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On Fri, 7 Nov 2008 23:55:23 -0600, "Highflyer" wrote:
"Alan Baker" wrote in message ... In article , Stealth Pilot wrote: aircraft brakes were designed for use in holding the aircraft still while the engine was started. after the taxy out and the engine has warmed you do a run up check to make sure that the magneto circuits are up to the bit of work that lies ahead for them. the brakes are applied to hold the aircraft while the revs are bought up and each maggy checked in turn. from a design aspect that is the end of the use of a light aircraft's brakes until after landing and we wish to hold the aircraft still for shutdown and disembarkation. of course brakes are brakes and people will use them like they were driving cars. light aircraft brakes were never designed for slowing an aircraft when landing. I know that they get used for that by students of bad piloting technique but the design intent is a fact borne out by their diminutive size. Stealth Pilot Nonsense: complete and utter. What aircraft brakes aren't designed for is stopping aircraft *repeatedly*. The chief advantage of putting larger brakes on any vehicle is that it providess a greater heat sink to allow for more braking before the brakes overheat. Aircraft brakes need to be able to stop an aircraft *once* and then have an essentially infinite amount of time to cool down again. -- Alan Baker Vancouver, British Columbia http://gallery.me.com/alangbaker/100008/DSCF0162/web.jpg Right. Almost. Maybe..... My Stinson has 9 inch diameter drum brakes and has lots of area on the brakes, but low pressure applied. It uses a largish master cylinder to pump fluid into an "expander tube" under the brake shoes inside the drum. They work great for holding the airplane for runup and are essential for ground handling and taxiing because the tailwheel is a swivel and in not connected to anything that could allow it to be used for steering. As a result, all steering is by differential braking. You can apply the brakes on the landing roll and it will quickly slow the airplane. However, if you do so, you may not be able to leave the runway. Rubbing all that shoe area inside the drum makes it quite hot inside the drum. My little infrared laser guided remote reading thermometer gives temperatures in the 400 to 600 degree range. It takes a while to dissipate that heat from inside the drum and until it does the brake fluid inside the "expander tube" gets quite warm. When it does it expands and the brakes tend to remain quite "ON" until things cool down. Even a long and complicated taxi, like into a parking spot at Oshkosh, will generally result in a noticeable loss of "differential" in the braking activity and a substantial increase in the power required to taxi. I have found that it is wise to NOT attempt a takeoff if it requires over 1000 RPM to maintain a reasonable taxi speed. :-) At about 2300 or so RPM the Deb will happily accelerate with the brakes locked and the tires skidding. Once it starts to skid it feels just like it hit ice as the rubber begins to melt/vaporize. Of course, this airplane weighs generally two tons and lands at 70 mph at touchdown in a three point attitude. :-) Man, but that's fast!. :-)) The old Deb comes down final at 80 MPH minus 1 MPH for each 100 under gross. With me and half fuel, final is about 73 MPH. touchdown? Gotta be in the 40 MPH range. Those big flaps slow it quickly. Course it only weighs a ton and a half so you're hauling the equivalent of the entire useful load of the Deb on top of the Deb's gross.:-)) The general limiting factor in ALL aircraft brakes is heat dissipation. The wheels are relatively small and the brakes are in a small space. The more effective the brakes are, the more heat they produce. All that energy they are dissipating when they slow you down has to go somewhere. Thermodynamics tells us that most wasted energy appears as heat! Randomized molecular activity. :-) To stop the airplane you have to waste the energy. 1/2 M V^2. You can't get around it. Those big Cleveland's will fade if they get a lot of use, but it's rare to ever use them that much. Even after a short field, with maximum effort they still work. You can lock them up, but after that the taxi gets kinda bumpy. Steep final into the round out, firm touch down on the mains, immediately let the nose down, get on the brakes, and go full up elevator if the brakes will keep the nose gear down. The aerodynamic braking with that big wing and tail is very effective giving it the ability to stop shorter than many 172s (book figures) I'm certainly not that proficient at present. Fortunately, a taildragger with the flaps down and the tail on the ground takes a LOT of energy to keep moving, so you can waste a lot of the energy you have to get rid of by stirring up the air. Then apply the brakes to turn off the runway after you have slowed down without them. FWIW Department. You scrape a lot more rubber off you tires by landing and applying brakes vigorously while little weight is on the wheels than you would in many many miles of taxiing around or rolling out with the brakes off! :-) Highflyer Highflight Aviation Services Pinckneyville Airport (PJY) Roger (K8RI) ARRL Life Member N833R (World's oldest Debonair) www.rogerhalstead.com |
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