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Ail & Rud settings to slow landing by crabbing

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Ail & Rud settings to slow landing by crabbing

Old 12-20-2015, 09:02 AM
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Default Ail & Rud settings to slow landing by crabbing

I have a foamy electric aircraft that is great to fly but a pig to land due to can't bleed off the excessive speed. I'm using a DX7 and proposing to mix the Ailerons & Rudder, in opposite directions, to slow the aircraft on landing.using the two position MIX switch. To initially test it will be a quick flip ON & OFF
Anybody have any suggestions on the ratio (or movement) of rudder to aileron please
Old 12-20-2015, 09:15 AM
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Using opposite aileron and rudder will give you a slip, not a crab. Slipping will cause you to lose altitude faster and can keep you on track in a crosswind, but it's best done with your thumbs, not a mix, because to do it right you have to use constant adjustments, there's no fixed percentage that will always work right. Furthermore, most planes will stall at a higher airspeed when slipping than in coordinated flight, and if you stall when slipping you'll probably spin. The way to slow a plane down is by raising the nose. Then use your throttle to control altitude, and raise or lower the nose to reduce or increase airspeed.
Old 12-20-2015, 09:58 AM
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A simple way to slow the plane with a mix is to raise both ailerons slightly. Induces drag to help slow the plane and reduces the angle of attack which makes it less stall prone at slower speeds. I'd start with 5 degrees or so. You will likely need to mix in some down elevator to compensate.
Old 12-20-2015, 06:42 PM
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Raising the ailerons will create some drag, but the main effect will be a loss of lift. You slow a plane down by using up elevator. If you're having trouble slowing it by doing that, you may be nose-heavy. Try moving the balance point back (just a small amount at a time). Have an experienced pilot fly your plane and get his opinion.
Old 12-21-2015, 02:09 AM
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Thanks for both those responses. If it ever stop raining and the mud settles a little I will try idea's separately & see the effect. Will let you know in the new year what happened
Old 12-23-2015, 07:27 AM
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What plane are we talking about here?
Old 12-23-2015, 08:22 AM
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It's an all foam Acrowot sold by Ripmax in the UK. Will continually fly nose up vertical - until it's too high to see
Take-off is a breeze. Motor is an 820Kv, Brushless Out runner with an 11 x 8 APC-E
Weight = 212g (7.5 oz)

Wing Span 1200 m; Chord 250mm at root, 180mm at tip Lipo – Overlander Sport 25C 2200mah 3S 11.1V

Fully equipped weight 1139g (40.18 oz) –inc LIPO
Old 12-23-2015, 01:34 PM
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The foamy you describe doesn't sound like a model that won't slow down. But it does sound like it's got a motor/prop that won't let it.

It sounds like a foamy with an major excess of power and an extreme amount of prop pitch. That much prop pitch on such a light model with the power to fly straight up without slowing isn't about to slow down unless the prop isn't spinning at all.

Turn the power off completely at say 100' and see how long it takes to slow down. Look at how long it glides in that time. Then fly your next landing with those two in mind. A two pound foamy with that size wing isn't going to hold it's airspeed for long. Unless it's got an 8" pitch pulling it along, and that much pitch on that powerful a motor will pull it along.

That plane doesn't need side slipping, it needs less pull from that motor/prop. Lots of speed prop planes have the same problem. It's worse for them as their airframes are a low drag as possible. The AWot's isn't.
Old 12-23-2015, 06:11 PM
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An electric with power off and the prop freewheeling is a boat load of drag. I would have to agree that it won't slow because it's nose heavy.
Old 12-23-2015, 06:18 PM
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It would be a big mistake to try and put it on a switch or as a mix. You crab by giving rudder and enough ailerons to keep the wings level. The amount is fully dependent on speed and any cross-wind. It's not a fixed throw.

Some models need (or can support) 1/2 rudder and others may only need a few degrees. Depends on rudder and aileron authority and the wing shape. The Acro Wot looks to be a pretty hot model and maybe 1/8th rudder would do.

But as others have said it's not the answer for slowing.

Rather than cutting the motor go to a low power. The prop spinning slow causes some braking effect (more than just stopped or idling, surprisingly). Come in low at 1/4 throttle 100 ft out and fly to a few feet over the ground and then gradually drop speed and increase elevator to keep the nose up.
Old 12-24-2015, 06:10 AM
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Originally Posted by speedracerntrixie
An electric with power off and the prop freewheeling is a boat load of drag. I would have to agree that it won't slow because it's nose heavy.
Simply being nose heavy isn't enough. A boat load of drag is there whether the plane is nose heavy or tail heavy. There isn't enough nose heavy in the world to keep a boat load of drag going fast with the motor off. Although with the motor off, unless the elevator throw is so restricted it can't control the pitch.

