posted by dynamite dean
1.I have always been lead to believe that flying for endurance initially you would think highe but I was told at lower altitudes because the wings can produce lift easier..HBJ states that you go high I'm confused?
For a jet, higher is in general better.
Classically, jet thrust is just a function of EAS, and so is drag. So a plane which can hold 300kts EAS at 10,000ft can also cruise at 300kts EAS at 35,000ft, say. (Ignoring Mach effects for now).
But at 35,000ft your TAS will be more like 600kts, while at 10,000ft it might be only 400kts.
I'm making these numbers up, since I can't be bothered calculating the exact values.
The relationships are all you need to understand the general point So the aircraft at 35,000ft is actually travelling over the ground (assuming no wind) 50% faster than the one at 10,000ft. Hence higher is better.
It gets more complex when you worry about winds, and Mach limits, and time taken/wasted in climbing.
And prop engines don't come close to the classic jet thrust independent of altitude assumption, so they do tend to be better off staying lower.
2.EAS is something I don't use in my flying but Vs EAS rises slightly as you climb for example, in what use of EAS does it become significant to talk about it in flying I use IAS or TAS but I have to say my knowledge in terms of talking about EAS in Flying is very limited ! - HELP
OK, there are 4 (5?) airspeeds which are useful to different people in different ways:
True Air Speed (TAS) : this is your actual speed relative to the local air mass.
Equivalent Air Speed (EAS) : this is the speed which would, at sea level under standard conditions, give the same kinetic pressure (1/2-rho-v-squared) as you are currently experiencing. It's useful for any time you are worrying about e.g. airframe loads, because for the same aerodynamic coefficients the forces will be the same at the same EAS. It gets
significantly lower than TAS at higher altitudes.
Calibrated Air Speed (CAS) : this is the speed which an ideal airspeed indicating system would read for the current conditions. It is similar to EAS, in that it drops to lower values than TAS at altitude, for similar reasons. But there is a factor, called the "scale altitude correction" which is the difference between EAS and CAS.
Indicated Air Speed (IAS) : this is the speed actually indicated to the pilot, and differs from the theoretical CAS due to inadequacy in the design of all airspeed sensing systems. These include position errors (PEs) due to the pitot/static system not being perfect, plus computing lags and equipment errors.
The possible "fifth" airspeed I have seen used is ASI - "Air Speed Indicated". This is sometimes used when people wish to distinguish between PEs and other errors - in that case IAS is CAS plus PE effects, and ASI is IAS plus equipment errors.
When flying the only speeds that should matter to the pilot are those used in the appropriate aircraft manuals. If they are quoted in IAS (as they should be) then all the TAS/EAS/CAS stuff is irrelevant - any corrections will have been made in producing the manuals, and if you respect the IAS limits then anything which is "really" an EAS limit (loads, perhaps) will be taken care of automatically.
3.Why does a jet engine produce more thrust at higher rpm than lower RPM and a Piston doesn't say?
4.Why are those bloody performance figures for pistons only ever quoted in 65% and 75%values only? - only I have never deared to ask anyone for fear of someone saying you twit!
Have to let an engines guy deal with those
5.HBJ talks a lot about Vdf /Mdf I thought the fastest you could go in jet terms (remember I am still a piston pilot?!) was Vmo/Mmo so what is going on I talk Vne HBJ talks Mmo and you could goto Mdf if you want!!!!! aggghhh!!! I'm getting confused.
Ok, time for a list of speeds this time
Vmo/Mmo: Maximum Operating speed/mach number. Not to be exceeded intentionally.
Vfc/Mfc: the maximum speed/Mach to which FAR25 handling requirements must be met
Vdf/Mdf: the maximum flight demonstrated speed/mach number
Vd/Md: maximum design speed/mach number
Vfc/Mfc lies between Vmo/Mmo and Vdf/Mdf. It's a bit complex in practice, if you think "about half way between" you'll not go far wrong.
Vdf/Mdf MUST be less than Vd/Md.
Vmo/Mmo MUST be defined such that the aircraft will never exceed Vdf/Mdf for a variety of defined "upset" cases.
The statement "you can go to Vdf" should be treated with
extreme caution. You shouldn't be anywhere near that speed under any kind of normal conditions. It's a speed you go to in flight test to make sure the aircraft will not fall apart at that speed, but there may be no margin for error. If you intentionally flew there and any of several things happened (gust, control runaway, autopilot failure (if it could be engaged that is)) you could easily end up past Vdf, in a region never explored in test. That is not a good thing.
Typically I wuld expect max practical cruise speed to be a little under Mmo/Vmo. That way you won't get all kinds of warnings every time there's a slight disturbance.
6.I think this is an extension of Q5 but at high Mach can you by deflecting a control surface too much put the wing through the sound barrier is this what the great D.P Davies means when he says beware mach buffet at high Mach No. etc...
You can alter the flow on the wing by deflecting the control surfaces. That can alter the shock pattern, and that can cause bad things to happen - including near-total loss of aileron effectiveness, for example. Trailing edge controls are particularly susceptible to this effect.
I don't think that's what he referring to though - you can get Mach-related buffeting without control deflections.
edited for style