Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

landing bounce

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Old 09-13-2012, 12:00 PM
  #1
Indiomike
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Default landing bounce

A guy at our field has a 4Star-40 plane (tail dragger). It would bounce back up into the air every time he attempted to land and didn't seem to matter if it was a fast or slow speed at point of touchdown. I've had many planes over the years and thinking back, some seemed to stick to the runway at touchdown and others had a tendency to bounce back up into the air. So, what causes this bouncing with some planes. I have seen both nose gear and tail draggers do this. I'm thinking it has something to do with wing incidence but I'm just guessing. Could it be due to the particular wheels that are on the model? Landing speed? Explanation please.

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Old 09-13-2012, 12:34 PM
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Default RE: landing bounce

You are descending to fast while still have enough speed to be airborne. You need to flair and level out just above the ground, the plane will come down when the airspeed slows down enough.
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Old 09-13-2012, 01:12 PM
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Default RE: landing bounce


Quote:
ORIGINAL: Mr Cox

You are descending to fast while still have enough speed to be airborne. You need to flair and level out just above the ground, the plane will come down when the airspeed slows down enough.
Right. The 4-stars land just fine if you do it right. I tell my students to never do anything to try to force a plane to land. Line it up, slow it down, sure, but don't make it land: it will do that all by itself.
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Old 09-13-2012, 01:48 PM
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Default RE: landing bounce

As you've guessed, and others have mentioned, it could be a lot of things.

With taildraggers, it can also be the result of a poorly located CG. When the tires touch the ground the mass of the plane tries to keep going in the direction it was going, toward the ground. If the wheels are located properly in relationship to the CG, they absorb the landing shock in such a way that the pitch of the plane isn't affected. If the mass of the plane is too far aft that the mass has some leverage, that mass keeps going toward the ground. The gear keeps the front from going too. The pitch is changed. The plane's angle of attack pitches tail down. The wing has been forced to provide more lift than you had established and that was giving you the slow descent you had right up until the tires hit. The plane starts to climb in response to that new increased lift.

So how can that happen with our models? The designers worked out the location of the gear to the CG to give good manners landing, right. Well, they also worked out the location of the CG for flight manners too. You did work out exactly where the CG is in your model, right? And it's in the range the mfg suggested, right. And you've not had to repair damage and might have somehow added weight aft, or (I've seen this more than a couple of times) put the landing gear back on backwards, right.

It could be the CG. It's not often the case, but look at the model again and maybe just give the CG a look. Sometimes they change. Sometimes the location of the wheels change.
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Old 09-13-2012, 02:31 PM
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Default RE: landing bounce

Thanks for the replies. This guy did say that his 4*40 had a tendency to "float". I watched him land several times and he touched down at about the last third of the runway each time. And sure enough, each time the plane bounced back up into the air. However, it looked to me that the landing speed was quite slow at the time of touchdown. If he changes the CG to make the plane at bit more nose heavy, could that solve the problem? I don't think the problem is simply too much speed at landing although I can see where too much speed can cause the plane to become airborne again. His landing speed is slower than some of the other planes and they are not bouncing back up in the air.

Mike
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Old 09-13-2012, 03:35 PM
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Default RE: landing bounce

The plane is tail heavy. Doesnt take much either. Ihave a 4*120 and the first couple times I landed it, it would bounce. Turned out to have lost almost 2 ounces of nose weight from the time Ifirst ran up the engine, to the time of the first flights. Ifound the weights in the back of the truck and glued them back on, now the plane lands good, without bounce no matter what the speed is.
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Old 09-13-2012, 03:44 PM
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Default RE: landing bounce


Quote:
ORIGINAL: acdii

The plane is tail heavy. Doesnt take much either. Ihave a 4*120 and the first couple times I landed it, it would bounce. Turned out to have lost almost 2 ounces of nose weight from the time Ifirst ran up the engine, to the time of the first flights. Ifound the weights in the back of the truck and glued them back on, now the plane lands good, without bounce no matter what the speed is.

This is half of the equasion, Its a combination of CG and gear placement. bringing the CG back will increase the distance between CG and wheel axles. I initally set up my designs so that the wheel axles are about 20% to 25% forward of the CG. After test flights and CG adjustments I usually have to adjust the gear as well. Too far forward will cause a sudden change in positive AOA and the rusultant lift will bring you back in the air. Too far back and you will nose over. In this case, I would bend the gear back about 1" and give it a try. For trike gear adjust the nose wheel so the airplane sits slightly nose down at rest.

