Northrop Grumman X-47B Design & Build

by Rocketman1092 | May 21, 2014 | (117) Posted in Projects

Hey everyone,

This is my first article on Flite Test. I generally post over on RCGroups, but figured that this community might be interested in seeing my latest project - an EDF powered model of the X-47B UCAS. 

 

 

Some background about myself - I'm a recent graduate of the aeronautical and astronautical engineering program at Purdue University and work at a major aerospace company. I used many of the design and optimization tools I had learned in my studies to determine the important aspects of the aircraft, so sizing, battery weight fraction, wing and power loading, airfoil, and wing twist distribution were all found mathematically. In addition, I perfomed some wind tunnel testing after the build in order to assess the performance and control. I've done my best to document the entire process, and will update this post as flight testing continues. Enjoy!


Weight Estimation

I needed to figure out how big the airplane would be before I could start CAD design and pick a power system. The first step to sizing the model was estimating the total weight and empty weight. I first put together a flight profile in order to determine how much total energy the batteries need to contain. It's pretty much a big list of guesses and estimations, and it's usually repeated a few times over the course of the preliminary design. The profile I came up with is:

- A 10 second takeoff run to a speed of about 60 ft/s
- A 30 second climb at a speed of 75 ft/s
- 3 minutes of cruise flight at about 90 ft/s
- About a minute of turning flight at 90 ft/s (this needs to be accounted for separately than cruise, since the airplane produces more lift & drag while turning)

There are some performance approximations as well, such as:

- A lift-to-drag ratio of 16
- Speed controller efficiency of 98%
- Average motor efficiency of 60%
- Propulsive (fan) efficiency of 40%

My code then calculated the energy required for each phase of the flight profile and then added them all up. The result is a fraction that represents how much of the airplane's weight should be made up of battery. It comes out at 16% for the values listed above, which seems reasonable as a first estimate.

Next, I made a plot in Excel that's just a straight line of payload weight plus battery weight versus total aircraft weight. In this case, "payload weight" is the Ardupilot along with all its sensors and wiring. I plotted another "historical data" line that represents average battery weight versus airplane weight for a list of similar R/C models. The point where the two lines intersect indicates the AUW that I should design for. The result is between 5 and 5.5 lb. You'll see later that the actual AUW ended up around 6 lb, which fortunately was close enough to the design value to meet all of the performance constraints.



Constraint Diagram

The next step in the process was generating the constraint diagram. This takes advantage of the fact that pretty much every performance parameter depends on wing loading and power loading. It essentially produces a range of feasible wing and power loading combinations for my model. The inputs are:

General Assumptions:
- Aspect ratio of 3.99
- Zero-lift drag coefficient of 0.03
- 40% propulsive efficiency
- 60% motor efficiency
- Oswald (span) efficiency factor of 0.60

Takeoff Performance:
- Zero elevation (for air density)
- Maximum lift coefficient of 1.3
- 75 foot takeoff run

Landing Performance:
- Zero elevation (for air density)
- Maximum lift coefficient of 1.3
- 75 foot takeoff run
- Rolling friction coefficient of 0.2

Ceiling Constraint:
- 1000 ft ceiling (this constraint is almost never relevant for R/C models)

Rate of Climb Performance:
- 1000 ft/min

Maximum Speed Performance:
- 300 ft altitude
- 90 ft/s maximum speed

Turn Performance:
- 300 ft altitude
- 70 ft/s airspeed
- Load factor of 2 (sustained 60 degree bank)

The program solves the equations for each constraint in terms of power loading over a range of wing loading values. The design space is the area where all of the constraints are satisfied, and it's marked out on the plot. From here, it seems like a 25 oz/sq.ft wing loading and about 150W/lb will fit comfortably in the design space. The active constraints are landing, rate of climb, and maximum speed, while the rest of the constraints are easily satisfied. However, a good rule of thumb is to multiply wing loading for a flying wing by 0.7 since the twist causes the tips to be very lightly loaded. So, I'll be designing the airplane for a wing loading of about 17.5 oz/sq.ft. For this planform, it'll end up around a 55" span.



