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Air Brakes

Old 09-15-2013, 12:37 PM
  #1  
ButteredCat
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Default Air Brakes

I have a Mamba (pictured).

Although it's great to fly I don't take it out much mainly because it is very difficult to land. It is a heavy model and glides incredibly fast - and the site where I fly isn't really suited to long shallow approaches. (The second generation Mamba's had spoilers on the upper surfaces obviously to combat this problem).

I am trying to work out a way of fitting some kind of air brakes to this model. I don't really want to be cutting into the model too much as I would be concerned about affecting the structural integrity. Also there is very little room to work with inside.

I was thinking perhaps building something that resembles a cockpit that raises up (hinged at the front). That I think would be the easiest solution as I could have the servo inside the cockpit. My fear though is how that would affect the aerodynamics of the fin.

I was wondering if anybody else has had similar problems and come up with their own solutions. Or even if there's something suitable already on the market.
Does anybody have any ideas?
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Old 09-16-2013, 03:21 PM
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BMatthews
 
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For something like this you won't find anything "on the market" simply because each solution is so custom to a given design.

I may have a fairly easy option but it depends the answer to a question first. I can't tell from the picture if it has inboard elevators and separate outboard ailerons or if the outboard surfaces are combination elevons. Can you clear that up first of all?
Old 09-16-2013, 09:55 PM
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ButteredCat
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I think I know where you're goiong with that question.
Unfortunately it has just elevons - a single control surface and single servo on each wing.
Old 09-17-2013, 06:37 AM
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Have you considered a split rudder?
Old 09-17-2013, 11:07 AM
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ButteredCat
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It doesn't have a rudder.
Old 09-17-2013, 01:57 PM
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Originally Posted by ButteredCat
I think I know where you're goiong with that question.
Unfortunately it has just elevons - a single control surface and single servo on each wing.
Actually that is perfect for what I'm going to suggest.

There's a way of adding speed brakes to a wing that only requires cutting out a fairly narrow portion of the trailing edge. You use that lower portion for the main part of the brake and glue a layer of thin but stiff material on the top that is twice as wide as the lower portion. Or in your case you may want to hinge along the top line and glue the sheet to the bottom face. Just don't forget to retract the speed brake a little before touchdown or the bushes or grass will tend to rip away the extension flap.

Another option would be to cut away part of the fixed fin and add back a split rudder/speed brake. You don't need to actually use it AS a rudder. Just set it up as a clamshell speed brake.
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Old 09-17-2013, 05:16 PM
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Originally Posted by ButteredCat
It doesn't have a rudder.
I know.

And for the same reason, a brake at that point cannot introduce much disturbance in yaw or roll, ........maybe some nose pitch up, .........maybe.

I like BMatthews' idea applied to the root of the wings as well.

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Old 09-19-2013, 10:39 AM
  #8  
ButteredCat
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As you can see from this pic it is a moulded wing with a thin profile and a curved reflexed TE, so I don't think I want to be messing about with that.
So if I go that route it will have to be on the rudder.


I'm still thinking about the cockpit idea. I'm think of a cockpit with a centre section that is hinged in the middle so the front would lift up but the back would drop down. The front lifting up would act as a brake but also as the back went down it would expose the rear of the cockpit which would then also act as a brake.
And being hinged in the middle it would put minimal load on the servo.
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Old 09-20-2013, 06:58 AM
  #9  
rmh
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Split rudders - takes a fair bit of work - the associated pitch especially on that design can wreck control unless it is mixed to pitch control
been there . Changing the vertical center of drag on a really short coupled design such as yours can be an adventure -no matter how you try it.
A bit of history-- An old full scale design had a wide canopy (Dyke delta) and loosing the canopy was extremely dangerous
- being a reflexed wing -it also was very numb at low speeds and pitch control was the same - down was down and UP was also----down.
a deadly combination.
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Old 09-20-2013, 12:35 PM
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Years ago, Hanno Prettner and Wolfgang Matt, trully outstanding pilots and model designers, originated the idea presented by Matthews. I first saw the concept in an old MAN article about one of their designs, about 30 years ago, and incorporated the idea into one of my wings.The idea works extremely well....

The wing area was around 700 squares, with a span around 65". The brake's ratio of areas I used was just a guess back then, 35% of the total area (spoiler) above the wing and 65% of the area (flap) below. I essentially added a flap on the wing that was 8" in length and about 1 1/2" wide, and later added the spoiler piece (1/32" a/c ply) over the full length of the flap. Except the spoiler extended forward of the flap hinge about 3/4"

The result was practically pure drag with no pitch change. The speed difference before and after deployment was dramatic, at only half deflection or about 25 degrees. Unfortunately a bulky engine claimed that model way too soon but I still remember the experiment as very worthwhile. The two spoilerflaps added about 25 squares to a 700 square wing, or about 3 1/2%. Try it, I think you will get what you are after
Old 09-20-2013, 03:00 PM
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rmh
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Originally Posted by MTK
Years ago, Hanno Prettner and Wolfgang Matt, trully outstanding pilots and model designers, originated the idea presented by Matthews. I first saw the concept in an old MAN article about one of their designs, about 30 years ago, and incorporated the idea into one of my wings.The idea works extremely well....

