Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Horizontal velocity constraint in Engineering applications

  1. Jun 1, 2013 #1
    Case 1:
    An airplane should not exceed upper limit horizontal velocity while on takeoff roll(or even during taxiing on certain roads).V-Rotation(VR)is an indication of the maximum horizontal velocity,a nose-wheel can handle.Some people claim that nose-wheel has an upper limit to normal reaction it can handle.But,the question is, how does constraining the horizontal velocity set up a constraint on the normal reaction by the road on the nose-wheel.(Assuming the conditions of the runway to be not "Ideal" i.e. with "minor" issues).Does it have to do with the classical physics of turbulence during takeoff roll.

    Case 2:
    I have also noticed a similar constraint on the horizontal velocity for vehicles travelling on army
    field-bridges.ex:a tank should not exceed a particular speed when travelling on a bridge setup by a bridging vehicle.

    Explain the reasons for these constraints in a rigorous manner(with equations if possible)
     
  2. jcsd
  3. Jun 1, 2013 #2
    Case 1:
    It's not very clear what exactly you're trying to say. First of all, in aeronautical engineering when you say "roll" the mind automatically thinks of the rotation of the airplane around the longitudinal axis. So it's not "take-off roll", it's just "take-off".
    Secondly, the take-off speed depends on the take-off weight. Anyhow, as the aircraft accelerates, lift is generated so the weight the wheels support gradually decreases until the airplane is airborne when the wheels carry only their own weight.

    Case 2:
    the speed limit on bridges is enforced to maintain the vibrations to a certain level so that their frequency does not match the bridges eigen-frequency and avoid resonance which would eventually destroy the bridge. For the same reason, soldiers do not march over bridges but rather they walk freely.
     
  4. Jun 1, 2013 #3
    Rolling takeoff perhaps.

    In addition, the wheels could be rated for a maximum speed, just the same way your car tires are, especially the donut spare tire at a maximum 50mph, so that they do not experience excessive heat build up from flexure and stresses from rotation, leading to a failure of the tire.
     
  5. Jun 1, 2013 #4
    Not quite so. In airplanes, you can't place a restriction on the take off speed because of the tyres. You have the maximum take-off weight and you compute the speed required for take-off and then you make tyres that are able to cope with that. Not the opposite.
     
  6. Jun 2, 2013 #5
    Answering the question

    First of all ,i'm sure the acceleration to takeoff speed on the runway is called take-off roll
    second of all the question has not been answered for at least case 1.The popular idea is that the nosewheel should not taxi at above certain speeds because of its fragile nature.how can that be?

    The second case focuses on the speed of the crosser rather than the type of marching of the crosser which induces resonance.Resonance is attributed to the marching of the soldiers and not their speed.that is why to avoid bridging accidents the soldiers(British)were told to break step.
     
  7. Jun 2, 2013 #6
    the question is about the connection between the horizontal velocity and the normal reaction under non-ideal conditions if any?
     
  8. Jun 2, 2013 #7
    You can be sure of anything you want. That doesn't mean you're right too.
    In none of the 5 year degree in aeronautical engineering that I followed I heard about take-off roll :P Anyway, that's not the point.
    The point is that you cannot impose restrictions on speed because of the wheels. I explained that the take-off speed is certain and you make the wheels based on those loads. The problem is at landing, not at take-off. At landing the nose wheel should touch down smoothly and not crash on ground.

    As for the second case, every vehicle moving induces vibrations. I'm not an expert on vibrations but I believe their strength and frequency depend on the speed of the vehicle.
     
  9. Jun 3, 2013 #8
    For takeoff-roll
    https://www.google.co.in/search?q=t...e.0.57j0j62l3.2681j0&sourceid=chrome&ie=UTF-8

    there are several pages which use the word "take-off" roll.( i do not mean the rolling takeoff by take-off roll)
    "but anyway that's not the point"

    My query is about the horizontal speed above which the nose-wheel is not advised to travel on the ground not the vertical speed you are talking about.In that case it is obvious that the aircraft has to be flared to land at a convenient vertical speed.
    The question is about the relationship between horizontal speed and normal forces on the nosewheel,it's a classical physics problem.
     
