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Friday, March 16, 2012
PSP "profile slope plane" design - The Original
Profile Slope Planes are beginning to show some promise for the future of slope VTPR aerobatics though it’s a slow pace in the big bad world of “foamy” glidering. Since I introduced the concept in 2011 with the launching of the prototype ZEPPELIN above, there is an interest of getting back to wood construction techniques. I am seeing a few projects across the globe of modelers designing their version of a PSP, and that gives me great delight. Wood is here to stay folks, no doubt about it. Balsa wood is an amazing thing if one can understand just how wide its capabilities are in a glider design. And since I am primarily involved with the rough and tumble world of VTPR aerobatics, this is especially true. There is naturally great apprehension in light of foamy durability to building a woody type “crunchy” model which at first, even I was skeptical. But my love for building crafty things beyond just glider models guided me to move forward and come up with something with greater durability and flying performance that could attempt to re-energize wooden modeling and have fun doing it.
Originally, the primary thought in designing a wood based VTPR glider was that it needed to be a tool, a model one could go out and not worry about its beautiful cover getting marred up. Something the pilot could use without limit to hone his skills, have fun and be easy and inexpensive to build. It needed to be a two piece unit sized in a popular wingspan, and it needed to be very lightweight and durable. A daunting task in wood for sure. The span size chosen was 68” (1.7m) and the target weight was 25-28oz equaling a 7.5oz wing loading. Secondly, the idea was to be able to fly slow and low yet have an efficient wing to be able to zoom back to a good manageable kinetic energy state to be able to cleanly set up for the next aerobatic figure with confidence. It is very important that the VTPR pilot once he enters below 72” off the deck in all manner of aerobatics whether it be inverted or slow rolls and such, he or she needs to know his craft is going to perform, and with ease. So I went with a true airfoil 100% of the chord length not using the typical aileron stock we see in generic wing construction. The airfoil of choice by Sarge Barth is his SB96V/VS, but with a modification I chose for a little bit more efficiency at 7.5oz WL.
The fuselage construction had to be very simple for the average modeler. Using 1/4” balsa stock in a 3 layer ply arrangement turned out to be a promising design given the total “low mass” of the model. Granted, balsa wood will never be as durable as EPP foam. But I found that once the design enters sub 30oz in the 1.7m span range we have something much different than the conventional age old wooden box fuselage design. The plywood balsa fuse plank becomes quite formidable yet is very lightweight. Another primary reason for going with a plank fuselage was to save weight and complexity in assembly. I wanted something that if crashed hard, and a broken tail boom the result, the modeler could repair in the field or on the bench in 30 minutes. He wouldn’t need to reconstruct a box anymore. And as it so happens during deliberate testing this occurred a few times and it’s pretty cool to see the repairs went quite easily and quickly. Ultimately we want to fly unabated of any fear of breakage but as mentioned above, this concept is a leap forward in the wooden RC world when it comes to durability. It takes a lot of abuse having a low mass component. The more I fly the design the confidence level grows exponentially similar to flying a foamy.
Next and I think I’m one of the first in slope VTPR to do this, all the radio and servo gear is mounted externally on the fuselage open to the elements. At first one might think, “that’s not going to last, it’s going to get banged up”. This isn’t the case. After many hours of field testing deliberately crashing the prototype PSP there were no effects to the gear. That is unless you enjoy flying in the rain or plowing into a tree or the ground at full speed. It’s quite amazing and that’s good news. The open design has a couple nice features. Ease of maintenance on the gear, and the protruding gear doesn’t increase any noticeable degree of parasitic drag. These PSP’s fly slow and low VTPR and are designed for sub 25 mph air speeds but they accelerate from 0 to 25 very quickly due to their efficiency. The flyer who normally enjoys big and fast aerobatics won’t be thrilled about this PSP design once he experiences the lagging high speed he might normally enjoy. He will be forced to fly in closer proximity to himself or land it and fly something else. These are purpose built machines that will deliver maximum performance for which they are designed, nothing more.
Lastly, this PSP being a two piece design is easily transportable and incorporates the tried and true rubber band wing mounting system. This feature is very effective in saving the wing and fuselage. I have deliberately crashed the model with no breakages in the wing saddle area of the model. It simply twists on the center axis of the wing or pops off on a hard ground hit. I usually install four #64 rubber bands. They are rugged performers and are not intended to look totally awesome unless you really enjoy the more mechanical looks of a machine over sleek and beautiful lines. They won’t win a beauty contest but they get a lot of looks on the slope. It is true most slope pilots generally have to like what their bird of choice looks like to help him fly better and prouder. My thought about this though pales to the performance beauty these little wood models can produce. I find that experiencing flying an economical, a little unique looking and aerobatically efficient model in VTPR mode is quite an exciting thing to experience. Performance beauty IMO trumps external beauty any day. I hope some of you reading this will try wood building again in a new light of understanding that I have shared here over the old fashioned way of box building. You could be positively surprised and encouraged of the results.
If you need any assistance feel free to contact me.
Good Lift… Eric
PS: As of this writing the second in the PSP line called STICTIONis now in the air flying beautifully below ;)