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Boeing B52 Stratofortress build update

#1
Hello guys. I posted a thread previously about the challenges I'm facing in building a B52 with a max wing span of 60cm. From my calculations, this would require a ducted fan with a diameter of not more than 16mm. I have never come across such a ducted fan as the smallest i see online are about 30mm diameter. I need the plane to be scale and i do not want to use pusher props etc. Please I need suggestions on how I can propel this airplane, please what are my options. Thanks guys
 

DamoRC

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#2
Hello guys. I posted a thread previously about the challenges I'm facing in building a B52 with a max wing span of 60cm. From my calculations, this would require a ducted fan with a diameter of not more than 16mm. I have never come across such a ducted fan as the smallest i see online are about 30mm diameter. I need the plane to be scale and i do not want to use pusher props etc. Please I need suggestions on how I can propel this airplane, please what are my options. Thanks guys
"For every door that closes...." The rest of this saying claims that another one opens, but in reality, each door that closes simply limits the number of doors you have to choose from. The same applies when trying to design and build a plane. As you make design decisions, these decisions will have the effect of limiting your options as you move further into the design process. At this point in the design it seems that you have made the following decisions:

(1) Wingspan can't be more than 60cm
(2) Plane needs to be scale (so the fuse diameter is limited by the wingspan to 31 mm at its widest point)
(3) Plane needs to be powered by EDF (based on the "no pusher props" decision).

This basically leaves you with, and locks you into, the 30mm EDF option mounted in the fuse at its widest point with the exhaust going through the end of the fuse which needs to be open (although there might be some 25-28mm options also available, but as you start looking at smaller and smaller EDFs, the thrust is going to fall short of what you need for the build).

That being said, I think this is a doable project at 60cm and a 28-30mm EDF mounted in the middle.

So some questions would be:
Do you have a budget for the build?
Is there any flexibility on the wingspan (can you go bigger)?
Would you consider micro motors (like the really small quad brushed motors) with props mounted on the nacelles?
What build material are you planning on using (I recall you said you were in Africa in a different thread and I don't know what foam / foamboard you intend to use).

DamoRC
 
#3
Thank you Damo RC. The answers to your question
My budget is about 120 Dollars
I can't go bigger on the wingspan as I have a limited space
Using micro motors with propellers will make it no longer look like a B52
I plan on using foam obtained from foam plates (the type used in packaging meat and other foods)

I would like to find out please
1.Are there other options of propulsion I can use e.g a simple compressed air can in the middle with flexible pipes directing the air to exit through the scale jet engines under the wing
2.I have motors and electronics from a quad, do you think it would be practical to attach a 3d printed fan and duct to one of the quad motors and use it as a custom EDF as i dont have easy access to edfs over here. Do you think this custom edf can generate enough thrust? I dont know the kv of the motor as nothing is written on it but it has a diameter of 6mm
3. I have attached a picture of a scale c17 globemaster fuselage i built in the past using meat foam packing plates but didnt complete, it is about 50cm long
4. Please help me guys as I want to make this a reality. Dont know why I suddenly became obsessed with the B52
5. Do you guys know of operation power flite where 3 B52s flew nonstop round the world as far back as 1957?
6. I really admire the technological advancement of you and your people while I am still struggling to get a model B52 into the air in this year 2018, something you guys can do with your eyes closed :)
 

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d8veh

Well-known member
#4
Compressed air is not going to work. I think you've put too many constraints on your project. I can't think of a way to achieve it. What I would do is build the B52 with dummy engines, then either fit two electric motors with propellers pushing the back of the wing or a single one in the nose pulling. Nobody will notice when it's in the air. It'll still look like a B52.

When you make a scale plane, you can't make a perfectly scaled replica that flies. The tail has to be bigger for a start. You can use some poetic license in your build and it will still look like a B52. Look at this cartoon one, which is instantly recognisable, though all the proportions are wrong:


Look at this picture. You can ask anybody what it is, and everybody will say it's a cat, though not a single feature of it bears any resemblance to a real cat. It's head is the wrong shape, Its body is the wrong shape. It has no fur nor whiskers. It's eyes are wrong:

The trick is to exaggerate the features of the B52 which are unique to it, like the engine pods and the shape of the wings, then nobody will notice that it has propellers instead of jets.
 
#5
Thank you. you guys have finally convinced me. I have decided to build the B52 with two pusher props behind the wings. This will be obtained from a quadcopter. The only control the B52 will have is varying the speed of the throttle for take off and landing and differential speeds of both props for turning left and right. I dont have servos so I cant install controls for rudder, elevators, ailerons etc. What do folks think about my plan?
 

DamoRC

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#6
Sounds like a good compromise between your design requirements.

Earlier in a different thread you had asked about where to pick up electronics including a transmitter and receiver. If you don't yet have a transmitter ensure that whatever one you get can handle differential thrust. You can pick up 5g or 9g servos for a few of bucks each. Having an elevator would help a lot.

The motor you have sounds quite small. Any pics of the motor or the quad it can from. Sounds like it could be brushed, not brushless (2 wires versus 3 wires going into the motor).

