wing loading should be very low from what you're describing which should make it pretty manageable. And it sounds like you're taking a reasonable approach, starting with a larger wing and a light airplane. I'm expecting you to be pleasantly surprised with how easy that plane will be to fly.

"Consider yourself already dead..." - Twelve O'Clock High

I had the same problem with your projects... seeing pictures, I was thinking pretty small and when you finally had the F-104 with something to give context I was surprise it was larger.

F-8
The current weight of known items...

A few more pieces are outstanding... components, the foam board portion for fuselage and wings, horizontal stab and elevon gear. I'm taking on a WAG of being somewhere around 700 grams.

Length = 44"
F-8 Wingspan = 28"
F-8 Wing Area = 200 sq-in

Training Wheels Wing Area = ??? sq-in

 

[L Edge]

I haven't seen something like that available commercially. I think that for anything more complex than a simple pitot static probe you would end up needing to just build it yourself. It shouldn't be too hard, but you would need to figure out a way to tell which way the servo would need to go and therefore might need multiple sensors or more complex control logic. Inq had some great recommendations on pressure sensors.

I'm curious now. What kind of application are you thinking of?

I automated leading edge flaps, but looking to do it by differential max high static pressure between upper and lower surfaces.

Played around with different techniques dealing with high alpha, stalls, and ducting with EDF's inlets as well as exhaust areas. Did testing to back it up. Interesting one was the SR-71 were I picked up good info on inlets and exits. Best project was trying to take a single 5 bladed EDF and bleeding off pressure to the end of the wing to counteract yaw so it will hover. Some success, watch the video.

Interested on your problems with stalls and ducting. You should see how I choked the EDF and it performed well.

 

[Inq]

I automated leading edge flaps, but looking to do it by differential max high static pressure between upper and lower surfaces.

Played around with different techniques dealing with high alpha, stalls, and ducting with EDF's inlets as well as exhaust areas. Did testing to back it up. Interesting one was the SR-71 were I picked up good info on inlets and exits. Best project was trying to take a single 5 bladed EDF and bleeding off pressure to the end of the wing to counteract yaw so it will hover. Some success, watch the video.

Interested on your problems with stalls and ducting. You should see how I choked the EDF and it performed well.

Outstanding piloting also didn't hurt!

On my first EDF build F-8... where piloting experience will be an issue. I'm concerned about having enough control authority at low speeds of landing... a belly lander. Have you considered putting say... elevon and rudder type surfaces at the trailing end of the exhaust tube? Vectored thrust kind of thing?

 

[L Edge]

Outstanding piloting also didn't hurt!

On my first EDF build F-8... where piloting experience will be an issue. I'm concerned about having enough control authority at low speeds of landing... a belly lander. Have you considered putting say... elevon and rudder type surfaces at the trailing end of the exhaust tube? Vectored thrust kind of thing?

Look at the video above, in fact, watch the landing at high alpha. It has saved my butt many a times.

Here is my suggestion to you. Flying EDF's is another breed. Before you trash your designs, learn to fly EDF's Got some foam. My EDF trainer consists of 6 pieces and is a flat plate. Add one 70 EDF(and best of all, it's protected) and practice ,practice and practice. Build light, and you will/could do this.
The key of flying is to feel your plane and be one with it.
Can you land twice on a short field? Anybody can with practice.

 

[Inq]

Before you trash your designs,

I'm not too worried about trashing it. I've embraced the F.T. concept of celebration when an accident happens. Gives me a chance design and build another plane. Maybe the same one, maybe a new one.

But your video does suggest something. Instead of concern about having enough speed on launch to get usable control reactions... just "shoot" it up and completely rely on thrust until it gets up to speed.

I wanted to benchmark my EDF anyway so I can see how much ducting losses I might have and incrementally improve them. But it will give me an idea if I'll have a better than 1:1 trust to weight.

 

[Inq]

It looks like your AC is actually pretty correct. Intuitively it looked too far back, and to be honest, I'm not sure I'm 100% following your approach, but it seem like the AC is in about the right place by my own calculations so it looks like your approach works. Guess I need to remember to trust the math.

Could you share the way you perform the calculation?

The less stable the plane, the more forward the CG, so for my F-104 I tend to shoot for 10-15% MAC.

I'm going to assume conservatively since you mentioned the F-8 tends to be bad handling. I'll go 10% of my A.C. chord line... unless your calculation method incorporates some factors for tip losses Reynolds number differences.

 

[L Edge]

I'm not too worried about trashing it. I've embraced the F.T. concept of celebration when an accident happens. Gives me a chance design and build another plane. Maybe the same one, maybe a new one.

But your video does suggest something. Instead of concern about having enough speed on launch to get usable control reactions... just "shoot" it up and completely rely on thrust until it gets up to speed.

I wanted to benchmark my EDF anyway so I can see how much ducting losses I might have and incrementally improve them. But it will give me an idea if I'll have a better than 1:1 trust to weight.

My video suggest two things:
1) Pick airplanes that are designed light and can handle stresses. Hence vertical takeoffs, dual landings and are very stable .
2) Too see if you have enough thrust and ducting losses, take the plane ready to go, hold vertically, add power until it hovers. My rule of thumb is if throttle is 1/2, go chuck it. If it takes 2/3 to 3/4 throttle, put landing gear and you need to be a good pilot to get it up and down.
3) If you are close to 1/1 ratio, you can chuck it and nurse it into flight, if you throw too steep, roll over is the result.

There are very little ducting losses for what we do. That is including ducting in the front. Only problem is if inlet area is smaller than fan duct area, yes it happen, can use cheater area under the ducting. Used pitot system to verify. This duct is over 12".