TAS change With Altitude - The "Fastest Altitude" for a Turboprop
Hi all,
I DID read the old threads and hopefully this will add to the discussion! By way of introduction I am a Herc driver in Canada. This whole discussion originated with the following statement found in a manual discussing flying into a headwind: "Under certain conditions it will be advantageous to descend to lower levels (descending increases both TAS and Fuel Flow.) Increasing TAS and decreasing headwind results in an improved SGR (Specific Ground Range.)" So, thus began the investigation... I have read in various threads that, relative to given IAS, TAS roughly increases 2% per 1000' of altitude (this Rule of Thumb is even on Wikipedia.) So we can say that at 10,000' for an IAS / CAS / EAS of 100kts, the TAS is (R.O.T.) 120 kts (10*2=20) (A calculator shows it's actually a few knots less. i.e. 116kts.) This deals with the relationship between indicated and true airspeed. As was stated by smarter people, this is all because there is less pitot pressure force (due to less dense air) on the aneroid capsule moving the dial at higher altitudes = relatively less IAS than TAS as you climb. All that, however, doesn't speak at all to how fast the plane can or will fly. The next threads I read pointed out that since the air is less dense "up high" there is less profile drag. So, for the same reason the IAS is less, the drag is less. This is good - it allows us to go faster for less profile drag. I also seem to remember from somewhere that turboprops start to encounter compressibility effects on their props, and this limits how fast the props can pull the plane. However, here is where I started to get lost… My question became: What is the "fastest altitude" for a turboprop - regardless of fuel consumption? What altitude offers the maximum TAS for fuel consumption? Or, what altitude can I descend to in order to get the maximum SGR / SAR (with nil winds or headwinds)? For the adventurous, feel free to answer for pistons (turbo vs non-turbo) and jet. Here is some data from a turboprop AFM to help us out: Vd (maximum speed (IAS) from chart; TAS calculated using calculator) SL - 318 KIAS / 318 KTAS 5k' - 320 I / 342 T 10k' - 322 I / 370 T 14k' - 324 I / 394 T 20k' - 293 I / 391 T 25k' - 266 I / 385 T Cruise Charts - Fastest TAS; Heaviest Weight SL - 292 KTAS 5k' - 298 KTAS 10k' - 302.5 KTAS 15k' - 302 KTAS 20k' - 293 KTAS 25k' - Drops off dramatically ("hanging off the props") Fuel Flow: We know it increases linearly from high altitude to low altitude for a given ("max") power setting (in this case "max" turbine temperature). We all fly as high as we can because we get basically the same speed with much less fuel burn. (Read Checkboard's amazing post to grasp it totally. I am in awe.) Ok so in a sense we have our answer. The fastest the (heavy) turboprop will go in level flight (TAS or GS with no winds or headwinds) is at about 10,000' to 15,000'. As it turns out a light airplane preferred 22,000 to 27,000'. Interestingly, the dive speed maxed out at 14,000ft as well, at 394 KTAS. So I guess I can say that:
So TAS does not really increase with altitude (it just bulges slightly in the middle.)! (However TAS DOES increase relative to a given IAS with altitude. The achieveable IAS is related to the achieveable TAS, which as I just said is relatively constant.) In terms of getting there, nobody cares about IAS anyway. Whew. I am not an engineer. My brain should be starting to hurt slightly. Mods- did I earn a sticky post?!? :) Related Posts: http://www.pprune.org/tech-log/32881...air-speed.html http://www.pprune.org/tech-log/10655...-altitude.html http://www.pprune.org/tech-log/46529...t-benefit.html -TNTDJ |
you can read out the highest TAS in your performance tables. on flat rated engines is mostly occours in the region of the highest altitude where the engine is able to maintain max torque and /or IAS redline is not a limitong factor. - usually in the mid tenths for turboprops.
flights are conducted usually higher than that because fuel flow in high altitudes drops more significantly than the TAS does so the overall efficency is higher. the same applies for tubocharged pistons. cheers |
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