The wing area was around 700 squares, with a span around 65". The brake's ratio of areas I used was just a guess back then, 35% of the total area (spoiler) above the wing and 65% of the area (flap) below. I essentially added a flap on the wing that was 8" in length and about 1 1/2" wide, and later added the spoiler piece (1/32" a/c ply) over the full length of the flap. Except the spoiler extended forward of the flap hinge about 3/4"

The result was practically pure drag with no pitch change. The speed difference before and after deployment was dramatic, at only half deflection or about 25 degrees. Unfortunately a bulky engine claimed that model way too soon but I still remember the experiment as very worthwhile. The two spoilerflaps added about 25 squares to a 700 square wing, or about 3 1/2%. Try it, I think you will get what you are after
Hanno stuck em on his Curare - inboard setups
he also added little plates to the landing gear doors
Old 09-24-2013, 09:31 PM
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As a private pilot with limited RC experience do you trim up when landing? a heavy plane will obviously pick up speed in a nose down descent, but lifting the nose and shedding airspeed close to but not stalling will give you a slow and steep decent. It seems strange to lift the nose when you want to descend but it works.
Old 09-25-2013, 06:05 AM
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MTK
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Originally Posted by jordanorville
As a private pilot with limited RC experience do you trim up when landing? a heavy plane will obviously pick up speed in a nose down descent, but lifting the nose and shedding airspeed close to but not stalling will give you a slow and steep decent. It seems strange to lift the nose when you want to descend but it works.
It doesn't work quite the same way in models because the loading is so much lower than man carrying size.

With current computer radios many guys set-up their models with dual aileron servos driven by a master-slave arrangement (AIL-AUX1 or FLAP). Some of us reflex ailerons on landing which tends to dump some lift and create some drag. But still the most effective set-up on models is the airbrake I discussed earlier, first seen on Pretner's planes, 30 years ago. They are simple to make but the hardware adds some weight....YMMV as always
Old 09-25-2013, 06:26 AM
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rmh
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Prettner's setup worked- but is a fair bit of work for even good builders to get right

The Multiplex Fun Cub used simple plates which can be dumped to 90 degrees down- I have one of these - and this setup coupled with mixed down elevators really works
The best part is that with only some tweaking on the flap to down elev mix, the plane has no pitch either way and just elevators down at a very flat attitude.
I use the Spektrum radios - DX 8,, 18- and the required flap /elev mix is all on one page- others have similar setups. -an adjustable deployment speed also makes the whole thing easy to predict.
Old 09-25-2013, 02:19 PM
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The picture of the wing's trailing edge tells me that you're right and using a trailing edge brake isn't a workable option.

That leaves you with chopping the fin and making a split rudder drag brake or hinging the canopy.

I'm having a little trouble visualizing what you described. It almost sounds like you're considering hinging the canopy in the middle so it flips up at the front and down at the rear? If so it's going to be an issue when it tries to drive itself down into the guts at the rear I would suspect. Also to produce enough drag to be effective with a model you really can't use small effects. We really need to open up a barn door to see much effect.

Years ago before wing flaps became popular in the FAI F3B class it was popular to hinge the canopy at the rear edge and use a servo to raise it up to around 70 to 90 degrees as a speed brake. But while it worked to some extent it was no where near as effective as a proper flap. So the competition aspect soon drove this option out of existence. But for what you're trying to do it may well provide just enough drag to get the job done.

Just be careful to try it the classic "3 mistakes high" the first time. The canopy hinging open on a conventional design is not a big deal. On a plank style flying wing the raising of the center of drag when the canopy hinges up like this could well produce excessive pitch up force.

