The Air brake Airbrake dive brake divebrake thread

[Hot Wings]

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?

 

[PTAirco]

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.

 

[Hot Wings]

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.

 

[Vigilant1]

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).

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

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.

 

[autoreply]

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

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

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

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...

If so, how?

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:

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

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...

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

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.

 

[Topaz]

...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.

That's in the neighborhood of what I had in mind. What I want is to simplify the controls for thrust and drag production in a motorglider-type aircraft. I may apply this to my own project, but it needs a lot more work in terms of both looking at possible contingencies and also at the mechanism itself. Here's the broad-brush outline:

Imagine an in-cockpit control that looks like a large throttle quadrant. There is a detent approximately mid-range in the travel. That detent position constitutes idle for the engine, and retracted drag brakes. Pushing the control forward from the detent progressively increases throttle. Pulling it aft from the detent leaves the engine at idle and progressively extends the drag brakes, precisely how they're handled in a sailplane.

The thinking here is two-fold: You should never have a situation where you need power and drag brakes simultaneously, at least in motorglider usage (again, I need to expend some more thought to make sure that's a true statement), so adding them to one control simplifies the cockpit a lot. Also, in terms of situations where you want to immediately increase climb under power (go around, etc.), I think it's a benefit to make sure that the drag brakes are automatically retracted by the intuitive control motion of adding more power. Initiating a go-around should require nothing more than shoving the throttle forward. Doing so from an aft-of-detent position with deployed drag brakes automatically retracts them on the way to opening the throttle. Operation of the "thrust/drag" control then becomes intuitive from the point of view of the pilot - the "natural" way to use it is reflected in the correct manipulation of the engine and drag brakes.

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 don't like the idea of making the drag brakes electrically deployed - one more system to fail. But that's just me. Perhaps someone else is less sensitive to that.

One thing about drag brakes that is frequently overlooked by power pilots is that, to be at all useful for flight-path control on approach, their degree of extension needs to be able to be smoothly modulated by the pilot. You don't simply "set them" like flaps. In sailplanes, drag brakes take the place of the throttle in adjusting the flight path on approach - and you use them in the same manner, continuously modulating them to set the glide-path you want. In a powered aircraft, what you'd do is set some small throttle opening (or idle) and then modulate the flight path with drag brakes. The only time you'd touch the throttle is either after touchdown or to initiate a go-around.

The other option, which I don't like, would be to instead "set" some position of drag brakes to steepen the approach, and then modulate the throttle to adjust the glide-path. But that would require some means - preferably automatic or "natural" - to retract the drag brakes for a go-around. If they were separate controls, I'd put the drag brake next to the throttle on the quadrant, so that pushing the throttles forward "naturally" has the pilot's hand falling on the drag brake control as well, with "forward" meaning "retracted" for the brakes. Shoving the throttle forward for a go-around should mean that the drag brake control is shoved forward also. Retracting drag brakes restores lift (the opposite of flaps), so there shouldn't be any "settling" of the aircraft when they're retracted.

I think trying to modulate both throttle and drag brakes would be a mistake. Too complicated, and too easy to get them working opposite each other. And I'm concerned that any design where they're separately controlled is conducive to that situation - and probably in some high-stress environment where it's easy to forget to retract the drag brakes, with a very probably dangerous effect on rate of climb, just when it's needed most.

 

[Hot Wings]

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!

 

[Hot Wings]

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.

 

[autoreply]

That's in the neighborhood of what I had in mind. What I want is to simplify the controls for thrust and drag production in a motorglider-type aircraft. I may apply this to my own project, but it needs a lot more work in terms of both looking at possible contingencies and also at the mechanism itself. Here's the broad-brush outline

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.

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.

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.

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.

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.

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?

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

 

[Topaz]

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.

A good point, and a practical one. I've been thinking of having a mechanical button on the quadrant handle that must be pushed to pull past the detent, but this is another option in that regard, and possibly a more simple one.

 

[Topaz]

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

I Googled that and got some sailplane stuff - in German - regarding what I call "Spoilflaps" (picked up that nomenclature from Rutan, who named his version of those devices that on the Solitaire.)

I'll Google Translate that page later - that kind of mechanism never really interested me since, for the type of thing I'm looking at, flaps should not be required, even in camber-flap form.

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.

That's pretty much as I'd envisioned for a system with separate drag brake and throttle controls, except that I hadn't foreseen the pin you describe. Which is brilliant.

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.

Yes, I'm not surprised. If you present the opportunity for the controls to be abused, they will be, no matter how unlikely that may seem on the design table.

You know, since this discussion is about ultralights, 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. 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. 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.

 

[Hot Wings]

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.

 

[Topaz]

...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.

Gotcha. Just throwing it out there.

So, more of an ultralight motorglider, then? In that case, I'd fall back on either a single combined throttle/drag brake control as I described above, or separate but rigged so that the drag brakes cannot be extended when the throttle is above idle, as Autoreply described. I personally prefer drag brakes to flaps in any situation where flaps are not required for performance reasons. My detailed preference is for the Schemmp-Hirth style "blades", since they're so easy to build. I have a schematic diagram of them in another thread somewhere. If I can find it and post a link here, I will.

EDIT: I found the post I was thinking of, but the diagram I posted was apparently stripped out during one of the forum software updates. I've got the diagram around here somewhere. If I can find it, I'll link it in this thread.

EDIT 2: Found it. See below. This view is from aft looking forward, in the right-hand wing. The tan rectangle is the back of the spar. The green dots are pivots, attached to the spar. Everything else is either labeled or probably very obvious. There's a mistake in the drawing - the blades of the brakes would probably be aft of the arms, not forward.

 

[Hot Wings]

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.

 

[SVSUSteve]

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.

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.

 

[Norman]

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.

Several points in that paragraph sound so familiar:depressed

 

[cluttonfred]

Gotcha. Just throwing it out there.

So, more of an ultralight motorglider, then? In that case, I'd fall back on either a single combined throttle/drag brake control as I described above, or separate but rigged so that the drag brakes cannot be extended when the throttle is above idle, as Autoreply described. I personally prefer drag brakes to flaps in any situation where flaps are not required for performance reasons. My detailed preference is for the Schemmp-Hirth style "blades", since they're so easy to build. I have a schematic diagram of them in another thread somewhere. If I can find it and post a link here, I will.

EDIT: I found the post I was thinking of, but the diagram I posted was apparently stripped out during one of the forum software updates. I've got the diagram around here somewhere. If I can find it, I'll link it in this thread.

EDIT 2: Found it. See below. This view is from aft looking forward, in the right-hand wing. The tan rectangle is the back of the spar. The green dots are pivots, attached to the spar. Everything else is either labeled or probably very obvious. There's a mistake in the drawing - the blades of the brakes would probably be aft of the arms, not forward.

View attachment 17677

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

 

[autoreply]

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.

 

[cluttonfred]

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?

 

[Matt G.]

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.