Weight & Balance

I planned to use XFLR5 to analyze aerodynamic performance and dynamic stability, but I needed inertia properties first. This is kinda like the "chicken and the egg" paradox, since I couldn't build the detailed CAD model until after the analysis is done but I needed the detailed inertia properties in order to run the analysis. The best I could do to start is to let XFLR5 calculate the inertia for the wing structure itself, then approximate everything else as point masses. I used a flying wing CG calculator to find a baseline center of gravity location, and then I found weights for all of the components and distributed them around the model. I tweaked some things at the end so that the actual CG would be close to where it's supposed to be. This should be accurate enough for the analysis, and it also helped me to locate components when I start the CAD process.




Aerodynamic Analysis

To choose the airfoil for this model, I started with the NACA 64(1)-212 and added twist and refex to control the lift distribution, which is important for tailless models. I elected to go for a bell-shaped lift distribution that's essentially a sine curve to the power of 2.5. I believe this approach was pioneered by the Horten brothers, and it not only creates longitudinal stability but some lateral stability as well. The design ended up with a range of twist from zero degrees at the root to -3.6 degrees at the tip. The resulting lift distribution in level flight matches the "ideal" distribution quite nicely, and the airplane is both statically stable and trimmed in pitch. 



CAD Design

After I knew the wing span and airfoil, I could get started with the CAD design in CATIA V5. I imported a 3-view of the full scale aircraft, and used it to outline the model. This process took several months, but it's pretty well summarized from start to finish in the set of renderings below.

 

Mold Fabrication

I started manufacturing molds right after the CAD design was completed. They're made from MDF, which was cut with a 5-axis CNC mill and sealed with Feather Fill primer. This method is durable enough to produce a few airframes, but not too expensive. 

 

Skin Layups

The composite skins are a sandwitch of fiberglass cloth with a thin foam core. It took some experimenting to determine that this method was best for stiffness and weight. Everything was done using a wet layup and a vacuum bag.

 

Internal Structure

The internal structural parts were cut by laser from some 1/8" plywood. They were assembled first, then glued inside the skins during the joining process. Much clamping ensured a tight fit.

 

 

Removal from Molds

De-molding composite parts for the first time always feels like Christmas morning. Here are the X-47B parts fresh from their molds.

 

Gear & Doors

It was easiest to install the landing gear and doors next, while the model could still be rested inside the molds. All of the hinges are laser cut plywood, and no two are identical. This is because the many complex curves mean that the hinge geometry must be precisly controlled. Each gear door has it's own small servo, and an Arduino controls the sequence of motions.

 

Ducting & Fan Mounting

The ChangeSun 70mm fan was mounted next. The inlet and exhaust ducting are made from fiberglass, shaped using a positive plug. A hatch in the electronics mounting plate allows access to the fan after the model is assembled.

 

Control Surfaces & Wing Mounting

The control surfaces, servos, and remaining electronics were the last to add. Pin hinges were used for elevators and ailerons, while tape hinges were used for drag spoilers. The outer wings slide on with a carbon joiner tube and are retained by a thumb nut inside each landing gear bay.

 

Wind Tunnel Testing

When assembly was complete, the entire airplane was mounted into a large subsonic wind tunnel. This let me gather lift, drag, and pitching moment data and allso allowed me to trim the model before it ever left the ground.

 

First Flight

Finally, the model was ready to fly! I installed some vertical tails for initial flights that will stay until the ArduPilot's yaw controller is tuned. The plane flew great, and looks awesome in the air. No trimming was required.

 

That's it for now, thanks for reading! I'll be doing some more flight testing over the next couple of weeks, and will also be painting and detailing the model. Stay tuned!

