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

landing bounce

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Old 09-14-2012, 03:02 PM
  #26
Granpooba
 
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Default RE: landing bounce

Simple remedy.

Pilot technique !

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Old 09-14-2012, 04:06 PM
  #27
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Default RE: landing bounce

Over and over in this thread the CG is mentioned. But in reality, all the posters are talking about the balance point when indeed, they should be concerned with the CG. The CG has a vertical component, not just a longitudinal aspect. The height of the CG does effect how an airplane reacts with the wheels make contact with the runway.

The original poster did not mention if he was landing on pavement or grass. Models that land well on grass often have the gear too far forward to perform as well on pavement. Since most fields in the west tend to be paved, while Sig stuff was most likely developed on grass strips, I am not surprised.
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Old 09-14-2012, 05:29 PM
  #28
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Default RE: landing bounce

As I mentioned before I have a 4*120 that did bounce the first couple times I flew it until I discovered it lost 2 ounces of nose weight. Now that I have the nose weight back on, it no longer bounces.  My 4* is balanced right in the middle of the range spec'd in the manual.  In addition the manual does state that the best way to land it is 3 point.

Right out of the manual
Quote:

Slow the model down during the entire approach by slowly feeding in "up" elevator. Just before the model touches, flare the landing by carefully feeding in more "up" elevator. Hold the model just inches off the ground until your elevator stick is pulled all the way back. The Four-Star 120 should settle in for a perfect "three-point" landing with a short rollout.

If your model always seems to make a nice landing approach but bounces when you touch the ground, you are simply trying to land too fast. If you're concerned about stalling during the approach, try doing some fake landing approaches about 50 to 80 feet high to build your confidence. You might be surprised at how slow the "120" will actually fly without stalling.
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Old 09-14-2012, 05:36 PM
  #29
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Default RE: landing bounce

Here is a section from the 4*60 ARF manual. The kit manual online is missing the last sections.

Quote:
Landing the FOUR-STAR 60 ARF is typically a pleasure. We
suggest using a standard landing approach, beginning with a
throttled back downwind leg and base turn to the final approach
into the wind. During final approach, keep just a little power on the
engine until the airplane is over the end of the runway. In
crosswind situations, a little rudder input will likely be needed to
keep the airplane lined up with the runway. The FOUR-STAR 60
is best landed in the three-point position
.
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Old 09-14-2012, 06:01 PM
  #30
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Default RE: landing bounce

I've never flown a 4 Star....so this may be irrelevant. But just wanted to share.
I fly a Pulse XT 40. It is very difficult to land tail high on the mains without a bounce or 3. My dad has a generic brand low wing sport plane,and it sticks to the runway as if the wheels were magnetic. He thought I had poor technique until he flew my plane...and later on got a Pulse XT .60 of his own. Then he realized it's more than just technique . I don't know what it is specifically about the design,but they are difficult to keep planted unless you stall into a 3 point. Still,I constantly try to practice landing on the mains because when you nail it and do a touch n go,it looks cool. It just requires impeccable timing......slowing it down enough,then letting off the elevator at the exact moment of touchdown.
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Old 09-14-2012, 06:11 PM
  #31
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Default RE: landing bounce

Most likely inherent with the design of the plane. The Pulse is very close in design to the 4 Star. Long fuse, wide wing, tail dragger. Looks like the Pulse LG is just a wee bit further back than the 4 Star. Sounds like you do the same thing I do, touch and go on the mains. Really helps to get the speed just right for a smooth landing.
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Old 09-14-2012, 06:20 PM
  #32
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Default RE: landing bounce

Your gears are to soft. Stiffen them up and you will lose the bounce.
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Old 09-14-2012, 07:35 PM
  #33
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Default RE: landing bounce

Quote:
Your gears are to soft. Stiffen them up and you will lose the bounce.
I agree, this is half of the problem, the other is the 4* landing gear is too far forward. I am very good at landing rc planes, it's something I practice a lot, and I try to make it look real smooth. I have a 4* now, I'm not a big fan...they fly ok at best. I have a very short runway with a fence at both ends which keeps you from coming in flat. But I can bring it in and have it so slow that it wants to yaw from side to side and you'd think it's just going to fall out of the sky, but it still won't be slow enough to get a true 3 point landing where the tail wheel touches at the same time as the mains. If you have a good breeze that cooperates right down the runway, then you can land it without a bounce! If I land it on grass, I can land it without a bounce, but on a calm day, or if the wind is across the runway I might get 1 out of 3 without a bounce on pavement. I can land my .25 size extra 300 every time without a bounce, also my Kaos (not fair it's a trike), or any of the various p-51's and spitfires I've owned. If I liked the 4* enough I'd build a new shorter wing, with D-tube construction, and wheels in the wing military style...then replace the stock pushrods...etc. Moving the C.G. forward would probably help, but I don't think it would fly at all to my liking.

