meritflyer
Well-Known Member
I had a lengthy discussion about what "zero thrust" meant on my initial.
How do ya'll define it?
How do ya'll define it?
zero thrust in a twin
- Thread starter DE727UPS
- Start date
ananoman
New Member
Did you mean Vmca?
greaper007
Well-Known Member
What am I getting wrong here. Indicated airspeed is supposed to be the same regardless of the altitude. It's just the difference between static and dynamic pressure. Only true airspeed changes with altitude. Or is this different with jets.
ananoman
New Member
Zero thrust is supposed to simulate a feathered engine. A feathered prop creates minimal drag at slower speeds, so 'zero thrust' is exactly that. Making the simulated failed engine run just fast enough that it creates no net thrust or drag.
ananoman
New Member
Well, there are several reasons that IAS cannot remain constant in jets (or any other aircraft for that matter). One is that available thrust decreases with altitude. If your engine can produce 22000lbs of thrust at sea level, you might only have 5000lbs by the time you get to FL400. The other problem is Vmo (max permissible IAS) usually decreases with altitude. For example in my airplane Vmo is 335KIAS up to 12000 ft and decreases to 310 KIAS at 29000 ft. Once you get into the high twenties, Mach effects start to come into play and will further decrease your max IAS. For example when I am at FL360-410 and cruising at M.75 my IAS is probably around 220 (TAS is around 440). If I decided to burn more fuel and go for redline at M.80, then my IAS would still be only 230 or 240. So, if you want the highest IAS readings, stay lower, burn way more gas and enjoy that wind noise!What am I getting wrong here. Indicated airspeed is supposed to be the same regardless of the altitude. It's just the difference between static and dynamic pressure. Only true airspeed changes with altitude. Or is this different with jets.
greaper007
Well-Known Member
That was a quick response. Thanks
Flysher
Well-Known Member
on a related topic, what exactly causes the coffin corner?
and so that i make sure i dont look like a complete idiot, coffin corners when you have a narrow window between stall and VMO, correct?
I assume that the one side is due to stall TAS increasing with altitude, but why does VMO decrease with altitude?
I need a good book on high altitude aerodynamics.:insane:
and so that i make sure i dont look like a complete idiot, coffin corners when you have a narrow window between stall and VMO, correct?
I assume that the one side is due to stall TAS increasing with altitude, but why does VMO decrease with altitude?
I need a good book on high altitude aerodynamics.:insane:
seagull
Well-Known Member
I sure did, thanks! Typing when I'm too tired. Actually, I'm tired now,but got woken up by something else, so figured I'd check my email.
ananoman
New Member
You are correct that the low speed side is due to stall TAS increasing with altitude.
The high speed side decreases somewhat due to the speed of sound decreasing with altitude. The speed of sound is solely dependent on temperature, which decreases from 15C at sea level to a balmy -55C in the stratosphere.
For most airplanes, coffin corner is not really that narrow. Especially the more modern airplanes. The classic example is the U-2, since it can fly at such high altitudes and is not designed for flying at high mach numbers.
ananoman
New Member
Yes, but in relation to Mmo. If your aircraft has a Mmo of M.80, then the TAS where this will occur decreases with a decrease in temperature.
WHOA!!!
I think you are confusing the knowledge seekers out there.
IAS...DOES NOT CHANGE WITH ALTITUDE/TEMP/ANYTHING!!! Indicated Airspeed limits, profiles, etc... DO NOT CHANGE!
If you are flying an airplane that requires 60kts to rotate...it will be 60kts anywhere. Even if you can magically put it at an airport at 40000ft...you still better rotate at 60kts!!
Indicated Airspeed is (in layman’s terms) what the airplane "feels". The wings will need to "feel" the same pressures to create the same amount of lift anywhere.
Now what ananoman is trying to say is that at high altitudes (top of troposphere and BEYOND ! ) A given IAS will warrant a certain Mach number based on the temperature the aircraft is flying around in. Way up high it is WAY cold!! As a result an aircraft will reach its maximum Mach number (Mmo) at a slower IAS. Remember Mach number is only a relationship with Temperature.
BUT!! In NO WAY does this affect your Indicated Airspeed or what your wings are "feeling". The Mach number limits at high altitudes come from transonic speed induced phenomena... i.e., shock waves, aileron flutter/buzz, etc... If you could take a 2000lbs C172 up to 60000ft and fly straight and level, you would still stall the airplane at the same critical AOA as at sea level, the airspeed INDICATOR would still read 46kts (or whatever a 172 stalls at, its been awhile for me
), and the control forces would feel exactly the same. Just because your C172 is going Mach .70 and has a True Airspeed of something like 250kts and a Ground Speed of 350 kts (nice 100kt tailwind) doesn't have anything to do with what the wings need to "feel" to keep flying.IAS limits and stuff are constant!! Don't take your Cessna 310 that you have been rotating off a sea level runway at 85kts up to Leadville, CO and try some crazy monkey math and think because of air density and what not that you should rotate at any other speed; because the airplane needs to "feel" 85kts of dynamic force in order to get off the ground. How do you know you are at 85kts IAS? Look at your airspeed INDICATOR!
While Vmca, Vmcg, etc, may change due to ENGINE OUTPUT changes due to atmospheric conditions, does not mean that anything to do with AERODYNAMIC FORCES change at all. 100 knots feels the same to an airplane at 100ft or 100000ft.
Clear as mud?
desertdog71
Girthy Member
Twin Comanche is 10" MP and 2200 RPM according to the POH. A feathered prop actually feels more comfortable to me, as opposed to the zero thrust setting.
Well.... Stall "speed" is a mis-nomer (airfoils stall at a critical Angle of Attack). I just tried to expand on the IAS vs. AoA at high altitudes. What do you think stall speeds, Vr, etc., are based off of? Yep, Aircraft Weight and AoA!! So the entire discussion is related. I was just saying that while discussing these topics, altitude/air density has absolutely nothing to do with IAS and AoA.
tgrayson
New Member
Not typically the case. However, you may be at lighter weights at those altitudes and that will increase Vmca. You may also be using smaller flap settings.
seagull
Well-Known Member
Excellent points. In fact, if I recall correctly, the DC-8 has a rudder lock-out at certain point on the flaps. I don't know if any of those apply to the takeoff range (did not to my recollection, but I was never trained for ops above 8000' field elevation).
Dugie8
Well-Known Member
Yup, the DC8 has a rudder limiter for flaps greater than about 8 degrrees, 10 being the min selectable setting, that Vmca is 210 for the long body and 220 for the short body. The discussion we were having was about 2 engine approaches into high elevation airports (bogata).
I may not be recalling this correctly, but I think for transport category aircraft Vmc is determined with wings level (no sideslip) so weight would not have an effect or nearly as much as it would with 5 degrees of bank. However, the reason I was getting from this check airmen, was that with thinner air the rudder is less effective at the same IAS...