Without the prop pulling that boat load of drag, using the elevator appropriately is going to have no trouble slowing the plane. That is, unless the pilot has waited too late to pull off the throttle and not given the plane time to slow down. A two pound, massively overpowered foamy is going to be more than touchy to fly. If the elevator throw is restricted to keep it from folding the wings, it's not going to have elevator command every time it's needed.

Light rocket ships aren't easy to fly.

One solution might be to set it up like most 3Ds, with full throws that have lots of expo.

Last edited by da Rock; 12-24-2015 at 06:14 AM.
Old 12-24-2015, 09:38 AM
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A nose heavy airplane requires you to maintain speed to keep the nose up. It needs airflow over the elevator right? Not to mention that the more elevator required is also more aerodynamic force being applied to the tail in a downward motion. This effectively increases the wing loading thus increasing the stall speed as well. Bottom line, a nose heavy airplane has to be landed faster then one with the CG closer to neutral.
Old 12-24-2015, 09:52 AM
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Originally Posted by speedracerntrixie
A nose heavy airplane requires you to maintain speed to keep the nose up. It needs airflow over the elevator right? Not to mention that the more elevator required is also more aerodynamic force being applied to the tail in a downward motion. This effectively increases the wing loading thus increasing the stall speed as well. Bottom line, a nose heavy airplane has to be landed faster then one with the CG closer to neutral.
He said "[it is] a pig to land due to can't bleed off the excessive speed." Not that he needed to land it fast. Wrong "bottom line."
Old 12-24-2015, 10:00 AM
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Originally Posted by da Rock
He said "[it is] a pig to land due to can't bleed off the excessive speed." Not that he needed to land it fast. Wrong "bottom line."

Congradulations!!! You and the OP have just discovered the worlds first perpetual powered model airplane. It continues to fly forever and won't slow down no matter what. LOL.
Old 12-25-2015, 07:04 AM
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It sounds like the OP's plane is just a bit more slippery than he'd like it to be. Before trying all these tweaks, why not try taking a little bit longer landing approach to see what happens? Get the plane down to 3-4 feet off the ground around 50 feet from the end of the runway. When you find yourself having to blip the throttle to be able to land in front of yourself you'll have found the right distance for the wind conditions for that day. If you are diving at the end of the runway from your normal flying altitude, you'll find that there are very few planes that will slow down enough to land properly without making you walk a ways to get them.
Old 12-25-2015, 10:19 AM
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Originally Posted by jester_s1
It sounds like the OP's plane is just a bit more slippery than he'd like it to be. Before trying all these tweaks, why not try taking a little bit longer landing approach to see what happens? Get the plane down to 3-4 feet off the ground around 50 feet from the end of the runway. When you find yourself having to blip the throttle to be able to land in front of yourself you'll have found the right distance for the wind conditions for that day. If you are diving at the end of the runway from your normal flying altitude, you'll find that there are very few planes that will slow down enough to land properly without making you walk a ways to get them.
Sounds like it's slippery, and if you are familiar with that model you know it is AND it's very light AND it's quite over powered.

Lots of 3Ds have those characteristics in different degrees. They're usually setup with full throws. That makes it really hard to give small inputs SO the setup includes significant exponential. They also prop the suckers toward pull/power instead of using lots of pitch. The OP has what is basically a 3D setup with an 8 pitch prop with a major power producer of a motor.

No wonder his plane appears to not want to slow down.

That is an excellent suggestion about trying a different technique on approach. It'd also be a good idea to keep in mind that 8" pitch prop is going to make airspeed control rather touchy.
Old 12-26-2015, 09:59 AM
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After looking up the airplane a couple things become evident. First, it is no 3D airplane. I would put it into the sport aerobatic category. Second, it is not a super clean design. It has a rather thick symmetrical airfoil. I guess I need someone to explain just how an 8" pitched prop still produces thrust without any drive behind it. The issue may be technique but my money is still on nose heavy.
Old 12-26-2015, 06:54 PM
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Didn't know there was money on this. Or there would be a winner.

Good thing is the OP has gotten some food for thought. Well, it's more like things to try.

The plane he's flying originally came out as a builtup ARF, not a 2.5 pound foamy. And it's powerful enough to do unlimited vertical, pretty much the usual for most 3D models. But it's got an 8 pitch on the sucker. Coming in on final is tricky enough for super light models. Add the overpowerful motor and ANY throttle at all and that speed prop is going to be doing anything but slowing down. 2.5 pounds descending really doesn't create induced drag. So the suggestion was made to try a throttle off landing. An emphasis should be placed on "off".

The winner here will be the OP if the thread actually includes advice that works for him. He's still part of the equation.
Old 12-27-2015, 05:32 AM
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I have electric planes that the prop windmills so much you would swear it must be pulling at zero throttle. I am betting its a efficient gliding plane that is slightly nose heavy as well.

Trixie had my hopes up that perpetual flight had finally been discovered.. but after awhile I figured we weren't breaking that law yet !

