drag brake configuration & sizing?

[Bob Chiang]

Hi all,

I was so impressed with this model that incorporates a drag brake http://tinyurl.com/6qx4bs for aerial photography that I decided to build something similar: a flying wing with underslung pod. Please look at the attached sketch showing the side view of my proposed fuselage pod and back view of the proposed drag brake.

What do you think of a drag brake consisting of a flat plate that is mounted to a servo hub? In normal flight the plate will be oriented flush with the back of the fuse pod. When deployed, it will rotate 90 degrees to expose two flaps beyond the sides of the fuse.

How big do the exposed flaps need to be to have a significant effect? Any comments will be welcome.

Thanks,
-Bob

[eflightray]

The drag brake I believe was just used for landing, it gives a much steeper descent.

I used to have a large flying wing with a 'trap door' brake, (a hinged section on top of the wing near the L.E.), it made quite a difference to the descent, but also made for a heavy landing if it wasn't controlled just right on the approach.

Some fliers use 'crow braking', usually flaps down, ailerons up, though some use just ailerons up. But it's more for landing only, probably not while taking photographs.

I would have thought your air-brake would put quite a strain on the servo, and could be at risk of damage when landing, (if belly landing).

What is your main use for the brake?.

[JetPlaneFlyer]

A couple of observations...

First even when 'retracted' the design in your drawing will give a blunt box like rear to the fuselage which will create drag all the time.

Second i dont think it will be very effective as a brake.The actual brake area is quite small and unlike the wing mounted 'spoiler' type brakes it wont effect the lift produced by the wing.

Steve

[Bob Chiang]

Thanks for the comments and questions.

The intended use of the brakes will be to land in tight areas, not while taking pictures.

Our simple hinged spoilers transfer the air load onto a servo arm and through the gear train being held by the servo motor. This design would transfer the load onto the servo hub bearing and not rely on the motor to hold the load. Maybe that will break or wear out the bearing quickly since servos aren't usually loaded this way?

Good point about risk during landing: I'll have to check if the flap would protrude far enough to catch when the fuse pod and wing tip contact the ground.

Also good point about the blunt back end creating drag. I do know there are other models with this type of pod configuration. Call it a Kamm back?

The brake size is the main question I had. I agree that the area seems too small, but I have no experience to base that on.

I'm still trying to understand the differences between spoilers and brakes and found this http://tinyurl.com/56p8he . If spoilers decrease the wing lift, then doesn't the model have to fly faster to remain unstalled? Thinking out loud, I guess that's why you can approach at a steeper angle. But shouldn't brakes that don't spoil the lift allow you to approach at a steeper angle AND slower?

-Bob

[JetPlaneFlyer]

But shouldn't brakes that don't spoil the lift allow you to approach at a steeper angle AND slower?

-Bob

Good point.. Spoilers are about making the model come down faster but do actually increase stall speed, which would explain the 'heavy landing' comment above. Air brakes that dont effect lift production wont bring the model down as quick but would not increase stall speed (they wont decrease it either)
If you want to come down steeper AND fly slower then it's flaps you want .. No good for a flying wing though :(

Steve

[Brandano]

Hmm, depends. inboard split flaps coupled with elevons moving upward might help by increasing the wing camber while augmenting the drag, and reduce the chances of tip stalls by adding washout. the problem with these is that ehy are vulnerable in a landing, but with an underslung pod for a fuselage they might be protected enough.

[nmasters]

Sure you can use landing flaps on 'wings. You just can't expect as much lift increase because of the pitching moment limitations. On swept 'wings you can do a self trimming trialling edge flap but on planks you're limited to a split flap with the hinge well forward as illustrated here (http://www.rcgroups.com/forums/showthread.php?p=10092995#post10092995) . One can argue that this is an air-brake but there is a small increase in lift. There's a picture of an AV-36 with the brakes open a few messages down the page from that drawing.

--Norm

http://uk.youtube.com/watch?v=S5ra48VPLos

[JetPlaneFlyer]

One can argue that this is an air-brake

Yep, that would be my argumant.

[HerkS]

Here's a system I've used with good results. Combines to some extent the effects of both spoilers and flaps.

[nmasters]

http://uk.youtube.com/watch?v=S5ra48VPLos
It seems that I have forgotten how to embed yuotube videos. Anyway that link is to the maiden flight of a big Fauvel AV-36 a few months ago.

[DT56]

It seems that I have forgotten how to embed yuotube videos. Anyway that link is to the maiden flight of a big Fauvel AV-36 a few months ago.

Thanks for that link! :)

I'd not seen any flight of the full scale AV-36.

