front wheel steering fork angle

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jany77

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Hi guys ,as most ultralights and trikes has the front wheel steering fork facing forward i would like to know if there is any formula to tell how much that angle should be compare to keel or any level surface ,as some planes has the angle more than the others such as sabre trike probably around 20 degrees compare to the nano trike around 10-14 ? degrees .
I have build and used both the caster type and sabre style in past im just trying to found out the advantages for the sabre style ,it also brings me to the question of the stability since bicycles and motorcycles has the same design thank you
 

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Dan Thomas

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Key point is locating the hinge line forward of the contact point ... er ... where the rubber meets the runway.
Yup. On most GA aircraft the steering pivot axis passes through the forward part of the tire's footprint, not entirely clear of it. You sure don't want it anywhere near the center of the footprint. The Cessna 182RG has a little more angle to the nosegear, and when you turn, the nosewheel wants to go further. That's because the pivot point is ahead of the footprint and the weight of the nose tends to increase the turn; the nose drops a little as the wheel steers left or right.

Compromises everywhere in airplanes.

One thing you surely want is that nosewheel to be perfectly aligned left/right with the nosegear strut. Not off center, not tilted left or right. That can cause shimmy that's impossible to fix.

Tailwheels rule!:popcorn:
 

GTX_Engines

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Just a side note: When taking off from hard-pack, wet sand beaches you don't want a forward angle nose gear since they are unpredictable and will dig in and flip the trike over. You need castering nose wheel for soft ground operation.
 

Dan Thomas

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Just a side note: When taking off from hard-pack, wet sand beaches you don't want a forward angle nose gear since they are unpredictable and will dig in and flip the trike over. You need castering nose wheel for soft ground operation.
No. You need a taildragger. Nosedraggers flip over regularly in soft surfaces. They're considerably worse than taildraggers for that. Right after I got my private ticket in the early '70s I taxiied a 150 through a puddle on a clay surface and got stuck mighty bad. Had to find a rope and several people to help me pull it out backwards. There had been aircraft tracks through that puddle, so I had figured it was OK. Looking at those tracks after we got the 150 out, I could see that it had been a taildragger that had run through there with no effort at all. A nosewheel has a lot of weight on it, and when it starts to sink, it becomes the pole for a pole-vaulting maneuver if you have enough speed. If it just get stuck during taxi, more power pulls just digs it in farther. A taildragger's mains are farther back, near the CG, and the load is almost straight down so that pole-vaulting doesn't happen so easily. The mains don't tend to dig in nearly so hard.

The RVs had flip-over problems with castering nosegears, too. The spring rod would start bending under when the nosewheel encountered drag, and the forward end of the wheel fork then dug in and it was all over. There's a stiffener mod to prevent that.
 

Dan Thomas

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The nose gear tyre on many aircraft is usually a lot smaller than the mains aggravating the whole (hole?) situation. The CH701's around here have the same size tyre on the nose as on the mains, seems to help a lot out in the bush. A taildragger has the big tyres at the front.
The taildragger's front tire location nearly under the CG is what saves it. When it encounters a hole, the CG, where the center of mass and its momentum is found, can't exert the ever-increasing downforce that it can on a nosewheel. A pole-vaulter doesn't stick the pole into the ground six inches ahead of himself; it's way out front where his weight and momentum will drive it in hard and lift him up and over.

On a soft surface, the same thing applies. Trikes can flip over real quick if that nosewheel is allowed to touch down at any real speed. Taildraggers aren't immune to it, of course, but they're more manageable, in my experience.
 

jany77

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Thanks for reply ,as i sad i have build and used both types in past ,im not building any trike now ,i was more or less interested to know why 98% of trikes and airplanes use front facing fork style landing gear ,even L.Pazmany book does not cover it or give any explanation .
 

jany77

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Only one think which i was able to found is that front facing style landing gear is less stress than caster style.
 

plncraze

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The BD4 used a castering nosewheel. You could look at that design for inspiration.
 

Aerowerx

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....Pazmany book does not cover it or give any explanation .
I beg to differ.

There is a section in Pazmany's book where he diagrams different configurations and angles for the nose wheel, and gives the pros and cons of each. At least there is in my copy.
 

TFF

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I think the main is steering. A small amount of positive castor is steerable by connection to the pedals. Negative castor for no connection. Trail to stay straight. Go too far either way and you have problems.
 

jedi

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Bicycle design includes a study of head angle as a design variable. Research that for your answer. "Scientific American" had a good article years ago. Like everything the angle is a compromise. There is no "right" answer. Custom motorcycles are a good example of extremes. All "ride" but all are different in ride performance and stability.

My opinion only. I am not a bike designer!
 

Lendo

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Agree with Aeroworx, I have that book it is very informative, but basically with Tricycle gear the front wheel must be behind the prop when fully depressed or engaged - it's logical.
George
 

Dan Thomas

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The Piper TriPacer has the tinest caster angle I've ever seen on any light airplane.

1586301740560.png

It works, too. The linkage to the rudder pedals is pretty rigid. This angle wouldn't work well at all with the bungee steering Cessna uses. Any shimmy would be awesome.

Airliners and other transport aircraft will usually have vertical nose struts. Some will have the axle set back a bit to give some caster, and others don't. They use hydraulic steering that permits no play, so no shimmy.
 

jany77

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Pazmany book has a section on it i agree but the section if i remember is on one page and does not cover what the offset or trail should be for slow and fast aircraft and has not enough details or explanations ,i found more articles and pages on bycicles design which cover it more in details ,as i sad at the beginning im not building anything im just trying to found out " WHY" all designs choose this style of landing gear specially ppg ang trike industry.
Too much time on my hands in this self quarantine time so please excuse me ,just trying keep my self busy and now is the time to found out more about designs im interested in and spend most time with .
 

flyboy2160

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...
The RVs had flip-over problems with castering nosegears, too. The spring rod would start bending under when the nosewheel encountered drag, and the forward end of the wheel fork then dug in and it was all over. There's a stiffener mod to prevent that.
My FEA analyses of the old, skinny Vans Wittman nose legs showed them buckling even without the edge of the caster fitting digging in. Check out the Antisplat compression test video:
Videos | Anti Splat Aero LLC
 

Dan Thomas

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My FEA analyses of the old, skinny Vans Wittman nose legs showed them buckling even without the edge of the caster fitting digging in. Check out the Antisplat compression test video:
Videos | Anti Splat Aero LLC
I did a prebuy inspection on an RV6 that had that thing. Looked good. But, sadly, it's one of those things that corrects an inadequate design while adding weight and complexity. The spring rod nosegear was supposed to be simple. Now it's not so simple. Some testing, as in the Anti-Splat video, would have revealed that the forward end of the rod should have been thicker. It looks like the engineers didn't take into account the drag on the wheel, just the vertical loading. See this:
 
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