The Air brake Airbrake dive brake divebrake thread

I did a quick search of the archives and found plenty of references to this subject but the were all scattered about as tangents to the original thread subject. So I thought I'd start a new thread that could be found easily with the search function ma\king it easy to reference in the future - thus the odd title.

This thread is not to be confused with approach control devices that are pretty well covered here - Precision approach control

My Duster has flaps that rotate to a 90 degree position and limit the maximum speed in a dive to the gliders Vne. It looks like I'm going to have a few pounds to play with on my part 103 project and am considering adding a similar device, but set to automatically deploy at some predetermined speed, maybe in the same way that an automatic leading edge flap does.

My questions are:

Has this been done in the past? (assuming yes)

If so, how?

Can anyone point me to a web source, or other source, where I can study up on ways to do what I'm considering?

Hazards, other than unwanted deployment or asymmetrical deployment, that I may not have considered?

 

With a Part 103 ultralight, is such a thing even necessary? They are already very draggy machines, with their large wing area if nothing else. I doubt a drag producing device has any kind of benefit.

 

With a Part 103 ultralight, is such a thing even necessary?

For most, no, it wouldn't be of much benefit.

I know the word ultralight brings up a mental image of a wire braced flying lawn chair, but think more along the lines of a Sadler Vampire, or a Swift glider. 500 pounds of drag is a lot for one of these slick designs. Edit: Hummelbird too.

But don't take this thread to be only about ultralights. That is just may particular interest at the moment.

 

I've considered the advantages of drag-inducement for a US-spec LSA. To meet the US requirements, they have to have low stall speeds (45 KCAS) without use of flaps, and for a two place LSA keeping the empty weight low is always an issue. So, eliminating the conventional flaps may be an appropriate design choice (they add weight and complexity, and you've already been forced to have more wing area to meet the no-flap stall speed). For approach angle control, a speed brake/split flaps mounted under the fuselage and for a foot or two out onto the wing can be implemented at low weight and complexity and can generate a lot of drag. As a bonus, compared to conventional flaps they should have lower drag at cruise (no gap to spill air to the top of the wing).

Retracting them is another thing to have to remember and accomplish during a missed approach/go around. I think Topaz may have suggested the advantages of having any retraction switch linked to or proximate to the throttle for this reason--when the power is advanced to the firewall, the boards are retracted in the same motion.

 

Why not? There's no significant difference between the two is there?

Yes. Every certified gliders has them because they have to have them. (They pop out above VNE). Not a pleasant surprise at such high Q; expect +1 G deceleration and your head smacking into the instrument panel. Don't ask how I know...

Schempp-Hirth type air brakes (not limited to Schempp-Hirth sailplanes) have a cap that seals the divebreaks. They are connected to the actual top drag plate by springs and set such that above VNE the caps are sucked upwards a bit. That will tilt them backwards; the "leading edge" of the cap stands proud, the rear of it is pushed on the wing. That's enough to suck them upwards through the overcenter lock.

The TE devices I mentioned in that other topic:

can also be made to automatically deploy; simply enlarge the part fwd of the hinge, such that it's instable enough to pull through the overcenterlock above a certain speed. Might be hard to retract again then though...

Too far inwards (close to the fuselage) and they'll be completely buried in the boundary layer of the fuselage; that's why they're so far out on sailplanes.
Deployment reduces the lift locally. Pulling the same amount of G's with them active will seriously increase your spar loading.

Belly plates (like in the Long-EZE's) generally perform poorly and take a lot of force to deploy. If you have doors; reinforcing them and opening them in flight (upwards to the wing?) might be an interesting option.

 

Why not? There's no significant difference between the two is there?

Generally probably not, but what I'm specifically thinking about is an automatic device that pops out if someone does something stupid and keeps the airspeed below the 138 moh limit BRS has on their parachute that matches my plane. I have the winglet rudders that can be used for approach control by deflecting both.

Might be hard to retract again then though...

This may or may not be a problem. I'm thinking I may not need an automatically retracting device, or even one that is easy to retract manually. If someone has screwed up badly enough in an ultralight that the airspeed is in the 140 mph range than maybe they should be letting the BRS do it's thing and ride it out.

The only (mechanical) system, capable of doing this is the Ventus/Mini-Nimbus/Libelle system:

This is pretty much what I was thinking. Link both sides mechanically and use a spring to keep them down below the target speed?

Thanks for the info!

 

I don't yet know if the mechanism can be built in such a way that the forces are complimentary - throttles usually take quite a bit less force than drag-brake controls, which are usually a manual mechanism.

I think I'd actually like the different force levels. It would give a distinct tactile feed back on exactly when the drag brakes are starting to be deployed. If the detent is light enough to not be an annoyance during routine use it might get over looked at higher stress levels.

 

Hänle-tüten will answer all your problems. :gig:

Normally they're used to block the drag brakes if the landing gear is retracted. For your proposition, imagine 2 levers, hinged on the same axis, being throttle and drag brakes, throttle on the "inside. Now have a pin on the throttle that makes it impossible to pull the drag brake lever behind the throttle. That gives you full control of all scenario's, while making the dangerous one (full throttle plus drag brakes) impossible.

The same solution is now proposed for sailplanes with an extending engine. Several have crashed because the drag brakes were unlocked during launch and were sucked out during climb.

