This week I got a great start on building the Raptor - a 48 inch wingspan flying wing. I learned an enormous amount of information:
1.) 3M 90 Spray foam Melts XPS Foam...
2.) Loctite Foam Board Adhesive Is a great foam, give it plenty of time (24 hours+) to dry/cure
3.) When cutting out flying wings with a hot wire cutter, both sides of the wing (if using a two template method) need to communicate very well
4.) A 12v car battery or 12v small power supply isn't really enough to cut a wing with a 30in of Ni-chrome wire, you need more power (more on this next week)
As you can see, I wasn't able to finish it this week, but I think we've got the hardest part figured out.
See you next week!
Comments
Paul, I'd like to discuss few details regarding a flying wing that i'm trying to design. Could you tell me how to contact you? Thanks
Regards,
Saad
saadtiwana (at) gmail.com
You should make at least two Trent and forget the AP for a while and go do some combat sloping, your piloting skills will increase dramatically and its a heck of a lot of fun.
Great work BTW keep going.
Saad,
Dihedral provides lateral/directional stability, which is needed for a stable airplane, but this is not necessary on a flying wing. Flying wings are a bit tricky. Lets discuss longitudinal stability first.
Every wing has a reference point about which the wing has a tendency to rotate, this location is a balance of pressures all around the wing, and it is called the "center of pressure" (CP). To have a stable airplane, you need to add the CP of both the wing and the tail to arrive at the location of the CP of the vehicle. This is important because to have a stable airplane, you want the "center of gravity" (CG) 10-15% ahead of the CP of the vehicle (not the CP of the wing! though it may be close). In the case of a flying wing, you do not have the tail to help you arrive at the vehicle CP, only a wing CP. Thus you need sweep in the wing (30 deg or more), and you will find that the location of the CP will actually be located at some distance behind the wing surface, and 10-15% of what is called the Mean Aerodynamic Chord (MAC), will help you arrive at the vehicle CG, which will be located at some location on the center root chord. There is much more to these calculations, but I will keep it simple for this post. Email me if you need more detailed answers with spreadsheets and equations. Provided you have arrived at a good longitudinal stability, your flying wing will respond very well in pitch, without nasty stall characteristics and with good overall controllability.
Now lets discuss lateral/directional stability. This is a topic of primary interest to me because I did my PhD dissertation on this very topic. A flying wing can be stabilized in really three ways: 1. dihedral to mid-span and then anhedral to the tips (like the wings on a bird) but these are big angles and hard to support with a wing spar. 2. Winglets on the tips or a vertical tail on the centerline. 3. active "drag-rudders/ailerons" and several active control effectors with lots of bandwidth and computing power (think of the B-2 bomber here). So with three options, why are most flying wings option 2? Because it is easy to manufacture (not #1) and control (not #3). So what about dihedral? Well you can add dihedral, but at the cost of controllability. You will find that well designed wings (like the CombatWings that I pointed out earlier) do not have dihedral and they perform great! Dihedral will not allow you to easily command an "aileron roll" where the airplane rolls about its x-axis, but rather (with dihedral) it will perform a "barrel roll". To me the "aileron roll" is more desirable, and is more responsive to control inputs. Winglets also have the advantage to apply the lateral/directional stability to a flying wing because of their location (in the x-axis) relative to the vehicle CP, meaning that they are located behind the CP and not ahead (this is stabilizing, think of a rudder on a boat and its location). While you can also use a vertical tail on the centerline of the flying wing, you will need some form or a fuselage to locate the vertical tail aft of the CP. The vertical tail is not desirable on most flying wings because of the motor and propeller being located there and a camera up front.
I hope that this helps. Like I said, if you need more info, just send me an email.
Cheers,
Paul
Paul,
It was nice to read your detailed explanation. Could you tell us why dihedral is a NO for flying wings?
Thanks
-Saad
Trent,
Dude, I like your enthusiasm.
I have some recommendations for you. Go to www.cstsales.com and look at their FeatherCut hot-wire bows. You need to keep the wire very tight to get a smooth wing and not have ripples in the surface.
http://www.cstsales.com/tekoa_hot_wire.html
They include hot-wire instructions and you will avoid many of the simple mistakes. The best part is that the wing machine uses a set of pulleys and a mass to pull the hot wire at any possible root to tip ratio and it does it autonomously! You don't need to pull it along with a buddy to help.
For your foam, you should use 3M Super 77, and apply it 12" away from the surface and in 3 lite coats. The problem you had with melting the foam is due to the Acetone propellant that is in the 3M Super 77 and Super 90. The only way to avoid melting the foam is to use less and from a further distance so that the Acetone will evaporate quicker. I have used this technique on several thermal duration gliders and flying wings.
Also, I would recommend the MH-45 airfoil for a flying wing, and check out Martin's website
http://www.mh-aerotools.de/airfoils/
I have been building planes from scratch for 20 years now, and I do aircraft design professionally for a large US aircraft company. For longitudinal stability (pitch) on a flying wing you need "reflex" in the airfoil. It is completely different than a regular wing. For lateral/directional stability (roll/yaw) you need winglets at the wing tips or a rudder along the centerline. Consider also to add 2.5 deg of negative twist, or incidence at the tip, to reduce your tip-stall onset. And be sure to not have any dihedral. Look at the CombatWings.com website for a good planform. I have flown Zagi and CombatWing and a few others. The planform and airfoil on the Combat wing is much better. Key point here is the elevons. Shorter chord at the root and larger chord at the tip, about a 2.5:1 ratio.
I hope that these recommendations help you, I hate to see anyone fail. Feel free to contact me if you need additional technical help on your aircraft design endeavors.
BTW, did you get Raymer's Aircraft Design book yet? You can also download the pilots handbook of aeronautical knowledge. This is a basic introduction to aircraft for beginning pilots. I used this many years ago and when I was starting out, and I highly recommend it.
http://www.faa.gov/library/manuals/aviation/pilot_handbook/
You can also get Nicolai's book of RC airplane design. I know him and have worked with him, and his work in the aero industry (30+ years at Lockheed Martin) is solid. You can download it here
students.sae.org/competitions/aerodesign/rules/aero_nicolai.doc
You might also consider buying Phil Barn's DVD's on composite wing building techniques. I have these and each time I watch them I learn something I did not see the time before. He truly has a great nack for making foam core wings.
http://www.rcgroups.com/forums/showthread.php?t=190214
His email is listed in this thread.
-Paul
Hi Trent,
Nice work. I'm following all you episodes.
You may also have seen this video. It shows how a single person can cut it. And we do not have to worry about running in the same speed at both sides, since it automatically guarantee that.
I don't have any experience on this. But this seems a nice approach.
https://www.youtube.com/watch?v=R0ZCnR_g6ZU&feature=related
Am I missing something?
-Ruwan
Hi Trent,
Nice video. I would suggest that you mark the..what i call "timing points" on both airfoils rather than the foam. Just removes one step from ur build process for future. Me and my brother, we use this technique and call out the numbers to each other to time between ourselves and have cut come very nice wings.
-Saad
Nice work man! Here is how to make your wings come out "perfect". No bumps, no waves. I cut some glider wings like this back in the day and they turned out amazing! It is really worth the time to build up a quick jig like this. Plus you can be a one man shop!
https://www.youtube.com/watch?v=S2TfNYSYLC8&feature=related
-Beall
@trent that is cool , looks like you are making good progress, looking forward to maiden flight.
@Dany: It is a new camera, funded by the Kickstarter project... I'm using to now to get familiar with it. Its a Panasonic HDC-TM900K.
@Mark: Thank you! Can't wait to see how it flies..
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