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GEV-2.0 Grasshopper, Part 2: Build Update

#1
A brief mid-build update (I tried hard to have everything finished before I came home for Christmas, but then it didn't quite work out that way). The only pieces I still need to cut out is the lower fuselage/battery pod and the winglets which connect the wings to the T-tail. With a build this complicated it is important that I make sure all the major assemblies fit together properly before I plan on going nuts with the glue. I altered the design and some of the parts from my original sketches and/or needed to trim to a proper fit, which added time.

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Since I teased y'all with it in the previous thread, I'll point out a few of the more unique features of the plane. The wing design is meant to improve stability in ground effect. A flat canard and lifting T-tail combined with a thick, lower AoA, forward swept, negative camber, reverse delta main [inner] wing should provide good center of lift (eg. longitudinal/pitch stability) while flying close the ground. Also washout in the main wing should improve stall characteristics, thank you to Ryland Research LLC for that tip! Experience from flying the previous plane (the Mayfly) led to design decisions here: wood reinforcement on the front and outer edges of the canard with cross-brace (see above) and doubling up the foam board on the motor mount to greater sustain impact damage. Another feature is the removable wheels that slide into the skiis (see below), which should allow relatively easy switching from wheels to water capable takeoff/landing. Once I am able to finish the plane, satisfactionally test it, and make any needed changes and if a noticeable amount of people express interest, I will consider sharing the plans.

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BATTLEAXE

Well-known member
#2
A brief mid-build update (I tried hard to have everything finished before I came home for Christmas, but then it didn't quite work out that way). The only pieces I still need to cut out is the lower fuselage/battery pod and the winglets which connect the wings to the T-tail. With a build this complicated it is important that I make sure all the major assemblies fit together properly before I plan on going nuts with the glue. I altered the design and some of the parts from my original sketches and/or needed to trim to a proper fit, which added time.

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Since I teased y'all with it in the previous thread, I'll point out a few of the more unique features of the plane. The wing design is meant to improve stability in ground effect. A flat canard and lifting T-tail combined with a thick, lower AoA, forward swept, negative camber, reverse delta main [inner] wing should provide good center of lift (eg. longitudinal/pitch stability) while flying close the ground. Also washout in the main wing should improve stall characteristics, thank you to Ryland Research LLC for that tip! Experience from flying the previous plane (the Mayfly) led to design decisions here: wood reinforcement on the front and outer edges of the canard with cross-brace (see above) and doubling up the foam board on the motor mount to greater sustain impact damage. Another feature is the removable wheels that slide into the skiis (see below), which should allow relatively easy switching from wheels to water capable takeoff/landing. Once I am able to finish the plane, satisfactionally test it, and make any needed changes and if a noticeable amount of people express interest, I will consider sharing the plans.

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I tried building a simple and crude ground effects sled in the spring and had fun with it in the yard, which was wet from rain so it did fall apart after a couple packs though it and some impacts. I am definitely interested in seeing how this machine goes and getting plans for it, if you are willing to share that is. Such a cool design (y)
 

speedbirdted

Well-known member
#3
I've always wondered, mostly because I've never bothered to actually try to build one, but what kind of design work goes into determining what makes a good ground effect vehicle?
 
#6
I've always wondered, mostly because I've never bothered to actually try to build one, but what kind of design work goes into determining what makes a good ground effect vehicle?
I am not an expert, but I was an aerospace engineering major my first year and college and I have read a lot of research papers on the aerodynamics in my own time. The math is VERY complicated because as the wing approaches the ground the center of lift shifts forward with respect to height from the ground. The design principle of any good GEV design is to use this aerodynamic quirk create a damped feedback loop that as the craft nears the ground it increases AoA and lift to self correct flying height (commonly involves a high mounted lifting tail or tandem wings of different stall angles). The other design goal is to have a wing design which is less sensitive to this shifting center of lift (reverse delta, forward swept, and/or negative camber).
 
#8
Finished the building process today! Let's start with some the positives: once everything was glued together the main body of this aircraft was solid as a rock, and initial chuck tests were stunning (tracks straight, inherently balanced, stable glide). The downsides: assembly ended up complicated/time consuming (even once the pieces were cut it took a couple of days to glue everything together) and it is heavier than I would have preferred (546g, water and snow landings/takeoffs are going to require more buoyancy). Therefor, I have decided against posting the designs for this particular plane. Instead I will be further developing this design by incorporating the lessons and ideas I have gleaned from putting this prototype together and [soon] flying it. I have already sketched out multiple design improvements that should reduce build time and and increase lift and buoyancy. I will post more once I have had a chance to fully test this plane out. [ Do you guys have any recommendations for inexpensive CAD software I can use to draw the plans with? ]
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Attachments

#9
Before I could fly it I had to set up the transmitter. Choosing elavon configuration disables the rudder input so I had to mix it to the elavons to the rudder get it to respond, and even then it took some fiddling to work properly. Then I did the maiden flight; it mostly ended up confirming most of the concerns I had at the time of my last update. I found it more difficult to takeoff straight with these wheels than I did with the skoats on my last plane. I was disappointed that getting the plane fully airborne required full throttle and a bit more distance to get up to takeoff speed than the Mayfly, then when in the air it did not turn as sharply or smoothly as that plane either. Two things that contributed to this was my CG was a tad nose heavy and my control surfaces were undersized for a slow flyer (as Josh says: always better to oversize and risk trimming later). Once again my lack of experience as a pilot was probably the biggest factor in the many crashes. I attempted to fly it until the prop started to show signs of cracking (I regretted not bringing a spare prop). I did an full assessment of damage to the plane: a little wrinkling and cracked seams on the tail and winglets and that was it. A new prop and re-gluing some seams and it would be ready to fly again another day. After I got back from flying I removed the wheels and covered the openings with tape for a buoyancy test... I suppose I should make the floats a wee bit wider next time.

Speaking of next time, once the CG was dialed in it sat ~0.33 of the center cord length. According to a research paper I had on hand, the ideal CG on his reverse delta with lifting tail RC plane was .28 cord length. I also found paper that included some useful data on lift to drag coefficients for different AOA, which proves I can get away just fine with a little more AOA on the redesign. The new design is going to be a little bit less long or tall but with more wingspan. Instead of elavons it will have a single elevator and ailerons on the upper winglets. I'll also be upgrading the to the lighter 2205 2300kV motor paired to a FunFly 4s 1300 mAh battery. Between that and ditching the wheels I am already saving 60g without before improving the airframe! I did [strongly] considered to mounting the propeller ducted in the middle of the plane where it would be less likely to be damaged like this time, however I opted for to keep with the puller set up here until the next complete re-design (GEV-3.0).

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