Ok. The only "mechanical" reliable measurement you have to measure speed is at the LE or TE. A spring-loaded "trigger" that is connected to a pitot seems like the easiest way to do it reliably.

That makes things a lot easier. Why not simply have a drag chute? Reliable and simple. The ASW 12, Janus and several other gliders have them. Works fine, is fairly light and you only need a small chute at such speeds.

Normally those are balanced, such that even at VNE actuation force is almost zero.

 

Why not simply have a drag chute?

Because I'd not yet thought of it. Being a pusher complicates things a bit, but it's still an option.

You know, since this discussion is about ultralights,

That is just my interest, don't let that limit the discussion on drag producing devices.

I have to fall back and agree with PTAirco on the subject of drag brakes or flaps of any kind. An ultralight (Part 103), by its very nature, lends itself greatly to simply using slips and throttle to adjust the glide-slope.

True for most, but I'm not expecting slips to be a practical alternative for my project and one of the main parameters is to make it as low drag as possible.

If the opportunity exists to simplify the design, take it. If there really is "unused weight margin" under the 254lb We maximum, take it, and use that as an opportunity to reduce wing area and power required.

I'd really like to reduce the wing area and get the best L/d up a bit, but to meet the letter of the law I'm already at the lower limit. If I could use the the A.C' 103.7 formula I could cut off a little but I don't want to get caught by a strict interpretation of the "noticeable taper" language. The FAA's A.C. assumes a Cl of 1.6 for the wing, which just isn't possible in 3D unless it's a conventional plane with good flaps. Mine isn't.

Power required is already under 15 for legal cruise. Any more and the speed limit becomes a problem.

Adding flaps and drag brakes to an ultralight seems like a needless complication, IMHO. An ultralight ought to be, in my mind, the ultimate expression of the KISS rule. That's only my opinion, of course.

Agreed. Remember I'm one of those that thinks a yaw string and an AOA are the only required flight instruments..

But then we get into the definition of how simple is as simple as possible. Maximum economy of operation, while still using all of the legal cruse speed, and much better safety than the average ultralight are 2 of the driving design parameters. Considering that this is a very slick part 103 and may be flown by less than properly trained "pilots" some kind of speed limiting device just seems prudent.

 

So, more of an ultralight motorglider, then?

Kind of. Maybe leaning more to the motor side. If I get 20/1 I'll be happy.

What would make me real happy is to find a way to get the best L/D up closer to the maximum legal cruise speed, but I'm at the point now where it's time to quit designing and get the drawings started for the making stage. I'm taking advantage of the long weekend to get my 2D CAD moved into 3D for a final check of things like cockpit fit and weight and balance.

I personally prefer drag brakes to flaps in any situation where flaps are not required for performance reasons.

I think we think along the same lines. I think of flaps as variable camber devices. Glide path control is best done with another device. I was just thinking about a third device to limit terminal velocity when I started this thread.

 

Poor man's TOGA?

There's a modification of that with the throttle quadrant of my turboprop. Once the power levers are advanced past a certain point, the spoiler lever is disengaged which forces them back into their retracted position. It turned out to be much simpler from a mechanical standpoint than I initially expected.

 

Several points in that paragraph sound so familiar:depressed

 

Old thread, new questions....

--Structurally, wouldn't it make more sense for this type of air brake to be in front of the spar and pressed against it by the air loads rather than pulled away from the back of the spar?

--Is there any reason that you need to have top and bottom blades to this type of design? I could see using just a top blade as a spoiler to prevent landing float in a relatively low-powered design with large wing area. I think that would also be easier to seal for drag reduction.

--I could also see this as a bottom surface only lift and drag device as I have seen 90-degree lower surface flaps placed forward like this as no-pitch-change flaps in flying wing design texts.

--Is this type of blade brake ever perforated like some WWII dive brakes? Pros, cons?

Cheers,

Matthew

 

Getting it to fit in the wing (retract) under load is the biggie. Bending loads with a single actuation direction (only above the wing) are enormous, which makes construction of them complex, expensive, a lot of labor and heavy.

You don't want them fwd of the spar, since they trash the laminar flow into a draggy turbulent one. Even on "turbulent" airfoils, you want to have them at or behind the 50% chord point.

"Drag brakes" are superior in many regards to Schempp-Hirth drag brakes like the ones every modern glider has. Higher drag though due to the boundary layer being tripped a bit earlier.

You need the holes in the drag plates. Without them you can have a heavily fluctuating flow, like a Von Karman vortex streat. Unpredictable, shaking and can be fairly uncontrollable, notably with the tail in the flow of the drag plates (only sailplanes can move them far enough outboard to not influence the fuselage flow or the flow over the tail.

 

Thanks, the potential applications for non-gliders are very interesting to me. As I understand it, the gap between the brake and the wing surface is also intended to keep the flow more uniform and predictable. That suggests to me that these are best rigged without intermediate postions--deployed all the way or retracted all the way, nothing in between. In that case, I wonder if push-pull actuation and an over-center spring arrangement makes sense so the brakes "want" to remain in either position?

 

For the same reason it is not desirable to have an engine that is either off or full throttle, I would not want 'binary' airbrakes that are either closed or 100% open. The vast majority of the time, a varying degree of partial airbrakes are used when flying in the pattern to land. An over-center lock full open would likely be dangerous. Top-only airbrakes seem to have a tendency to want to go to full open by themselves and they have to be held back, or at least that's how the Grob Twin Astir I've flown works.