My dad made his a trike years ago, that cured the bounce.
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Old 09-14-2012, 07:57 PM
  #34
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Default RE: landing bounce

High Plains: The runway is paved, no grass.

Over the past couple of years I have gotten away from glow and now mostly have gas (20cc to 55cc engines). I still have 3 planes powered by Saito 4 strokes though. One of them is the Hangar 9 Pulse 60 XT powered by a Saito 100. I've never had any bounce issues with that plane.

The guy with the 4*40 usually comes out to the field on Wednesdays. He does not bring it out every time but the next time he has it I will offer to test fly it and see if I have any bounce issues with it. I'll post the results if he wishes me to check it out.

As I mentioned before, I've had models that tend to bounce and others that stick to the runway like glue. Since my landing technique is pretty consistent I was wondering what was the cause of the bounce. From the responses here, it appears the answer is not clear cut as several things may be at play. I appreciate the discussion.

Mike
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Old 09-14-2012, 08:20 PM
  #35
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Default RE: landing bounce

My old pattern plane sticks like glue from the second it touches.

My others do not
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Old 09-14-2012, 08:49 PM
  #36
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Default RE: landing bounce

Some planes have ground-effect "float," some don't. The original Gee Bee racers were known for it.

Sometimes the solution can be right in front of us. A crusty old aeronautical engineer showed this trick to me years ago, when my 4-star style plane was bouncing.

On these sport planes, sometimes the ailerons are a bit too far down at "neutral." That can cause a flap-effect, where the plane won't want to stick to the ground. It's an easy thing to check out. Just take each aileron at zero setting, and using a clevis adjustment, or sub-trim, raise the aileron maybe one thickness of the trailing edge, and then do the same with the other side. This will move the aileron slightly up and away from "flaperon" and a bit toward "spoiler." That's usually enough to help a plane to settle onto the runway and stay there, and otherwise you shouldn't notice any real difference, other than the plane settling down a bit more rapidly from altitude.

It's easy to do, it's free, and it usually makes a notable difference. If it does, it's priceless.

Side note. CG adjustments should be far down your list of things to mess with when adjusting landing characteristics. Not saying you won't do it at some point, but there are MANY other aspects which should rightly be addressed first.

I hope this helps.

~ Jim ~
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Old 09-15-2012, 03:02 AM
  #37
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Default RE: landing bounce

Jeffsend,
The bounce problem of my Pulse XT 40 went away after the tail wheel broke off and was replaced with a sullivan tail wheel bracket that raised the tail about an inch. Prop strikes are no more frequent than before.
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Old 09-15-2012, 03:44 AM
  #38
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Default RE: landing bounce

I use the Sullivan Steerable tail gear all the time. I find it drastically improves the steering over most stock gear. Maybe this is a fringe benefit!

~ Jim ~
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Old 09-15-2012, 03:56 AM
  #39
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Default RE: landing bounce

In my experience, these are the factors that contribute to a "bounce". They may operate individually, or in concert1.

1... too high a touch-down speed, coupled with too high a descent rate.
2 ... stiff undercarriage
3... undercarriage far forward of CG (on a tail-dragger)
4... geometry such that the 3-point attitude equates to a low A of A.
5 ... high touch-down speed (on a trike)
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Old 09-15-2012, 04:31 AM
  #40
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Default RE: landing bounce

For many, many years now, I have flown birds with the gear moved forward of the wing. Four Stars of all sizes, Dragon Ladies, Super Sportsters, Giant Super Sportsters, and all tail draggers. You name it, we've flown it, with aftermarket gear or the stock gear moved forward onto the fuselage. We flew on a grass field, with the requisite number of bumps and things you'll find on any grass field.

Neither I nor any members of my club have ever had any issues with bouncing from gear forward of the CG, though I read in here all the time that it's attributed to the gear being forward of the CG.

In my experience, that's just not so, if you land properly. I taught the guys in my club how to set up their planes so that they slowed down for landing, with proper low idles leading the list of requirements. Attention to detail, mostly, so you're not dealing with extra factors while on approach. Minimize those extraneous factors, and landings are a much friendlier process. Of course, I wasn't satisfied until every one of them learned how to land under the most demanding conditions, dead stick being the least of them. All of our club members had reputations for consistent greased landings, even under harsh conditions.

