Cessna 172 Nose Wheel Strut Spec: 5.15 - 5.30 inches?!?

[DCR]

Hello,

I have heard everything from "two" to "four" fingers of chrome showing on the nose wheel strut. Trying to see where this number came from I checked the maintenance manual (both pre 1976 and 1977 - 1986), both manuals list "5.15 in MIN to 5.30 in MAX".

I'm no bear, but the palm of my hand (thumb included) is about 4 inches. What's going on here? The only thing I can think of is perhaps maintenance may check the plane when it is suspended/jacked and there is no load on the nose. Any thoughts?

Picture from the manual attached. I also must admit I'm a but confused in the way they put the arrows for section D-D, but the section image looks like the nose strut to me.

 

[Ryan Klems]

That figure is the min/max when the strut is at full extension. The strut should be serviced IAW Section 2 of the manual, which sets the pressure to 45 psi in the strut. The amount of extension seen when the strut is serviced as such will vary based on the loading and weight of the aircraft. For example, if you have a light fuel load and have bags in the rear baggage, you'll have more strut extension than you will if you are fully loaded with fuel and an empty baggage area. There are other aircraft that do specify an extension, the Comanche for example specifies a certain extension with full fuel and oil, but the 172 is not one of them. The important thing is you don't want the strut locked out when on the ground (you won't have any nose wheel steering) and you don't want it collapsed. Of course, you also don't want the strut to bottom out.

 

[DCR]

Ok that makes sense - thank you! I found the table in section 2 you are referring to.

But now I'm wondering - why won't you have steering with the strut "locked out"? Does locked out mean overpressurized such that the strut is at max extension on the ground?

 

[Mikey52]

The strut has a built in centering mechanism to keep the nose wheel straight at touchdown. With weight off the nose wheel or strut at full extension it is locked in center position mechanically.

 

[Dan Thomas]

The 45 psi will give you a fairly high nose. There's a danger there that one must be aware of, and we had issues with it in the flight school. In flight, that nosewheel is a long way down, well below the mains. You can see it here:



Next factors are the airspeed and angle of attack relationship, and flaps. More speed means that the angle of attack is less, so the nose is lower. Adding flaps increases the wing's angle of incidence, so the nose goes down even more for a given airspeed. A pilot that is in the habit of landing fast is landing flat, particularly with flaps extended, so that the nosewheel touches the runway first, well before the mains touch. That can cause porpoising and/or wheelbarrowing, and both of those have wrecked many airplanes. If I put the specified 45 PSI in the nose strut, I saw too many close calls, so I softened it some. Sometimes one has to make allowances for poor training and habits. When I was instructing I wanted that nosewheel sell off the runway at touchdown, and that requires planning all the way back on downwind. Get the speeds and approach path right.

And yet, another instructor caught me landing flat one day. It's a pernicious thing.

Taildraggers rule. You won't get away with bad habits like that.

 

[midwestpa24]

The 45 psi will give you a fairly high nose. There's a danger there that one must be aware of, and we had issues with it in the flight school. In flight, that nosewheel is a long way down, well below the mains. You can see it here:

Next factors are the airspeed and angle of attack relationship, and flaps. More speed means that the angle of attack is less, so the nose is lower. Adding flaps increases the wing's angle of incidence, so the nose goes down even more for a given airspeed. A pilot that is in the habit of landing fast is landing flat, particularly with flaps extended, so that the nosewheel touches the runway first, well before the mains touch. That can cause porpoising and/or wheelbarrowing, and both of those have wrecked many airplanes. If I put the specified 45 PSI in the nose strut, I saw too many close calls, so I softened it some. Sometimes one has to make allowances for poor training and habits. When I was instructing I wanted that nosewheel sell off the runway at touchdown, and that requires planning all the way back on downwind. Get the speeds and approach path right.

And yet, another instructor caught me landing flat one day. It's a pernicious thing.

Taildraggers rule. You won't get away with bad habits like that.

While I understand your intentions Dan, I don't think the amount of pressure in the strut is going to change the extension of the nosewheel in flight or at touchdown. With no weight on it, the strut will fully extend. The importance of flaring and getting the nose up remains. Lowering the pressure it make reduce the rebound slightly, but doesn't make up for poor technique.

When I preflight and teach preflight, I demonstrate how to compress the strut by pulling down on the prop evenly. The strut should not be locked at full compression, and neither should it come even close to bottoming out. That is where the 2 to 4 fingers of strut visible come into play.

 

[Dan Thomas]

The strut has a built in centering mechanism to keep the nose wheel straight at touchdown. With weight off the nose wheel or strut at full extension it is locked in center position mechanically.

Yup. At the top end of the upper torque link (scissor) there is a steel block that mates with a flat spot on the collar at the bottom of the oleo cylinder. That's the extension stop, and it forces the nosewheel straight. Not just to make the wheel straight for touchdown, either; that centering is the centering for the entire rudder system, via the steering bungees. It straightens the rudder to add to yaw stability, and when you push right or left rudder, you are pushing against one of the other of the steering bungees.

That system is often misrigged. The service manual gives rigging instructions, but some mechanics are in the habit of fooling with the threaded cable connections at the rudder instead of adjusting the bungee forks.

 

[Dan Thomas]

While I understand your intentions Dan, I don't think the amount of pressure in the strut is going to change the extension of the nosewheel in flight or at touchdown. With no weight on it, the strut will fully extend. The importance of flaring and getting the nose up remains. Lowering the pressure it make reduce the rebound slightly, but doesn't make up for poor technique.