 

Update: 6/7/2014: 

Got the model painted and finished! Paint is Testors Neutral Gray, and decals were custom made by Callie Graphics. I'm very happy with the result! Hopefully the solid color won't cause any orientation troubles while flying. 

 

Update: Here is some additional flight video from the 4th and 5th flights.

 

Update 2: I've only flown the X-47B a few times recently, as I'm not making much progress on tailless flight. Debating a switch from APM to AS3X, as I only need stabilization and I think it might be an easier approach. Here are some more flight photos.

COMMENTS

Buddah on May 23, 2014
Just Wow.
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ShaunMartin on May 23, 2014
Holy moly. This looks like it should be classified. :)
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jhummel on August 10, 2014
X1
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mcwalkman on May 23, 2014
How many sheets of Dollar Tree foam board?
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Good Kebo on May 24, 2014
;)
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Drbudd on September 5, 2017
Lmao
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TehMaxwell on May 23, 2014
That is.....awesome! How much? XD
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Willsonman on May 23, 2014
Epic composite building. Love it.
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apnewton on May 23, 2014
Six stars. Superior article
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Battershell on May 23, 2014
Simply put....... WOW!
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LordVader on May 23, 2014
This blows my mind, so cool. I would love to see the plans for a DTFB build. I can't wait to see what else you have in mind. Thanks for sharing.
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Rocketman1092 on May 23, 2014
Thanks guys, glad you enjoyed the article!

I actually built a simple flat foam version of this airplane last year so that I could test the control layout. Uses a typical pusher jet power system and doesn't need a flight stabilizer, though some rudder input is needed here and there to keep it pointed straight. I'll put some plans together and add them to the article.
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engineer on May 23, 2014
Amazing. Great job! Looking forward to more articles from you!
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MT Alex on May 23, 2014
Absolutely fantastic! Looks like all of the money spent on your higher education wasn't in vain. :) Good luck with your new job.
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spetnazer on May 23, 2014
Waoooowwwww....really...really fantastic...awesome.....Your technique is very professional
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Yogenh on May 23, 2014
I love it you did a really great job with it. hehehehe could make me one????
You did great!!!!!!!!!!!

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#3 on May 23, 2014
finished product is fantastic pictures great if you ever sell a kit i would like to get one. the parts of your article that have charts and any words were way over my head ,but not the way my rc planes are over my head lol thanks for sharing
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FPVAdventures on May 23, 2014
Wow, and here I am thinking I'm cool for using a ruler to cut my foamboard in a straight line. That is an incredible build and I can't wait to see it fly without the vertical stabilizers.
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Carmatious on May 24, 2014
Sir, this piece of work is professional, hence fabulous.
You have further shed light into my pursuit into the field of
aerospace engineering; marvelous Sir. Thank You.
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Carmatious on May 24, 2014
Was this an independent project? If you don't mind could you include your RCGroups profile.
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Carmatious on May 24, 2014
What "code" did you use and did you program Arduino yourself? Sorry, a lot of questions.
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Rocketman1092 on May 24, 2014
I have a MATLAB script that I wrote back during my senior design course that constructs the constraint diagram plot. Regarding the Arduino, I found some code online that someone else wrote for a landing gear sequencer, then I just modified it to suit my needs.
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MT Alex on May 24, 2014
What kind of troll would give this article less than five stars?
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gmyers2112 on May 24, 2014
so this took a couple of days then?

It looked like it needed the whole runway. Was there power enough or do you think you'll up it a bit? Everything else was pretty much way past anything I'll ever be able to do and really cool. Are you going to mount a camera for FPV (or classified recon missions)?
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Rocketman1092 on May 24, 2014
Power is fine, it's just difficult for the model to rotate because the main landing gear is a good distance behind the CG. It's in the scale location, but the CG on the full size aircraft is pretty far aft, like many fighter jets. I'm hoping that I can get a shorter takeoff roll by lengthening the nosegear so that the model sits at a positive angle of attack.