Okay, thirty out of thirty-one did. You'll occasionally find a guy who is truly hopeless, but that's not the point here.

Anybody can bounce from coming in too hot, of course. Nobody likes landing a rocket.

If the details have been addressed, most planes cease being "rockets," and become much more user-friendly. There are usually reasons why a plane doesn't land well, and often those reasons have to do with the setup. I absolutely believe - through many years of experience - that having the gear forward of the CG is not a real reason for a plane bouncing. There are a dozen other factors that should be looked at first.

~ Jim ~
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Old 09-15-2012, 04:36 AM
  #41
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Default RE: landing bounce

To fully answer the question, we have to establish that there are two aspects of physics involved, mechanical and aerodynamic. The landing gear and wheels act like a spring. An aircraft with zero airspeed, dropped in a level attitude from any height, will bounce to some extent (may be tough to see, but it's there). The other force in play is aerodynamic. This is the result of a relationship between the center of gravity in relationship to the chord line and what happens to the angle of attack when the wheels touch down. As the wing approaches the runway, it has some angle of attack through the air. Generally speaking, when you increase the angle of attack, the lift increases, and the airplane goes up. The converse is also true. As the airplane touches down, if the center of gravity is ahead of the main gear, the nose rotates downward, the angle of attack decreases, and the airplane loses lift and thus tends to stay on the ground. Now, remember, I said generally. If you hit too hard, regardless of the aerodynamics, the airplane will bounce because of the spring like nature of the landing gear. Now, consider the case of a taildragger. Because the CG is behind the main gear, the inertial force tend to drop the tail, INCREASING the angle of attack, and therefore increasing lift, and the airplane appears to bounce. This is why speed control on touchdown is so much more important on taildraggers than tricycle.
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Old 09-15-2012, 05:48 AM
  #42
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Default RE: landing bounce

Just wow.

The secret to landing any tail dragger, and this learned in a 1:1 Cessna 140, is at the moment of touch down, ease off the elevator. Let your elevator go from up to neutral or VERY VERY slightly down.

The bounce as stated above increases the angle of attack of the wing. Holding up elevator only makes this condition worse.

When the wheels touch the top of the grass, let the elevator go to neutral. I've only had ONE model that did not like this approach and that was a Top Flite ELDER; on THAT original kit the gear is WAY too far forward.

MTC YMMV
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Old 09-15-2012, 05:53 AM
  #43
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Default RE: landing bounce

What it boils down to is how you land, nothing more, nothing less. The 4* manuals state that in plain english. Then again, not everyone can read english.   When you land a properly balanced 4* right, it doesn't bounce.  If the balance is towards the tail though, the plane has a higher tendency to bounce, as mine proved to me.
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Old 09-15-2012, 06:05 AM
  #44
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Default RE: landing bounce

acdii, even in a three point, getting off the elevator minimizes the forces that make bounces happen.
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Old 09-15-2012, 06:55 AM
  #45
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Default RE: landing bounce

YellowBlueBird,
Yeah,my original tailwheel was replaced with a Sullivan a long time ago. Seems like it ground loops a little easier now,but I don't like having the tailwheel mounted to the rudder anymore. I think it is what caused my ca hinges to begin to crack out(I fly off of really rough pavement). Luckilly,I caught it when it wasn't holding knife-edge quite right. Brought her in and discovered one of the hinges had broke. This was so long ago,I don't remember what affect it had on landing characteristics. The bouncing isn't a major problem for me......I just find it a sensitive bird to land on the mains.


Quote:
ORIGINAL: YellowBlueBird

Jeffsend,
The bounce problem of my Pulse XT 40 went away after the tail wheel broke off and was replaced with a sullivan tail wheel bracket that raised the tail about an inch. Prop strikes are no more frequent than before.
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Old 09-15-2012, 06:55 AM
  #46
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Default RE: landing bounce

Not arguing that.   The first couple landings on my 4*  were no different than I land it now, no elevator when the wheels kiss. It bounced the moment the wheels touched, but now that I have the nose weights glued on so they dont fall off sitting in the truck, landings are fine, settles in nicely, wheels touch and the plane rolls out on the mains then drops the tail. The Kadet does the same thing on landing, settles on the mains and rolls out till the tail drops.   The 2 ounces that fell off didn't affect flight that I could tell since I flew it for such a short time.  I still have some trim to do, it tends to yaw left on vertical, so I added some right thrust to the engine, and have yet to test fly it.  Overall the plane always handled well in the sky.

