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FlyingForFun
21st Aug 2002, 10:12
Could someone please tell me what will happen in the following scenario: taildragger, during the take-off run on a grass runway, has its tail raised and is close to flying speed, when it hits a bump. Will it pitch nose-up or nose-down?

My own thought is that it will pitch down. The centre of gravity is behind and above the main wheels, and all pivoting will take place around the main wheels. The momentum of the aircraft will cause the centre of gravity to continue to move forwards, and thus rotate around the main wheels, causing the nose to dip.

However, I've had an e-mail from someone suggesting the opposite - that the aircraft will pitch up. This comes from someone who knows what he's talking about - a very exerienced pilot, instructor and test-pilot who regularly flies everything from Tiger Moths to 777s. Although he said the aircraft would pitch up, he didn't explain why.

I will be meeting this person in a couple of weeks, and I'll ask him to explain why he thinks the aircraft will pitch up. Unless, of course, someone else can explain it to me before then!

Cheers,

FFF
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john_tullamarine
21st Aug 2002, 11:13
Your colleague may be considering the ramp characteristics of the said bump, the relative value of the horizontal and vertical impulse loads experienced by the aircraft as it transits the bump, the lift distribution and pitching moment characteristics of the aircraft at the point of transit, aircraft centre of mass, spring characteristics of wheels and undercarriage assemblies ... as these will influence the net moment generated about the mainwheels - only part of which is due to the bump itself.

Is the aircraft pitching prior to the transit due to pilot control inputs ? if the aircraft is a typical highlift wing taildragger, then maybe there isn't much load on the wheel assemblies at the point in the takeoff roll which you are considering ?

.. the list goes on ... and, like most such questions, the devil is in the detail ...

Final 3 Greens
21st Aug 2002, 12:14
This may be b@lls because I'm not technically minded, but as a supplemntary factor would the removal of ground induced friction from the landing gear cause cause acceleration and a momentary pitch up before trimmed airspeed was regained?

kabz
21st Aug 2002, 14:35
It'll pitch up ... assuming it's the wheels that hit the bump ...

There's two way to land these things ...

1) full stall, stick back, in which case the 'bump' of landing and any more bumps during roll out can't make it pitch up any more, or at least can't give it enough pitch up to fly ... most of my landings have a 'settling' time with the stick full back ... at least when I'm out of practice ...

2) wheel landing, in which case, the 'bump' of landing had better not really be a bump, and any reaction from the ground must be countered by a forward movement of the stick to counter any tendency to bounce and start flying again ...

Hope that helped ... ;)

FlyingForFun
21st Aug 2002, 15:39
Thanks guys.

John, thanks, it's obviously much more complicated than I thought it was! Confused by a lot the points you make, but it certainly gives me things to think about, and I don't know the answer to some of the questions you ask (and neither does the other guy). I hadn't considered the aerodynamic effects, and they will probably make a considerable difference to my over-simplified explaination.

F3G, why is ground induced friction being removed from the landing gear? If anything, I'd have thought there'd be more friction - the bump will apply a force to the bottom of the wheel, which would add an additional load to the bearings, etc, until such time as the body of the aircraft moves up by the same amount as the wheels have moved

Kabz, I'm talking about taking off, not landing! Although the situation towards the end of the take-off roll is very similar to the start of the roll-out after a wheel-landing, so there's certainly some cross-over. You said "any reaction from the ground must be countered by a forward movement of the stick..." If you make the assumption that the aircraft would pitch up without this forward movement of the stick, I agree - what I'm interested in is why the aircraft pitches up without pilot intervention.


Any more thoughts from anyone?

FFF
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john_tullamarine
22nd Aug 2002, 01:26
... apologies if I confused the issues .... don't worry too much about the points I raised .. the only intent was to suggest that it is not just a matter of considering the bump as a garden variety sort of bump in the runway of life ...

The main consideration relates to the overall pitching moment to which the aircraft is subject... if this is nose up then the pilot will need to correct with a touch of forward stick .. and the reverse if it is nose down. ... but the situation will, quite likely, vary among aircraft and specific circumstances ... while I admire the confidence shown by our colleagues who have put forth definite statements .. I feel that such statements have presumed assumptions underpinning them ...

If the aircraft is firmly on the wheels, then we are interested in the moments about the wheels ... if in flight (or with negligible load on the wheels) then the cg becomes very relevant.

If you reflect back to your PPL studies in PoF or weight control, draw a piccy of an aircraft in the circumstances you describe and then think about ALL the forces which might be acting on the aircraft and WHERE they are acting ... this permits you to figure some moments (the old force X arm trick) and the sense (nose up or down) of the moments ... figure the subtotals of nose up and nose down .. and the bit left over gives you an indication of what the aircraft is likely to do statically .... the subsequent dynamics of the situation get a bit more complicated, though ....

... and if you are in the typical leaf spring undercarriage Cessna .. then, unless the bump be of prodigious size .. you are not really going to notice it at all .... the wheels just keep on merrily bouncing up and down ....

The matter of undercarriage characteristics is very important ... most people have no idea just how involved, for example, something as apparently simple as altering a tailwheel assembly might be when it comes to the engineering assessment of loads and load paths in the back of the bird ...

At the end of the day .. if the aircraft is reasonably conventional in its characteristics ... the typical bump isn't going to present a problem ... the pilot will make whatever correction might be necessary typically without even thinking about it ...

If your experience has been limited to nice smooth runways, then I can appreciate your concern .... but do have a talk with some of your paddock operating brethren ..... at times the wheels can become quite a blur during the ground roll ...

Intruder
22nd Aug 2002, 02:06
While the main wheels are on the ground, any force from the bump will be transmitted through them to the airframe. Any such force will act as a torque around the center of gravity (CG).

The initial impulse will come from forward of the CG AND forward of the center of lift, so the forward part of the airplane will be "bumped" upward initially. If we just consider the vertical component of that force, inertia will tend to cause the center of gravity to continue in its previous direction (no vertical movement), causing a rotation around the CG -- a "pitch up" as the tail goes down to compensate for the rising nose.

OTOH, there is also a horizontal drag force acting on the bottom of the wheel, trying to "trip" the airplane and pitch it nose down. If the ramp of the bump is shallow enough, the horizontal force/moment will be relatively small, and the vertical force will predominate. If the drag force is large, though (e.g., running into a lip/ledge just less than half the height of the tire), it may predominate and cause the airplane to nose over.

Another consideration would be how "tall" the airplane is in relation to its length. The vertical component of the lever arm between a tall landing gear strut and the CG could be substantial, exacerbating the "tripping" of nose-down tendency. OTOH, in a low-slung airplane with an aft CG, the horizontal lever arm would predominate, emphasizing the pitch-up tendency.

This analysis takes no balance weights, pilot reaction, or reflex into account.