No plans for FPV, though I'd like to add a GPS module, airspeed sensor, and LIDAR Lite as soon as it's available for Ardupilot. That way I can do autonomous flight, including takeoff and landing.
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Force5 on May 24, 2014
WOW !! Congratulations! Where did you build it? That's a nice facility.

Great article, thanks for sharing. Looking forward to future reports.
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Kurt0326 on May 24, 2014
Good job! With a professional shop like yours you could design anything I'm sure. Now will the next upgrade feature the folding wings?
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gremlin88 on May 24, 2014
That is the most amazing plane I've ever seen! I would be cool if you worked with the FliteTest guys to make a small foam board version.
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gremlin88 on May 24, 2014
Also, do the wings fold?
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Rocketman1092 on May 24, 2014
I'm working on plans for a foam board version and will post them soon! The wings are removable, but do not fold.
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FTDude on May 24, 2014
WOW! Beautiful build. Completely amazing.
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earthsciteach on May 24, 2014
Fantastic project! Its great to see something other than DTFB on here! You have access to some amazing tools!

Are you performing the constraint analysis using a software package or did you set up and Excel spreadsheet to do that?

How much of the analysis were you able to do in XFLR5 and how accurate did it turn out to be?

LOVE THIS!
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Rocketman1092 on May 24, 2014
It was done in MATLAB, but the equations aren't overly complicated so I think Excel would be capable enough to do it. Now that you mention it, maybe I'll put together a constraint analysis spreadsheet and post it somewhere. Might help scratch-builders determine the right wing and power loadings to get the performance they want.

I used XFLR5 primarily to figure out how much twist and reflex I needed to achieve the bell shaped lift distribution that's desirable for tailless aircraft. I also did a stability analysis to see how the model would behave. I haven't yet tried to verify these things, but I suppose I could use the Ardupilot's data logging to try and look at the dynamic stability. At some point, I'll also get the lift, drag, and pitching moment plots from my XFLR5 model and see how close they are to the wind tunnel data.
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earthsciteach on May 24, 2014
Also, can you elaborate on the skin molding? What weight cloth did you use and did you use DTFB? What resin system?

Thanks!
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Rocketman1092 on May 24, 2014
The skins are made up of two layers of 2oz glass cloth, followed by a ~2mm sheet of XPS foam and finally a third layer of 2oz glass on the back side. I imagine that Depron or DTFB would work as well if it were sliced to the proper thickness - the pink foam is just what I had available. I initially tried some layups without the foam sandwich core, but couldn't get an acceptable amount of rigidity.
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itssean95 on May 24, 2014
i've never seen so much precision put into an RC plane
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Chad on May 25, 2014
Great work!
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spoonerisms on May 25, 2014
Holy @$#%! Never seen so much research and work put into a plane! Great job!
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val0production on May 25, 2014
Catia is so beautiful ^^ !
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Non Action Man on May 26, 2014
superb! absolutely amazing work!
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bigrocketnerd on May 26, 2014
Holy Cra*. Way to throw the technology at the problem. Impressive for sure! I can't wait for the Wave Rider.
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carsonpauli on May 27, 2014
Just wow is all can say. Great work
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danallen82 on May 27, 2014
Mind Blown! You just raised the bar too high for this site ;). You made the best use of the CNC, laser cutter, and fiber glass molding that I have ever seen. Bravo! I can't wait to see more, and what you do with ArduPilot, great choice btw.
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Skipp2Maloo on May 27, 2014
NASA is looking for you! That's Awesome!!!!
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PetterKien on May 27, 2014
People already said it, but just wanted to add my: Wowww...
Crazy job, congratulation!
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3DMad5 on May 27, 2014
Wow! Very impressive!
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JulietKiloMike on May 28, 2014
I created a FliteTest account just to rate your article and compliment you on your build; it's absolutely amazing...wonderful job! You blew my mind :o
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ndcappa on August 1, 2014
x2
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jhummel on August 10, 2014
X3
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hursto75 on May 28, 2014
Your shop is amazing and your work is better!! Love when graphs are included. Thanks so much, more videos.
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Ron on May 29, 2014
Great job in creating this. I had thought about making a RC model of this aircraft but don't have the tools or talent. Obviously you do though. I would love to see a longer video of this in flight.
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rcflyer729 on May 29, 2014
that is so cool great job
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Rocketman1092 on May 30, 2014
At some point, I'll definitely put a longer flight video up. I'm up to 4 flights now, and have mostly been working on tuning the gains for the Ardupilot. I can't decide whether to use stabilization on all 3 axes or on yaw only. Pitch and roll are pretty stable as is.
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11-Nov-11 on June 4, 2014
All is well !
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vwflat4 on June 5, 2014
Rocketman - Podcast 029 - you are mentioned at about 35 min in.
Great Article by the way - absolutely Wonderful. I look forward to seeing more.
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Rocketman1092 on June 7, 2014
Thanks for the heads up, the podcast mention was pretty cool.