On using back trim to bring the model down more controllably jordanorville makes a highly valid point. And it's well worth trying what he suggested. But much will depend on how easy it is to hold your model in this nose high pre-stall posture for long enough to land. Some models do this quite well. Others are simply too close to the stall before any real effect is noticed. So close that the risk of a final stall and tip drop to a cartwheel during the last few feet is just too high. But others can be "mushed" and flown under good control. Try your wing and see which response to easing back it produces. It may turn out that you don't need any drag brake but your thumb/fingers on the stick.
Old 09-25-2013, 02:50 PM
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rmh
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On extremely short planes - planks- which are speed sensitive to trim -for obvious reasons- a drogue chute is the best answer
I did not think this up - the military figgered it out long ago.
anything which compromises the vertical center of drag will be spooky
good luck.
Old 09-29-2013, 10:02 AM
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Originally Posted by jordanorville
As a private pilot with limited RC experience do you trim up when landing? a heavy plane will obviously pick up speed in a nose down descent, but lifting the nose and shedding airspeed close to but not stalling will give you a slow and steep decent. It seems strange to lift the nose when you want to descend but it works.
What you are referring to is "flying on the backside of the curve" which is quite effective in controlling descent with a clean airframes. But you have to remember that the elevator no longer responds the same as it does in normal flight speeds. There is no flair from the back side, you have to lower the nose to regain speed in order to land with a flair. It is also nice to have a rudder for directional control, since the ailerons tend to lose effectiveness when flying so close to stall. Over the years I have seen many fliers enter this mode of flight without knowing what was going on. Usually when trying to stretch a dead stick landing back to the runway. Seldom do they land, most are better described as an arrival.
Old 09-29-2013, 10:24 AM
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ButteredCat
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After putting much thought into this I'm half way to deciding what to do.

Seeing as the plane is all moulded and a complex shape, not to mention a lovely looking plane (just my opinion of course!) I've decided that cutting into the plane is not the way to go. If I get it wrong or it doesn't work I won't be able to put it all back.

Therefore the cockpit route is the way to go. Th advantage I have here is that I can bolt it on when I bolt the wings on. So iIf it doesn't work no harm done.

But I'm still undecided exactly how to do this.

@BMatthews;
In the initial idea you said you had trouble in visualizing it. So I will try to clarify this.
What I intend is to have a kind of rocker hinge near the top of the cockpit, about half way back.
The windscreen area will be fixed and will house the servo. Then a large portion of the cockpit between the screen and the hinge will lift up into the airstream. Behind the hinge the upper part of the cockpit will descend down into the lower part of the cockpit. Not actually into the plane itself.
This will have two benefits. The fixed part of the rear of the cockpit will also act as an airbrake. But also, as the air flows into the cockpit and under the hinge the forces on the movable part behind the hinge will go some way to neutralizing the the forces on the front of the cockpit that is up in the airflow. (This is important as I want to use the smallest servo that is practical).
This is basically the same principle as the idea you proposed yourself, although it won't be as effective (due to lack of space) and it's in the top of the cockpit as opposed to on the TE.

But there is another idea that I'm toying with;
The idea is to have a long slender cockpit. Again the windscreen will be fixed, but behind that the cockpit will be in two halves that open out laterally, scissor-like. When deployed these two halves would be sticking out laterally just above the wing. These would then act as spoilers, disrupting the airflow over the wing and therefore reducing lift.

But what I'm wondering is how far above the wing can I have them and still be effective. And also how effective would they be if they were some way in front of the wing. (The further forward I hinge them the longer the spoilers can be).
Old 09-29-2013, 03:12 PM
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rmh
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Don't.
Old 09-30-2013, 03:18 AM
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Would you care to explain why?
Old 09-30-2013, 06:13 AM
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Post 16--
when you upset the vertical center of drag - the model will pitch.
do it- see if it works for you
Old 09-30-2013, 10:21 AM
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To add a little to Dick's response, it will also destabilize the model in yaw, in addition to pitch. It isn't a good idea to be adding a lot of drag much foreward of the AC, without countering the aerodynamic effect with more vertical stability in the rear
Old 10-01-2013, 08:46 AM
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Spoilers induce a pretty strong pitching action. Which is why on full size gliders the spoilers extend from both top and bottom. So anything that works like a top surface only spoiler will produce a strong pitching force. If the V like halves end up positioned at or near the high point the effect will be to pitch the nose down. But the drag created by the split canopy halves will be fairly high up which will try to pitch the nose up. So all in all it's tough to say what will occur. But it's safe to say that SOME TO A LOT of elevator compensation will be required when the spoiler/brake is used.

Which brings me back to a previous suggestion to "just slow down". A free and easy way to steepen the glide slope is to ease back on the elevator trim to set up a slow near stall descent. Flying at this trim point steepens up the glide slope appreciably. And if flown into the wind will help all the more. The trick is to avoid the temptation to "dive" to the landing. When you do that all you get is a whole lot of speed energy which is tough to bleed off with a clean design.

Years ago I had a Quickie 500 pylon racer with a .25 on it that I flew for fun. But it was a clean enough design that the only way to get it to land decently was to set up for a slow, nose high approach. If I wanted to land from a shorter approach I had to dive down to just a little way from the runway and perform a tight and draggy high G pylon turn and exit the turn with the nose high and in the draggy glide trim. If I did that it landed just fine without any overshoot.

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