  10. Jun 3, 2013 #9

    sophiecentaur

    User Avatar
    Science Advisor
    Gold Member

    There is more to consider than the simple "normal reaction" force. Runways are not perfectly smooth and there will also be objects (litter) to contend with. These factors impose a horizontal stress on the undercarriage, which will be made as light as possible (consistent with these stresses) and also the wheels are fairly small diameter. I heard, 'somewhere' that the undercarriages on Russian military aircraft are (or were) built more robustly to cope with much worse runway clutter than Western designs because the runways are inspected with less rigour, prior to take-off. Is that just apocryphal? It could be.

    I think there is likely to be more damage to tyres during landing (vis the smoke when the wheels touch down) than overheating over a few hundred metres of straight rolling.

    I think the issue of speed limits on temporary bridges is more to do with the possibilty of setting up waves along the structure of the bridge. That is certainly a factor which governs the amount of wear on tarmac roads with heavy vehicles travelling at high speed.
     
  11. Jun 3, 2013 #10
    I'll try one last time to make you understand. After this, I give up.
    You have a A380-800 with max. take-off weight of 560000kg. In order for this aircraft to take-off when it's fully loaded, it needs to develop some certain speed say x m/s (SI units). Now, when the aeronautical engineers were designing the aircraft, they calculated that speed. With the take-off speed known (among other things such us the weight etc), the structural engineers, designed the wheels in such a way that they can cope with all the forces that act on them. Moreover, there's something called "safety factor" which comes to save the day if something goes wrong...
    So, in answer to your question, the maximum horizontal speed that the nose wheel can withstand is the maximum take-off speed.
     
  12. Jun 3, 2013 #11

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    I see what you are suggesting.. taxi too fast on rough ground and I suppose you could cause damage but I've never seen VR referred to in that context. Normally...

    http://en.wikipedia.org/wiki/Takeoff

    http://www.airbus.com/fileadmin/media_gallery/files/safety_library_items/AirbusSafetyLib_-FLT_OPS-TOFF_DEP_SEQ07.pdf [Broken]

    So VR would only give a very rough indication of the max speed a wheel needs to be designed for. Howeve since VR is only normally achieved on a nice smooth runway I'm pretty sure the vertical forces are likely will be higher under other conditions such as landing.
     
    Last edited by a moderator: May 6, 2017
  13. Jun 3, 2013 #12

    CWatters

    User Avatar
    Science Advisor
    Homework Helper

    Putting asside the examples you quote...

    Have a look on this forum for the post a few weeks ago about the forces involved when driving a car over a "sleeping policeman" aka speed bump. They were surprisingly high.
     
  14. Jun 3, 2013 #13

    cjl

    User Avatar

    The normal force on the landing gear of an aircraft will decrease slightly throughout takeoff, as the wings will begin to generate lift. This will be relatively minimal relative to the weight of the aircraft, but probably non-negligible (for most aircraft at least - high performance military and aerobatic aircraft may generate no lift at all during takeoff due to the symmetric airfoil used on many such aircraft). The normal reaction is almost definitely not the limiting factor however - the normal force on the wheel during landing is much higher than it is during takeoff, despite the fact that landing usually occurs at a slower speed than takeoff. Rather, the limiting factor will probably be the tire's design speed, and this is usually determined by the aircraft requirements (the tire will be designed to allow the aircraft to reach its full potential, rather than being the limiting factor). There are a few interesting cases where tire speed actually could be a limiting factor, but for the vast majority of aircraft, this will not be the case.

    As for your rather strange definition of VR, it has nothing to do with what the nose gear can handle. Rather, VR is determined such that (based on a standard takeoff and rotation profile) the aircraft will be lifting off the runway at the correct speed for good climb performance as well as some margin in case of engine failure (for multi engine aircraft), as Cwatters noted above.


    Oh, and it isn't terribly relevant, but for what it's worth, takeoff roll is a relatively standard phrase, and I did hear it multiple times during my education (I have a masters in aerospace).
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Horizontal velocity constraint in Engineering applications
Loading...