Now if I can just keep it out of the corn on landings......
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Old 09-15-2012, 07:28 AM
  #47
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Default RE: landing bounce

Quote:
ORIGINAL: Indiomike

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.

Mike
As alluded to by others, don't forget though that airspeed doesn't have much to do with it. It's descent rate that determines how softly a plane lands. My Kaos lands fairly fast because I like to do those pretty greaser landings with it, but it touches down very softly. On the other hand, when I was learning my trainer used to put the wheels down with almost no airspeed and bounce like crazy because I often stalled it out a foot off the ground.

On the topic of stiffer LG's, low-bounce tires and such, if you need those things on a sport plane then you're just not landing properly. Even LG placement isn't crucial if the pilot is setting the plane down gently on the wheels instead of slamming it down. The way lots of aeromodelers land would result in back surgery after a couple of years if they were full scale pilots. If you manage your airspeed and therefore lift so that the lift being produced is 99% of the plane's weight at the moment the wheels touch, there is no way for the plane to bounce. What has helped me is to keep flying the plane all the way until all the weight is on the gear, which is a 2-3 second process when you do it right. You touch down with no weight on the wheels, then you let the plane slow and kill the lift to transfer the weight from the wing to the wheels.

One thing that has not yet been mentioned is the value of managing the throttle during landings. If the pilot sees that the plane is settling in (losing lift) too fast, a bump of the throttle will save the landing by slowing the descent. Many new pilots go to idle and try to glide the plane in. Sure that works if you manage your approach right and the wind doesn't mess with you, but the throttle comes in handy quite often. What I've learned to do is intentionally come in a little short and establish a slight flare, then throttle the plane down to the ground much like a helicopter pilot does.
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Old 09-15-2012, 09:27 AM
  #48
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Default RE: landing bounce

It really DOES boil down to how well you land it. How well you land it depends on the setup, and your abilities. The setup is critical, and an engine/motor running at a high idle will screw your landing up more often than not. Learning to gauge your speed and bleeding it off is just as critical; and that takes time and experience. Knowing what it takes to stall your plane, knowing whether you can throw it into a fairly steep bank at a low speed, and at just how low a speed it can be done; knowing your planes tendencies is all a part of it.

I took shortcuts to get to my "happy place" for landing. I paid extra and bought Saitos, They tick-tick-tick at idle, and don't pull your bird past the field. Any good four-stroke can do the same for you.

Angle of attack and all that is integral, but not really something you're going to be able to use/ factor/formulate on approach. Once your plane is set up right (that means everything from C.G. on up is dialed in), your angle of attack will take care of itself.

For instance, a factor that can be "added in" for landings is flaperons, which I personally like, because most of today's radios allow you to not only set the degree of flaperons, but dial in simultaneous elevator as well. So, say you set up flaperons and they're set for 15% flaps. Great, but the flaps usually cause the nose to angle up. Not great, right? So, set in some down elevator, and it pushes the nose down, which gives you a change to your angle of attack AND the lift and speed reduction. Adjust the elevator down until you like where the nose is. Keep it "up" a little bit for a good flare on landing.

Most planes that handle great and land beautifully are no accident. Diligence and attention to detail are the reasons. In time, all this stuff becomes second nature. You know what it is, you know why you're doing it, but for the most part it's automatic in your building and setup, and the other guys lucky enough to fly them will covet them. I promise.

~ Jim ~[8D]
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Old 09-16-2012, 03:46 AM
  #49
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Default RE: landing bounce

A lot of interesting comments. I hope to test fly the plane possibly next Wednesday if he brings it. The more I think about it the more I suspect his bounce is due to his flying technique. I mentioned in post #5 that he tends to land long, usually in the last third of the runway. Our runway is 600+ feet with the pilot stations at mid point. The plane thus glides past his position and is going away from him. He can see that he is running out of runway and therefore may be anxious and forces the plane down which may cause the bounces. The whole problem may be he waits far too long to reduce power and thus has too much speed at the approach end of the runway. The plane must then "float" for quite a ways to bleed off speed. Anyway, that is one possibility.

Mike
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Old 09-16-2012, 04:27 AM
  #50
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Default RE: landing bounce

If the model is floating 130 yards beyond the threshold, then the approach speed must be seriously high. Only the very lightest of touch-downs would fail to result in a bounce.
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