One thing OP could try is setting his motor up to SOFT BREAK, possibly that would slow it down a bit...though its my understanding that once the prop stops it would still cause very little drag...
Old 12-29-2015, 05:56 AM
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Hi everybody, many thanks for all the comments and suggestions.
It looks as if my way forward is (obviously) to abandon crabbing & side slipping
My strip is a short grass runway with long grass on all sides – it requires a long low approach and it doesn’t lose altitude by chopping the throttle prior to crossing the threshold (it just keeps going). Others pilots have experienced the same problem and the best approach is to be about 2 feet high a long distance away (A nerve wracking approach). Great to fly between take-off & landing. I use 40% exponential on all control surfaces.
Most of you seem to believe that it is a mix of nose heavy CG & prop pitch. I will try moving back the CG – any suggestions on how much to reduce the prop pitch? – Keep the diameter to use as a braking effect?
Later on I’ll try raising the ailerons 5 degrees to see the effect.
Hopefully the weather will improve soon (LOL) and I can get back to you with the results

Meanwhile have great flying New Year
Old 12-29-2015, 09:02 AM
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It might help us get a handle on this if you'd tell us where your balance point is and where the plans or instructions say it should be.
Old 12-29-2015, 09:09 AM
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There's nothing wrong with crabbing. It's a useful pilot skill to have when you need to make a steep approach or just find that you've come in too high to make the runway. I think the point the others guys were making, certainly the point I would have made, was that trying to crab with a mix is the wrong way to do it. Absolutely it will be good for your flying to learn to crab with your thumbs, especially with the flying site you are using. As for your CG, there is absolutely no way we can tell you how far to move it back. The best thing to do is measure where it is now and calculate it's location relative to the mean aerodynamic chord. The normal range is between 25%-28%. A few scale planes like to be more forward than that, and a few 3D pilots will go back as far as 30%. I'll bet you'll find based on your descriptions that the CG is up around 25%, so moving back to 27% should improve the flying characteristics all around and give you enough elevator authority to flare properly for landing.
Old 12-29-2015, 09:13 AM
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Good luck on two things.... The weather improving, and the solution to your problem. We've been waiting out the rain for over a week now.

In following your posts, it sounds like all you and your flying friends are in agreement that your plane can only be landed one way. Sorry to hear that. And it sounds like nobody is familiar with how easy it is to trim any model's CG in flight. There are a couple of simple methods even beginners can handle. I'm surprised no one has described one. or two.

And there really aren't many models that can only be landed one way. Modelers new to pylon racing and thermal soaring often start with the belief in the long-and-low final as the only way in for those suckers. Humans have a really hard time judging speed, and our planes don't show us anything but ground speed, and it's airspeed that matters. So humans need tricks or loads of experience.

Full scale pilots are trained on the three leg approach for a reason. It has a couple of needed tricks. It breaks the landing up into segments we can shorten or lengthen to help us control airspeed. And the two turns also help control airspeed. And the two turn points also help us control altitude. Also, breaking the landing into parts helps us control the pilot workload. He doesn't have to deal with throttle, pitch, airspeed, location all together the entire approach.

Less experienced model pilots often try to get that last benefit by doing long, low approaches. They basically turn over control to the airplane.

Hope it doesn't hurt your feelings to ask if you've ever flown your AcroWot dead stick much. It's really useful to discover just how slow they go with no power, and more importantly, how fast they are going when they stall. Stall speed is mostly a function of wing loading, and you've got a design that started life much heavier and not much larger but is only 30 ounces made of foam.

On those straight in approaches, ask yourself when do you actually shut down the throttle? and what would happen if you did it farther out? That sucker comes in deadstick the whole long, low approach? Try shooting the same approach "normally" but higher up, with the goal to do a flyby at however much the model was higher up. The flyby of course will require a touch of throttle.

Another thing to try is to kink that straight in base leg (down wind, base, final) 45degrees with the turn right at the threshold. Turning any airplane creates induced drag which slows the airspeed. Then try 90degrees from base leg. Of course, do these "higher up" before trying a landing with them. Doing any of this as a flyby is safer, and the plane is close enough to see better what it's ground speed is. Working out flying problems and increasing pilot skill is the goal. And we also control how much risk we incur by choosing the altitude of the test.
Old 12-29-2015, 09:18 AM
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As for a suggestion of pitch...

With electrics, a tach and watt meter are going to give you the best advice. And e-calc helps a bunch as well.

But keep in mind that ANY model that will do unlimited vertical with an 8" pitch is seriously overpowered. Simply holding back on the throttle really won't give linear thrust reduction.
Old 12-29-2015, 11:30 AM
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It looks as if my way forward is (obviously) to abandon crabbing & side slipping
I wouldn't say abandon it. It's a viable technique for cross wind landings if they can't be avoided. I just don't think it can be a mix. Kind of had to be adapted to conditions.

But for the basics stick with taking off and landing into the wind.

Later we'll work on crabbing and then the knife-edge approach to lose altitude. ;-)

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