HarryC

My Feedback: (1)

Kimhey is right that the noseleg spring is very important, as well as the speed. Most models have a main leg as a noseleg with the result that the spring is too powerful and the leg acts as a solid wire leg rather than a sprung leg. So when it hits the tarmac it has no give, and rebounds immediately which throws the nose up into the air. For example, many people fit a Robart 3/8” main leg as a noseleg. It is designed to take about 6lbs per leg but your nosewheel weight will probably be around 1 – 2 lbs so the spring never compresses and you might as well have fitted a piano wire leg. My experience of legs sold as noselegs is that they still have much too powerful springs. I always calculate what spring rate and initial compression is required, remove the spring it came with, and fit a spring calculated for that model.
In full size design where they do it properly, design is complex but a basic rule of thumb is that the noseleg is proportionally softer than the main legs so that its frequency of bounce is lower than the mains. Thus if the plane touches down all 3 at once and they compress, the mains will extend again more rapidly and keep a nose low attitude initially. If the nose is proportionally harder, its frequency of bounce is higher so it would raise the nose ahead of the mains and risk the aircraft lifting off again, which is what happens with models.
When you put the model on its wheels does the nose leg compress a bit? If you press down gently does it compress a lot more? That’s how it should be. Or does it stay at full length on the model’s weight and take a hard push down on the nose to compress it? Then the spring is too strong.

BillJ

My Feedback: (19)

Good point Harry C....and you are indeed correct...The rigidity of the nose gear spring came to mind as I compaired the action of this gear vs a skymaster gear.... Thanks

VF84sluggo

My Feedback: (55)

Even with the correct airspeed on approach and landing, the OP does have a valid observation. It is not uncommon at all for a 'full scale' to have at best a zero, or even slightly nose-down, attitude on the ground. It does serve to take the AOA off the plane's wing as the nose comes down, and put weight on the wheels for better braking effectiveness. The P-3 and MD-88 come to mind. The problem with this setup in RC can be with 'unsticking' the nose on takeoff rotation. The jet might tend to 'pop' off the deck.

In any case, it is best if the the jet is on speed for landing...which for me I freely admit can be easier said than done!

turnnburn

My Feedback: (49)

Bill J, you never did tell us what airplane it is that you having landing issues with. Kangaroo's and to a lesser degree Hot Spots and similar airplanes are some of the worst "hoppers" if not landed correctly. One of the surest ways to get the hop type landing is by letting the nose gear be the first gear to contact the runway. This is almost always caused by landing too fast or too long and forcing the airplane onto the runway or allowing it to touch down well above stall. Drag devices like flaps, speed brakes will help get the airplane slowed down. Crow ailerons will also help, not so much for the added drag but because they are in effect reducing the angle of attach of the wing tips and therefore hopefully slowing the stall speed and reducing if not eliminating tip stall where one wing tip drops rapidly when stalled resulting in a wingtip first touchdown which is not likely to turn out well. I noticed consecutive posts above where one gentleman suggests a more forward CG and the very next post suggests a more rearward CG. I agree up to a point with the more rearward CG. A more rearward CG will be easier to get the nose up and hold the nose up but dont go too far rearward or you may have other problems like pitch stability and or nastier stalls when and if the wing is stalled. I tend to go to higher rate on elevator for landings on most of my jets in order to have enough elevator to get the nose up and keep the nose up as speed decreases. But once again dont over do it and you may want or need a little more expo to keep the approach and flare smooth.

BillJ

My Feedback: (19)

It was suggested by ROBART...that cutting a couple of coils off the nose gear spring would "soften it" What do you think ? Indeed they use only one spring for all gear (or gear group) and do not offer any other alternative..