DamoRC
 
#7
I intend to purchase a new quadcopter for this purpose as the one I currently have has been dismantled. It won't be easy for me to purchase servos as they are not readily available here. Quads are steered by varying the speeds of the motors right? I intend to steer the plane by leveraging this feature by applying differential speeds to both propellers using the quads controller. As for the motors being small, some quads have gears to increase the power of the motor. I intend to use the gear mechanism in the quad to ensure adequate power

What do folks think would be a good prop diameter for a 60cm wingspan B52 bearing in mind I will be using 2 props, one behind each wing. I see a lot of pusher planes on the internet with propeller diameters that are surprisingly quite small for the plane. See the link
 

DamoRC

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#9
The props have to be matched to the motor. Use the ones that came with it.
Agreed.

@Clement - do you happen to know what the weight of the quad is (all up with motors and battery)? This will provide some insight as to whether there is enough power in two motors.

Using the flight controller to drive the motors is fine but you will need to be able to tweak the settings on the flight controller to make it behave the way you want in a plane configuration.

DamoRC
 

DamoRC

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#11
Great! Do you know which one you are going to get (we might be able to find the specs online).

I looked at the vid of the 777 and then found his build vid for that plane. Wow! He did an awesome job. All up weight was around 7 pounds but his wing loading must be really low based on how slow he was able to fly.

DamoRC
 
#12
Thats true, the 777 flew quite slowly. I was surprised such small diameter props were able to power the 777

I don't know the quad i will purchase for now. It depends on what is available at the store. The wings of the real B52 are installed at a 6 degree angle of incidence to the fuselage and have a negative dihedral angle of 2 degrees. Should i maintain these figures for my model? what do you think
 
#13
If you're going to have it gain altitude by increasing throttle you'll definitely need some angle of incidence so that way it will climb better.

I've never used negative dihedral so I don't exactly know how it works, but I'm going to guess that it does the opposite of regular/positive dihedral, somebody correct me if I'm wrong.
 

DamoRC

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#15
Thats true, the 777 flew quite slowly. I was surprised such small diameter props were able to power the 777

I don't know the quad i will purchase for now. It depends on what is available at the store. The wings of the real B52 are installed at a 6 degree angle of incidence to the fuselage and have a negative dihedral angle of 2 degrees. Should i maintain these figures for my model? what do you think
I think the wing incidence is a good idea but honestly I don't know what a good angle would be. Perhaps initially leave the wing removable so you can add shims between the leading edge and the fuse to alter the wing incidence until you find what works.

On the quad, in order to save some time and money you should research what it's going to take to convince the flight controller (FC) to fly a plane. I have a feeling that this is not going to be trivial. The FC will try to adjust the motor speeds based on its orientation to the ground and the forces it experiences in the air. This is definitely worth putting some thought into.

DamoRC
 
#16
Thanks Damo RC. I will research on it. I actually initially thought quads change direction by varying the speeds of the propellers based on inputs from the controller.

I will like my B52 to fly at a very slow speed, just like the boeing 777 in the video. How do I achieve this? Please whats the secret?
 
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DamoRC

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#17
Thanks Damo RC. I will research on it. I actually initially thought quads change direction by varying the speeds of the propellers based on inputs from the controller.

I will like my B52 to fly at a very slow speed, just like the boeing 777 in the video. How do I achieve this? Please whats the secret?
You are correct, in general terms, that the change in motor speeds controls how the quad flies. The flight controller takes the inputs from the transmitter and receiver and converts them to the appropriate instructions to the motor. Bit it also uses onboard sensors to modulate the instructions to the motor based on what it senses the quad is doing and depending on what level of automatic control (for example self leveling) that it is using.

For example, you provide increasing throttle input to get the plane moving forward, this is initially fine because throttle input to a quad increases the speed of all 4 motors to increase the altitude of the quad. Then, because you are not using an elevator, but design the plane to climb with more throttle, the plane will pitch up. The flight controller senses the pitch up and will reduce the speed of the front two motors and increase the speed of the rear two motors because it didn't receive and elevator stick input from you, so the "quad" must be going out of trim. If you are using the front two motors to power the plane, this will reduce the power to these motors, and the plane will start to level off and then pitch down (leading the flight controller too increase power to these two again). This might work well for your plane or it could lead to significant oscillations - I don't know for sure. I think it could work, but it may require a lot of tuning of the plane to get the flight controller behavior and plane behavior to work well together.

On the slow flying, it's not really a secret. Weight is your enemy, particularly when building a smallish plane. The 777 had a lot of wing area for a relatively low weight. This ratio is key to building a slow flyer.

DamoRC
 
#18
Thank you so much for your detailed analysis and explanation DamoRC.

Found this url of an airplane built with quadcopter parts

The B52 has a lot of wing area, so I guess my plane should be able to fly slowly too :)

Since I won't be using servos, do you think there is any need to incorporate flaps, elevators, rudders etc. You know the flaps, elevators, rudders could be manually set before take off for example the flap could be set in the down position. Should I include these flaps, elevators and rudders that can only be set before takeoff manually and cannot be altered during flight (since no servos) or do I exclude them for simplicity?
 
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DamoRC

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#19
Thank you so much for your detailed analysis and explanation DamoRC. The B52 has a lot of wing area, so I guess my plane should be able to fly slowly too :)

Since I won't be using servos, do you think there is any need to incorporate flaps, elevators, rudders etc. You know the flaps, elevators, rudders could be manually set before take off for example the flap could be set in the down position. Should I include these flaps, elevators and rudders that can only be set before takeoff manually and cannot be altered during flight (since no servos) or do I exclude them for simplicity?
I would skip the flaps, you won't need it on this size build. I am tempted to say that a small rudder for trimming might be good although you should be able to trim the yaw with the differential thrust. And an elevator for trimming sounds like a good idea.

DamoRC