[BMatthews]

I've seen flaps used on flying wings but for them to work with little or no pitch coupling the sweep angle of the wing needs to be carefully chosen. And don't ask 'cause that's all I know about it.... :D

If I was to guess at a good starting point I'd alter the wing sweep and chords such that the center section flaps were positioned at the line through the 25% MAC point. This way the flap drag and any pitching is occuring at what should be the aerodynamic center of the overall planform.... but that's just a guess so don't quote me... :D

The drag flap shown by Herk is supposed to work well but it needs to be an all or nothing deployment. Angles of only a couple of degrees will still result in some spoiler effect on the upper surface so again it's all or nothing.

I'm also in total agreement with the comments on the blunt back end and drag plate of the PDF in your first post as mentioned by JPF. Any EFFECTIVE drag added by that small cross pivoting plate was pretty much a placebo. Notice in the first part of the video that the sink rate was pretty bad and power was added quickly in each case when gliding was attempted. Of course the rather pendulous camer pod sure didn't help but in light of all the rest of the drag that produced by the "drag brake" would have gone totally unnoticed.

[nmasters]

The one I pointed out yesterday works for planks or swept 'wings. It's not as complex as trailing edge surfaces. I show how to place one on a swept 'wing here (http://www.rcgroups.com/forums/showthread.php?p=9021614#post9021614) . I know of several 'wings that used them or a very close cousin. In spite of the fact that Lippisch used it 66 years ago some lazy patent clerk granted Rathion a patent on a very minor variation earlier this year. Trailing edge flaps can be talered for that job too but it's a bit more involved. Here's a video of a swept 'wing with large trialling edge flaps (http://uk.youtube.com/watch?v=33jU_g3XdL4&feature=related) that lands quite well. Ive also got some pilot reports.


--Norm

ps cant wright anymore tonight, the ambien is making it hard to control where my fingers go' :p

[ciurpita]

i've often wondered about spoiler sizing. the spoilers on models seem much bigger than what i've sen on full sized gliders.

on a full size schweizer 2-33 with a ~50ft wingspan, there is a ~3 ft x 6 inch spoiler on top and bottom of the wing. the top spoiler is hinged to open forward while the bottom is hinged to open rearward. airflow pushes the top spoiler closed while trying to open the bottom one. this balances the forces, making it easy to control with one hand. a fully opened spoiler reduces the L/D of the 2-33 from 22:1 to 5:1. a slip (rudder and aileron opposite) can be used to decrease the L/D even further.

since the gliders have to land high to account for wind, gusts and sink, the spoiler is constantly being adjusted to control the glide path to bring the glider down to the landing spot where it then flares and lands by slowing down to a stall a couple feet above the runway.

and why are they called dive brakes? they aren't used to slow the plane down on a glider. aren't dive brakes used on navy dive bombers to slow their airspeed in the dive?

[HerkS]

[QUOTE=nmasters]The one I pointed out yesterday works for planks or swept 'wings. It's not as complex as trailing edge surfaces. I show how to place one on a swept 'wing here (http://www.rcgroups.com/forums/showthread.php?p=9021614#post9021614) . I know of several 'wings that used them or a very close cousin.

Norm - I built a big plank sailplane some years ago - had the type of flap you illustrated here, but I put it on the upper and lower surfaces. I thought it was very effective and there was almost no issue with pitch changes. Later converted to electric - flew well in both modes.

[HugePanic]

why not mount a variable pitch prop an reverse it for landing???

or find a ESC with a break that can be switched on/off during flight....

[MarkusN]

and why are they called dive brakes? they aren't used to slow the plane down on a glider. aren't dive brakes used on navy dive bombers to slow their airspeed in the dive?
Well, you can use them to avoid exceeding vne in a dive.

[nmasters]

More than just avoiding exceeding Vne. The later model P-38s had this kind of forward split flap because it creates a nose up pitching moment. The P-38 had a very low critical Mach number and the controls would lock up if it exceeded it in a dive. Opening a trailing edge split flap behind the shock wave doesn't create as much extra drag as it does at lower speeds because the shock is already having that effect (like opening a split flap behind a split flap). By moving it forward they got the hinge ahead of the shock wave and restored some of its normal function plus got a positive Cm as a bonus. They called it a dive brake but the P-38 relied almost entirely on the the pitching moment of the forward split flap to pull out of a trans-sonic dive.

[mark963]

I've got a big glider that has a huge boxy fuselage (it houses standard size servos with room to spare) and no spoilers, ailerons or brakes. I've never flown it and I'm converting it to electric. I've wondered about putting a couple pop open brakes on either side of fuselage under the wings, behind the CG. Another thought was laying one on top of the fuselage behind the wing, maybe about 2" wide by 6" long. I'm not sure either of those ideas would work at all, but they seem a little easier to do than cutting apart wings.


It's just a wild idea that probably wouldn't work.

[MarkusN]

It's just a wild idea that probably wouldn't work.
It's been done and works. For some time in F3B popping open the fuselage cover was popular as an airbrake. That was before 4-flapped wings and crow.