Jgwright

I would not get too hung up on the angle of attack. In many early jets the planes had a substantial angle of attack sitting on the runway. On my Horten it had 7 degrees and the Boulton Paul is also 7 degrees. In the book it says the angle of attack on landing was 17 degrees. It takes some getting used to high angles of attack especially is the jet has a heavy wing loading. The Horten I was advised should have a nose leg that I could reduce the engine of attack on landing, but after making one I found that it was not needed and landed fine provided it was slowed up enough and landed on all 3 wheels at once and not the nose leg on its own. The approach was slowed up with use of the 3 surfaces on each side being used with the inner going down the middle up and the elevon slightly up. (effectively crow braking).

The Boulton Paul is very different being a delta. It is very heavy for its size and has only one speed on approach...fast. It like to be landed with a long flat approach with the nose being lifted prior to landing. There are petal air brakes but they do not do much. The full size was similar and had a chute as well to help slow it up, they even resorted to using it before reaching the threshold. The model plane also thunders in and takes a while to slow up. Normally landings only bounce if there is uneven grass. The full size apparently also was known as a 'hot ship' and would take the full length of the Farnborough runway to stop. I did have a problem with the nose oleo being stiffened with a grub screw that made the action very stiff and on one landing the nose oleo was effectively locked. It still did not bounce though.

These high angles of attack are used sometimes because the main wheels are well behind the CG.

There are differences between different aircraft designs. The Rafale will tolerate being landed very heavy and fast without bouncing but some deltas are a handful. The worst I have seen is the Velux. This is an attractive looking plane until it is landed. It takes some real skill and being well set up to land without bouncing in ever increasing bounces. The Kangaroo also had a reputation of being a bouncer on landing if not got right.

After building many jets I would say the most important thing is to get the Cg in the correct place. Too far forward and lack of angle of attack and too high landing speed can make bouncy landings a problem.

John

HarryC

My Feedback: (1)

Crow aileron increases stall speed, not reduces it. It's flap but upside down so it gives the wing an inverted camber causing either a reduction in lift or even a downward force. However the change in stall speed at model sizes is likely to be so small that it's not really a problem.

HarryC

My Feedback: (1)

The maths does not agree with that! What matters is the spring rate and that is not related to the length of the spring.
For example, if the oleo has a stroke of 10mm, and there is a nosewheel weight of 1kg, you might choose a spring rate of 1N/mm and pre-compress it by 7mm. Then when the model is on the ground the oleo will compress by 3mm which allows it a bit of spring out to cope with holes, and will take 1.7G load to compress it fully. Thus the oleo will ride up and down over bumps and will absorb the touchdown on landing.
I can't recall the rate of spring on a Robart 3/8" oleo but a figure of 6N/mm or more is in the right ball-park. Even if you cut it to the exact length of the chamber so it is just floating free when the leg is fully extended, it will compress by just 1.6mm at rest, and will require a load of 5G to make it compress fully. So unless you crash the model onto the runway it behaves almost like a solid wire.

HarryC

My Feedback: (1)

just for interesting background info, full size oleos are damped only on extension, not on compression, so that the springing element is undamped as the aircraft touches down but the rebound is damped so that the springing doesn't bounce the plane back up into the air. our tiny undamped oleos have no such luxury!

dionysusbacchus

My Feedback: (25)

Harry, you are correct on all counts. I always replace the nose gear spring in the Robart struts with a softer spring. I am always looking for springs, old family hardware stores and estate sales, whenever I see them I buy them! Not sure why Robart includes such strong springs in there struts, maybe they sell softer springs? I'll have to check. I even have to replace the springs in the Robart struts on the main gear for my Ziroli Panther, I'm afraid to compress them by pushing down on the fuselage because I would probably crack my plane trying to get them to move, they are that tight!

Soften those springs and land right! Good luck!

HarryC

My Feedback: (1)

That's not so surprising for main legs, for example how much G would you want the main leg to take on a really hard drop landing before it reaches max compression and bangs against the end stop? 3G? I do think they pre-compress them for the upper end of the weight spec though, if you put the model on the ground and the oleo does not compress say 1/5 - 1/4 of its travel, the pre-compression is too high regardless of whether the spring rate is correct or not. Pre-compression is fixed by removing a bit of length from the spring, but that doesn't fix the rate!