The 45 PSI doesn't change the extension. It makes that oleo stiffer, causing the initial bounce and starting a porpoise, or if the landing was really gentle, you're on the nosewheel only. The pressure will hold that oleo stiff for much longer. I once watched an instructor and student make just such a touchdown on the runway. I saw a couple of inches of daylight under those mains, and it stayed there for a long time. If there had been any crosswind whatever, a groundloop would have resulted.

Pressure matters.

 

[DCR]

Yup. At the top end of the upper torque link (scissor) there is a steel block that mates with a flat spot on the collar at the bottom of the oleo cylinder. That's the extension stop, and it forces the nosewheel straight. Not just to make the wheel straight for touchdown, either; that centering is the centering for the entire rudder system, via the steering bungees. It straightens the rudder to add to yaw stability, and when you push right or left rudder, you are pushing against one of the other of the steering bungees.

Ok - please excuse my ignorance, but, If the torque link is keyed to the strut when the strut is locked out - how can you still push on the pedals and turn the rudder? I thought this would effectively lock your pedals too. I'm guessing it has to do with those "bungees". Will research...

Really appreciate your answers!!!

 

[robin ardoin]

...... it has to do with those "bungees". ......

 

[DCR]

Lol thanks robin. So if I understand what I've read correctly, the bungee is basically a spring inline with the steering arm. When torque link is keyed at the top - or you are parked and there is too much friction to overcome - the spring will compress but the wheel will not turn. During normal taxiing however, since the wheel is rolling it does not need to overcome as much friction and the spring will transmit enough force to turn the wheel.

This explains why my instructor insists I straighten out the wheel BEFORE we come to a stop for engine runup or parking.

Thinking out loud here so let me know if I misunderstood. This is the best video I could find of the bungee:

edit: Wow I also just re-read Dan's comment and realized what you meant by "centering the entire rudder system". I hadn't even thought of that! Without the bungees I guess the rudder would have no reason to return to center when you release pressure (except maybe some aerodynamic effects). Learning so much thanks.

 

[brcase]

Wow I also just re-read Dan's comment and realized what you meant by "centering the entire rudder system". I hadn't even thought of that! Without the bungees I guess the rudder would have no reason to return to center when you release pressure (except maybe some aerodynamic effects). Learning so much thanks.

This hadn't occurred to me either. I knew about the centering system, but hadn't thought about it centering the rudder as well. It makes sense that it likely also dampens out adverse yaw from the ailerons . i.e. keeps the rudder basically straight to effectively make a larger vertical stabilizer.

My spiel, that I mostly made up from observation of the system and bit of reading is... Cessna did a lot of engineering on the nose wheel to make it steer just like a taildragger. They were trying to convince the population of mostly tailwheel pilots and instructors that nosewheel was a good idea. Making it as familiar to them as possible seemed like one way to do this.

Piper on the other hand just made a simple Hard connection to the nosewheel and let the pilots deal with the difference.

Brian
CFIIG/ASEL

 

[Magman]

Related to this is the fact that an adjustment to the Rudder may well

have an effect on nosewheel shimmy or adjusting the nosewheel system

can adversely affect the Rudder System.


These 2 areas are related and very easy to check per SM. A sharpened

piece of welding rod is utilized so it is not intuitive.


The “ hard connect “ on Cherokees and Tri-pacers works well;

but trying to turn the nosewheel while standing still WILL cause

problems with the linkage. Cherokees are noted for this.

I have pieces in my hangar. Scary!


I believe servicing a strut is allowed as Preventive Maintenance.

Like many other things it is not tolerant of errors.

I have looked at aircraft that were supposedly just re- rigged and

in 30 SECONDS found they were dangerously out of rig.

 

[DCR]

I believe servicing a strut is allowed as Preventive Maintenance.

According to the FARs:

FAR 43, Appendix A
"(c) Preventive maintenance is limited to the following work, provided it does not involve complex assembly operations:
[...]
(3)Servicing landing gear shock struts by adding oil, air, or both."

 

[Dan Thomas]

From a 172 manual on rudder rigging:




Those steering bungees get worn out. The springs get weak, making steering sluggish. There's also a washer inside them, retained by a crimp around the tube, that wears and if it manages to pop past the crimp, it will seriously restrict rudder control.

McFarlane sells new bungees.
https://www.mcfarlaneaviation.com/search/?q=cessna+172+steering+bungee

 

[Magman]

As you can see per Dans post; it’s not Rocket etc.

So why does it change?

In addition to wear, I’ve found a few with the cables stretched and

unable to adjust to spec.

 

[midwestpa24]

Piper on the other hand just made a simple Hard connection to the nosewheel and let the pilots deal with the difference.

Brian
CFIIG/ASEL

I've helped a few pilots transition from learning in Pipers to flying Cessnas, and usually steering on the ground was the most difficult part. They are so used to push the pedal and the airplane turns, and in the Cessna it is push the pedal to suggest to the airplane to turn.

One advantage to the Cessna is the ability to turn tighter as the nose wheel can steer much further. I first learned in Cessnas and was used to using differential braking to whip the plane around, something that doesn't work so well in a Piper.

 

[Gary Ward]

 

[Magman]

When holding Rudder in a Cessna the nosewheel will be straight at

touchdown due to the Centering System. Pipers will move move with

the Rudder Pedals. I think zero time to become accustomed.