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Old 09-13-2012, 04:03 PM
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Default RE: landing bounce

If its an ARF the landing gear is pretty much set.  Mine is an ARF, but now that I have the CG dead on again, it lands fine. The one the OP posted about didnt say if it was an ARF or kit.
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Old 09-13-2012, 04:52 PM
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Default RE: landing bounce

One could always shim the leading edge of the gear to move the axle location back.
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Old 09-13-2012, 08:38 PM
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Default RE: landing bounce

I don't know if his 4*40 is a kit or ARF. It is a black fuselage with yellow wings. Don't know if ARF's came with that color scheme or not. Also, it is electric powered. I've never seen him fly anything but electric power. Next time I see him I'll ask about how he has the plane balanced and will suggest moving the CG forward a bit.

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Old 09-14-2012, 05:14 AM
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Default RE: landing bounce

Before he moves the CG, he needs to test it in flight to see if it's right. CG isn't determined by how a plane lands, but rather how it flies. There lots of good aerobatic trim charts available that give good information about how to get he CG set right.

There are two possible causes for a bounced landing. I'll call them gear bounce and tail bounce. Tail bounce is what was described above where the tail starts to settle because you put the plane in a little hard which makes the wing rotate up and suddenly make more lift. Moving the gear back a bit helps with that at the expense of a greater tendency to nose over on takeoff. Gear bounce is when your gear is in the right place, but you put the plane in too hard and the springiness of the gear shoots it back up into the air. If the pilot doesn't gun the throttle after a gear bounce he'll get 3-4 more bounces often resulting in the plane coming to rest on its nose. If you're watching you can tell which one is happening. A tail bounce looks like the pilot jerked up elevator just as the wheels touched down, so it's somewhat smooth. A gear bounce a very abrupt and will cause the plane to come back in nose down for the second landing. You can't do anything in the setup to fix gear bounce because its cause is landing technique.

It sounds like your friend is a fairly inexperienced pilot, so maybe the only problem is that he is stalling the wing a foot above the ground or either not flaring properly? A common mistake for newer pilots to make is to judge the quality of a landing by how fast the plane is going in forward speed. That really doesn't have much to do with it though, because what really matters is descent speed. A good landing is one where the pilot manages his descent speed using the tool of forward speed properly so that the wheels touch lightly then gently add the planes weight to them. With a good landing, even a badly set up LG won't cause a bounce, although it will be less forgiving of a bad landing than a properly set up gear will.
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Old 09-14-2012, 05:33 AM
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Default RE: landing bounce

daRock's comments are right on. Location of the gear is quite important on tail draggers.
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Old 09-14-2012, 05:34 AM
  #13
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Default RE: landing bounce


Quote:
ORIGINAL: jester_s1
CG isn't determined by how a plane lands, but rather how it flies. .

Exactly right. However, once the CG is worked out for the flying, the gear might have to be moved for the ground handling.

BTW, the CG trimming tests are pretty much exactly like the landing trimming test. You do something with the plane that tests pitch recovery and that tells you where the CG is for flying. You land the airplane and how it reacts upon hitting the ground tells you where the gear needs to be.

There are many ways that the gear can wind up "in the wrong place".
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Old 09-14-2012, 05:35 AM
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Default RE: landing bounce

Well, 2 ounces in the nose fixed my 4* bounces. 
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Old 09-14-2012, 06:23 AM
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Default RE: landing bounce

He's probably holding up elevator after the plane touches down. Took me awhile to learn to relax the right stick after the first touch.
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Old 09-14-2012, 07:43 AM
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Default RE: landing bounce

As jester mentioned and I forgot to describe, his landing are of the bounce type. His plane will usually bounce back up into the air 2 or 3 times before staying on the ground. It sort of looks like his gear has springs on them. This may suggest he is touching down with too much airspeed but it doesn't look like the plane is going that fast.