I painted the model and updated the article with some new pics. I do have more flight video as promised, but haven't gotten around to editing and publishing it yet. Sorry about that!
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Nathan_116 on June 10, 2014
Speaking of he podcast, I just listened to it and I heard something about Northrup Grumman. Good luck on that as that is the company that I dream of working for one day.

Also, when you were designing and doing calculations for you model, did you use he scale airfoil in your design or did you alter it. I have this thing for he P-40 Warhark and am in the middle of designing it. I have done boatloads of research on and and want to build a scale model (or as close to scale as you can get with foam and a limited income as you can get) and was winding if I should use the scale airfoil or if I should modify it. A lot of the other designers here on FliteTest just stick win the simple under amber or flat bottom airfoil, bu with me wantin to make it scale I was honking about using something different. Also, what design program did you use, and how much does it cost? Do you think someone could do it in a free program like SketchUp?

Thanks, Nathan
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Rocketman1092 on June 11, 2014
I made some changes to the airfoil, but mostly for pitch stability. For a conventional model like the P-40, the scale airfoils should do just fine. It'd also be a safe bet to go with a common model airplane airfoil like the Clark-Y, NACA4415, or SD7062.

As far as design software goes, I had access to CATIA v5 at college. A license is on the order of $30k from what I've heard, so I'll need to figure out something else now that I've graduated. I've seen people do some pretty elaborate things in SketchUp, but I haven't used it much. There's an open-source program called FreeCAD that also looks promising. http://freecadweb.org/
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sergey300c on November 13, 2017
hi
i have some questions on this project
i have some questions on drawing
so i would be happy to get answer from you
sergey_top_1@mail.ru
friends from Russia
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BuckE67 on June 9, 2014
WOW. You have done an incredible job!
Respect!
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GB-air on June 12, 2014
Great Article!
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theteeheeguy123 on June 19, 2014
test it for it's stealth capabilities XD
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spidal3 on July 2, 2014
Crazy Good!!!
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Flying Fox on November 23, 2014
Holy Smokes awesome design
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Frankenfoamy on October 23, 2015
That plane has been on my back burner to build. I was not confident that a ducted fan could pull it off. Great job!
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mhmod2222 on August 5, 2016
can you provide the link for model you used to make your plane or even your model

because I am interested in building one like this

Thanks in advance
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Ian Legend on August 24, 2016
Two years later and your creation is still impressive! Great job Rocketman1092.
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wanabeRCexpert on December 4, 2016
that is probably the best looking scratch-build i have ever seen
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andy2096 on July 12, 2017
Can you provide an article showing us how your wind tunnel is set up?
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Razor7177 on July 10, 2018

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Northrop Grumman X-47B Design & Build