The big problem comes with so many models using the smaller robarts such as 3/8" as noseleg when they are main legs expecting up to 6 or 7lbs weight on them. But even legs sold as noselegs by other brands have, in my limited experience, been given main leg spring rates. The worst case I have had is the Mick Reeves legs for my MR EE Lightning, where the mains were so soft they collapse entirely and the noseleg so strong it didn't at all! this meant the mains offered no suspension at all as they were up against the end stops just sitting on the ground, plus it is the opposite of the required rule that the mains are relatively stiffer than the nose. With the MR Lightning legs the mains have a lower frequency than the nose (in fact the mains have a frequency of zero!!) Naturally I did some simple calculations and bought new springs for all 3 legs.


love your message!!!!-
"Don't believe everything you read on the internet"
--Abraham Lincoln

BillJ

My Feedback: (19)

FYI Robart does not sell any softer or replacement springs other than stock

kimhey

Just recalled that on my EuroSport (Delta wing), I replaced the spring with a length of Tygon (fuel line). It will compress nicely, but not kick back like a coil spring.

mr_matt

My Feedback: (10)

If you cut the spring (reduce the active length) the spring rate goes up.

Of course a shorter spring might help with the angle of attack problem depending on other factors.

I agree most of this is just landing too fast. And I have helped with planes where the guy has the CG so far forward you can't get them to slow down easily.

sgtmonte

What is "crow"?

BillJ

My Feedback: (19)

A thing that flys around the corn field in Kansas...(sorry couldn't resist) .. CROW is where both ailerons Lift at the same time....kinda Like flaps ...but in an opposite direction.....creates a washout on the wing tips causing them to stall quicker.....Used by sailplanes to get them down

rrembert

So BillJ, two others have asked what kind of airplane you are having the problem with...so, third try...which plane is it it you're having landing issues with? Bob

HarryC

My Feedback: (1)

No it doesn't. The spring rate is inependent of its length. It's expressed as a force per distance compressed or extended e.g Newton per mm or N/mm. Notice that the length of the spring is not in that quantity.

mr_matt

My Feedback: (10)

Play around with this calculator.

http://www.engineersedge.com/calcula...ring_k_pop.htm

Reduce the number of active coils (equates to cutting the spring). Look what happens to the spring rate....the spring constant (k)

kimhey

This is getting too complex now.... If you cut the spring in half, it will bounce a given weight to a lower height than if the spring had it's full lenght.

HarryC

My Feedback: (1)

I take it back, you are correct. Force is the same at any point so each coil deflects the same, not a share of the load.

gsmarino2000

My Feedback: (2)

I have spring trailing link gear on my Navy Cat, which will absolutely bounce if you land too fast or stall the plane with the mains more than an inch or two off the runway. I also have a Viperjet with expensive damped struts. They are amazing. You can plunk it down soft or hard and it sticks like glue. I've even hit it hard enough once to crack the main gear flex plates and it still did not bounce.

Greg

basimpsn

My Feedback: (5)

I have the same jet... my fix was low bounce wheels.

mr_matt

My Feedback: (10)

umm...not sure what you mean. Earlier in this thread someone said the bounce problem was related to too stiff of a nose strut spring. IF this is the problem, cutting the spring is just going to make it stiffer, not the other way around.

As I said maybe cutting the spring (and NOT putting in a spacer to make back the length) has some sort of other consequence like lowering the static angle of incidence, I don't know. But cutting a spring down makes it stiffer.

kimhey

So if you cut the spring to the extreme...ONE coil left in the strut, what would be the result?

I agree that the spring will be stiffer when cut, but the stored force (the force that decides how high the nose wheel bounces) put in the spring when compressed, will be less.