A lot of the guys ask me to test fly their planes for them as I am one of the more experienced pilots in the club. I think I will ask this guy if he would like me to test fly the plane to see if it bounces when I land it. I'm not trying to brag but I'll say that if a plane is flyable, I can handle it. I haven't crashed anyone's plane that has asked me to do the maiden flight and that has been at least 30+ planes. For me, the landing is the most fun phase of flying and I take great pride in making a good landing.

Mike




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Old 09-14-2012, 07:51 AM
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Default RE: landing bounce

You my sir have summed it up in a nutshell. I have a hangar full of taildraggers and no two land the same. Some require landing on the mains were others' require a three point stall.
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Old 09-14-2012, 08:36 AM
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Default RE: landing bounce

Quote:
As jester mentioned and I forgot to describe, his landing are of the bounce type. His plane will usually bounce back up into the air 2 or 3 times before staying on the ground. It sort of looks like his gear has springs on them. This may suggest he is touching down with too much airspeed but it doesn't look like the plane is going that fast.
Airspeed won't make you bounce, it's descending too fast that will do it. If the problem isn't CG/gear placement, it could be that he's trying to fight the plane's tendency to float by giving it down elevator at the last minute. Doing that just a little will get you a bounce, and it's hard for beginners to resist the urge. (Doing it a lot will get you a busted prop or worse.)
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Old 09-14-2012, 09:19 AM
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Default RE: landing bounce


Quote:
ORIGINAL: flycatch

You my sir have summed it up in a nutshell. I have a hangar full of taildraggers and no two land the same. Some require landing on the mains were others' require a three point stall.
The 4* manual does recommend a 3 point landing to reduce bounces. I found that a slight nose up when the mains touch goves the smoothest landings.

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Old 09-14-2012, 11:03 AM
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Default RE: landing bounce


Quote:
ORIGINAL: Top_Gunn

Quote:
As jester mentioned and I forgot to describe, his landing are of the bounce type. His plane will usually bounce back up into the air 2 or 3 times before staying on the ground. It sort of looks like his gear has springs on them. This may suggest he is touching down with too much airspeed but it doesn't look like the plane is going that fast.
Airspeed won't make you bounce, it's descending too fast that will do it. If the problem isn't CG/gear placement, it could be that he's trying to fight the plane's tendency to float by giving it down elevator at the last minute. Doing that just a little will get you a bounce, and it's hard for beginners to resist the urge. (Doing it a lot will get you a busted prop or worse.)
Top_Gunn

My thinking was with enough airspeed and AOA a plane will lift. So, if the AOA at touchdown is positive any excessive airspeed will take the plane back up unless held with down elevator. I do agree descending too fast will cause a bounce, similar to what a basketball does when you drop it to the floor.

If I test his plane and can land it without bouncing, then we will know the problem is his technique. If I bounce it, then we will look at CG, landing gear placement, and maybe going to the "low bounce" tires sold by Dubro or Sullivan.

Mike
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Old 09-14-2012, 11:38 AM
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Default RE: landing bounce

Going back to the OPs concern - so what if it bounces? I bounce landings as often as I don't. The bounces are small. Haven't damaged an airframe. Don't have prop ground-strikes. I suppose that CG could play a role. But I wouldn't adjust CG to make it LAND better, I'd adjust it to make it FLY better. If you have to go as far as modifying the gear, that seems like overkill. I'd say, keep practicing and try different approaches (mains vs. three-point).
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Old 09-14-2012, 11:43 AM
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Default RE: landing bounce

Quote:
If I test his plane and can land it without bouncing, then we will know the problem is his technique. If I bounce it, then we will look at CG, landing gear placement, and maybe going to the "low bounce" tires sold by Dubro or Sullivan.
Yes, that will tell us a lot. Probably shouldn't have been speculating in advance. Let us know how it turns out.
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Old 09-14-2012, 12:04 PM
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Default RE: landing bounce

Go to your hobby shop and buy anti- bouncing wheels
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Old 09-14-2012, 12:36 PM
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Default RE: landing bounce

Practice some touch-and-touch-and-touch-and-gos.
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Old 09-14-2012, 02:35 PM
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Default RE: landing bounce

Slow the plane down so that she is not flying at time of touchdown. Low idle helps.
I have a four star 60 (kit), that I used to try to land at a slow flying speed -> bounced landings.
With a LOW idle, so that airspeed is slower, she flares nicely to a three point.
Get a really LOW idle, perhaps mixed on a switch for landing.
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