View Full Version : Spoiler Alternative? Variable pitch dive brake.
12-15-2005, 08:20 PM
Just fishing here. I wanted some input on the feasibility of using VP as an alternative to the weight and drag- and extensive wing mods required- for adding spoilers to my Bird of Time electric convert particularly, or any sailplane really, for emergency use or glideslope control.
I have seen VP with a hollow shaft outrunner and specialized hub for much smaller planes and it occured to me that with a sailplane's large prop it would be a very effective dive brake when flatpitched for escaping the clutches of an overachieving thermal with the virtue of keeping the wings on the aircraft!
Is such a creature available in a scale such as a 200- 300 watt or so motor? Is it even a good idea? Might be quite destabilizing possibly...
Even retracted, spoilers cause some drag unless they are masterfully crafted and add weight and outboard mass to the wing. With all the simple 2 meter ARF types out there it could be a popular retrofit.
12-16-2005, 12:50 AM
20-odd years ago, the rage was "talking bird" spoiler. The top front half of the fuse opened up, and the model would react to how far open the hatch was. Closed, there was virtually no drag, and allowed easy access to batteries/servos/etc. It requires a good servo, but it sure works.
12-16-2005, 04:41 AM
Not to sure how you would get the glider down without damage to prop.
But, Castle Creations has released a new ESC that has dynamic braking on it. You can use the throttle to use a regular prop as a brake.
12-16-2005, 11:50 AM
To Teslawinger, your idea might work, but I have a hunch it would be de-stabilizing if a "flat" pitch was selected. A 12" solid disc (aerodynamicially solid, that is) suddenly appearing on the front of a glider might be akin to hitting a wall. My experience with powered gliders has been, if low throttle is applied (lower than needed for flight) the model will lose altitude quickly but not violently. Bear in mind that is with a fixed-pitch prop. So you should be able to exit strong lift by either "Freewheeling" the prop or applying enough power to pull your way out. The complexity and expense of a VP may not be needed.
I agree that spoilers are difficult to retro-fit, but it's not as necessary on powered gliders. They are heavier than TD (unpowered) gliders and the prop can be used to exit lift. Also if the model has ailerons (dual servo driven) a bit of reflex will speed up the descent. Just my opinion, Ron
12-16-2005, 03:19 PM
I think spoilers would be a safer solution. When I have had controller problems and the prop kept windmilling the plane did not seem as stable.
12-16-2005, 04:36 PM
I think you're over thinking this one.
I've got several large gliders with spoilers, the couple of grams extra weight in the main panels isn't noticable. On my ancient Olly II I use a bit-o-string to actuate the spoilers so all I have in the wing is a couple of pieces of balsa and a magnet (tiny, rare earth) to help hold the spoiler down. On another I use two hs-55s and that isn't so much weight either.
Of course for getting out of monster thermals I don't use spoilers I just peg the control stick in the bottom right corner and spin the thing down.
I get a nice tumble that kills the lift pretty well and keeps the speed down to where it doesn't rip the wings off, and on that thirty year old OlyII I could easily rip the wings off.
12-16-2005, 05:03 PM
Thanx for the thoughts, guys.
Having seen the degradation of glide from a windmilling prop (when I had inadvertently switched off the ESC's brake) got me thinking along these lines.
I have experimented with various "underspeeds" for glideslope control, and yes, if it is not spinning fast enough to provide thrust, it is creating drag- but not enough to prevent a dive disaster I would think, like spoilers or flaps for emergency use in heavy lift.
Of course the prop would need to be stopped near the ground, after helping on approach and now it will float forever in ground effect, all cleaned up again, too low to bank with that long wing... as options dissipate.
Fighterjets pop up that barndoor on top, the space Shuttle has a split rudder. Drag chutes are too messy to consider. "Talking bird?" Interesting- Yawn for Drag? :O
Having lost a few gliders to monster thermals- and experiencing the sheer willingness of my new Bird of Time electric convert to soar- as well as the challenges of landing it in my small field, got me thinking of DRAG CONTROL!
Most likely I will add the spoilers since it MUST be very effective at killing lift and speed-limiting. Graupner makes some popups that look like they seal cleanly, appear well made and reasonably easy to install. It's gonna be a long winter, so....
PS: Ron, thanks for the input- but my 10 ft Bird of Time is no heavier than the glider version since the first thing I did was saw off the huge hunk of steel weight in the nose and add a modest 8 cell NIMH battery (ZAGI 1800 HiRate pack and a lightweight 200 watt outrunner) for just a few climbs, unlike my early efforts where I used very heavy batteries in 2 meter redrive gliders. 3 meter ships are less burdened by the powerplant weight than the 2 meter ships and essentially get a 'free ride' but lipos and brushless motors are improving that equation these days for the smaller ships. Also my intentions were for simple, non articulated wing alternatives or I'd simply Crow it on down.
PS: Rob- Another Gassbagger heard from! I love the Oly and had one many yrs ago. The Tailbiting manuever is useful and I've used it but the descent rate might not exceed the lift! I was just looking for someone to talk me out of cutting into those sweet wings but theres only so many ways to skin a weasel... ;D
10-27-2008, 05:29 PM
Lets see if it works
vBulletin® v3.8.3, Copyright ©2000-2017, Jelsoft Enterprises Ltd.