Rental fleet & other nose gear designs
Steve said:
Item: The nose gear in the ubiquitous Cessna 182 is attached directly to the firewall. Cessna sells many replacement C-182 firewalls and boot cowl skins because of botched landings directly attributed to pilot technique (PIO, dropped in from 5 ft., wheelbarrow landings, etc.). Steve
Steve very good point. When I was a CFI at a large school/club, we had about 35-40 airplanes, most Cessna from 152-172/rg-182/rg-t210-340. The rest where Mooney or Piper (Seneca II). I can tell you I would check the firewall very well during pre-flight. I instruct my students and pilots I checked out to look for signs of hard landing. I saw the aftermath on occasion. The few times a firewall was buckled, the pilot would always fess up, except once the guy parked it, tied it down and told no one. On occasion over a 3-year period I saw one or two Cessnas in the maintenance hanger on jacks, with a bent firewall coming off. This gets into pilot error, but from a structural standpoint it is relevant.
Nose gears and their support structure can only be so strong. Even an Airbus Jetblue made the news with nose gear issues recently. Mooneys are known for prop strikes due to POI and low prop clearance. With Cessna's the firewall buckles. With RV's the gear leg folds and seems to be related in part due to (may be a little) pilot skill/technique and in part to catching the nose wheel or bottom of the gear leg on an uneven surface.
THE ULTIMATE NOSE GEAR
This reminds me of my friend with a 50's something Piper Tri Pacer, the first GA tri-gear plane sold. Being an engineer he researched the history and found pictures of how they "sized" the nose gear. Since nose gear's on small planes was a new thing, they where forging their way, making it up as they went along. He found pictures of an Early model being tested by being pulled across a plowed field 90 degrees to the furrows with a tractor. Look at a tri-pacer nose gear. The support looks like bridgework. Look at the size of the tire. Look at the fork, which is traditional, going vertical. They (over) built the tri-pacer to take some serious punishment. The tri-pacer is of course so homely it is cute.
RV NOSE GEAR
Clearly the spring steel gear of the RV is clever and works. The short stout Cessna nose gear is made of a steel Oleo (gas/oil) strut bolted to a large fitting attached to the firewall. Clearly the latter is stronger. Would a Cessna gear work on a RV. Well for sure it would be aerodynamically draggy and heavy. The moral of the comparison is accept the limitations and operate accordingly.
NEW AND IMPROVED
I think the stock RV design is an acceptable solution. I also think is could be improved, however some who fly's off of hard surface or improved soft runways (they know) may prefer to keep it as is. I also think a bigger tire (and wheel fairing) and some kind of anti-dig skid on the front of the wheel fork pivot would reduce the tendency to catch on uneven soft surfaces. A larger change is stiffer (bigger) heavier gear leg with less taper and more "meat" through the end/mid section of the strut. All the damaged nose gear pictures show a bend at mid section. However the thin end allows the wheel fork pivot to rotate down (as Van described in the RVator).
TOO FLXIBLE AT THE END?
As the nose wheel is pushed up the front of the gear leg at the wheel fork/pivot goes down. The "load path" is offset or eccentric. On a Cessna the wheel axial and strut are in line. Any offset in the load path makes secondary bending moments. This bending moment at the end of the RV gear leg is what causes the end of the gear leg to be driven down, reducing clearnace to surface objects. A stiff gear will ride harder. Also in the event of a hard landing the damage may travel from the gear leg to the firewall, like a Cessna (probably not).
STABILITY?
RV trikes have a long flexible nose gear leg. Also with it slanted forward it has stability issues. As it deflects, especially sideways the loads increase as deflections increase, which promotes more deflection, which increases load (stress). At some point a small increase in load results in an infinity larger deflection. In other words you are pushing the wheel across the ground vs. pulling the gear along the ground. Think of a nose gear leg that is slated back from the direction of travel. These are self correcting in that as the wheel gets "out of line" with the supprt it wants to return in trail. This is OK but you have to design the gear for deflection and not strength. I think the reason we feel some anxiety, we can't do anything about it. It is not like we can use just beef it up with a thick aluminum doubler and add more large rivets. The gear is a very highly engineered part that builders can not modify or fabricate themselves.
(As noted by RV6ejguy the free castoring swivel nose wheel does not add to the side stability of the gear leg because it FREE to swivel. Ground irregularity or other side forces can act on it, causing the nose wheel to track or drift left or right. As the wheel goes to the side it deflects the gear leg to the side, while the aircraft goes straight. Additional side or drag load can cause further deflection of the gear leg. The best way to reduce the castor effect is a large brake out force that keeps the tire going straight, but than turning would be difficult. In most planes the nose wheel is connected to steering linkage, which stabilizes it and keeps it from BEING turned.)
LIMITATIONS
Can pilot skill overcome the limitation of any design. Is the limitation too restrictive or acceptable. Note, limitation is NOT a bad word, it only means that there is a limit and all structure has it. Is it safe to operate on soft-fields? I think the answer is yes, BUT extra care obviously must be taken on soft surfaces. One of the biggest issues is KNOWING the surface. Many of the issues where found on fields that the pilot never had been on. Was it the fields fault of the pilots? Proper maintenance is critical. Proper installation is critical. The margin in pilot technique and skill is narrower on soft fields. In other words soft field operations are less tolerant to deviation from ideal on any plane. However the average pilot can operate safely on smooth grass and dirt fields. For example Mooneys CAN not operate on soft fields due to prop tip clearnace. I know because I had to fly out and pick up renters who dinged the prop while taxing on soft ground. It happened TWICE to differnt pilots. Just be careful.
George