propeller axis inclination in tricycle landing gear planes

[Nicknick]

Hi folks,

Is the propeller axis usually totally horizontal or slightly inclined e.g. to aid rolling or prevent prop strikes? Are there different marks for checking to oil filling for different plane types because of diferent inclinations or are they all the same (except for taildreggers)?

Thanks in advance,

Dominik

 

[riggerrob]

Oil dip stick levels correspond to normal parking attitudes.
If you install the same engine in a tail-dragger (say Cessna 180) and a nosewheel airplane (say 182) you might need to replace the dip stick.

Civilian certification standards set propeller axis (e.g. crankshaft) high enough to prevent prop strikes even when the nose tire and nose oleo strut are completely deflated.

After that, landing gear lengths are set more to align wing incidence for take-off. For example, Dyke Delta might look like it is parked too nose-high, but Dyke pilots report that the angle is perfect for take-off. They just advance the throttle and it flies itself off the runway. That angle is also ideal for flying final approach. Deltas are very sensitive to angle of attack at low airspeeds. If they get too steep an angle of attack, deltas generate huge amounts of drag, fast descent rates, steep descent angles and hard landings.

A third variable is the need to maintain a 15 to 17 degree angle between the main wheels and the tail wheel/skid to allow the wing to fully stall during landing. Fully stalling the wing helps when landing on short runways as it allows you to touch down with the minimum velocity to use the minimum amount of runway.
Fully stalling the wings applies all the weigh to the main wheels and improves braking.

Larger, more complex airliners use wing top spoilers to destroy lift as soon as the main wheels touch the runway. Spoilers are activated by weight-on-wheels switches built into the main landing gear legs.

Lazlo Pazmany wrote an excellent manual on how to design landing gears for small airplanes. Sadly, Pazmany never completed his second volume on retractable landing gears.

 

[Nicknick]

@riggerrob thanks for your detailed answer1
The point is, there is a give engine design which has the generator driv shaft in the front. I don't want the shaft sealing to stand under the oil level if the plane is parked (potential leakage). The max. oil filling is thus limited by this shaft sealing, which should't be a problem if the engine is normally level. If there are planes out there which require the propshaft to be inclined downwards, the oil pan would have to be adjusted...

 

[Zoo Tycoon]

With the tricycle light e-aeroplane design I was involved in, we had the thrust line horizontal to the fuselage horizontal datum. The trick is to align this with the drag centre so as to minimise any moments which give power induced pitch and yaw effects.

As for propeller tip clearance this is always a bit alarming when you think about it as these always look marginal. However when I checked the minimum tip clearance (flat oleo/tyre) for the plane I normally fly and settled on that being a good figure, something like 15mm.

I understand the thinking of propeller clearance but I’d prefer neutral power balance moments, vs adding a few more mm of propeller tip clearance.

On the tail-draggers I’m familiar with, for oil there’s only one mark on the dip stick. I never flown one with an oil tank contents gauge (a bit too up posh for me) but just like the fuel gauge, I would expect a dual scale, one on ground and one in flight.

 

[riggerrob]

The finer points of marking oil dip sticks depend upon whether the dip stick is located in the front, middle or rear of the oil pan. In the middle, you should not need separate dip sticks for nose-wheel versus tail-dragger.
It also matters whether your oil pick-up is at the front, middle or rear of the oil pan (see Nicknick's comment).

For comparison, early, straight-tailed Cessna 182s have fuel pick-ups in the rear of the wing tanks. Since the tanks are shallow and wrinkles can increase the amount of unuseable fuel, it is possible to u-port the fuel pick-up in a shallow descent ... even when fuel gauges say that you still have a half hour of fuel on board. More than one Cessna 182 jump plane has been forced to land off-field because of fuel starvation.