zero thrust in a twin
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Dugie8
Well-Known Member
The seminole and the baron had settings in the POH for zero thrust, but we always used a "standard" setting for ease of use.
On a side note, sitting in groundschool today and we were talking about a change to the two engine profiles. Long story short, we had some hard numbers for Vmca put in for the different models and we were told to be careful at the higher altitude airports because Vmca goes up with altitude????
I questioned this, since I don't have access to an AFM, saying Vmca should go down since the engines put out less power (not much less, but less) and the rudder maintains the same authority at any given IAS. The check airmen told me that as you go up and the air gets thinner, the rudder has less authority because the air is thinner.
I buy the air thinner argument, but 165 knots (Vmca for the -70 series) is an IAS number and at any altitude that 165 knots IAS provides the same "airflow" over the rudder, TAS is what goes up and down.
Am I missing something here?????
Flysher
Well-Known Member
VMCA goes down with an increase in altitude because the operative engine doesn't "pull" quite as much due to the reason you stated. I have never thought of the thinner airflow over the rudder affecting VMCA, maybe its so small that its not even mentioned in our manuals.
The only case in which I could think in which VMCA goes up would be in a turbocharged airplane up to its critical altitude... Because technically the engine would develop the same power through a certain range of altitudes, therefor VMCA would remain the same indicated airspeed but the TAS of VMCA would increase?
Im just speculating there and thats not from any book that I have read. Comments/thoughts welcome.
The only case in which I could think in which VMCA goes up would be in a turbocharged airplane up to its critical altitude... Because technically the engine would develop the same power through a certain range of altitudes, therefor VMCA would remain the same indicated airspeed but the TAS of VMCA would increase?
Im just speculating there and thats not from any book that I have read. Comments/thoughts welcome.
Skrob
New Member
"...throttle adjusted to produce the appropriate engine speed shown below..." and yeah Prop RPM dependant on airspeed
i dont believe the throttle was at zero, at least for my training but for the life of me i dont remember the setting. usually my instructor would zero thrust it after he "killed" an engine and i followed up on the appropriate checklist... unless he got lazy and made me do that too.
i dont believe the throttle was at zero, at least for my training but for the life of me i dont remember the setting. usually my instructor would zero thrust it after he "killed" an engine and i followed up on the appropriate checklist... unless he got lazy and made me do that too.
seagull
Well-Known Member
Well, I don't have access to DC-8 numbers, however, it would seem odd to me. I can see some factors at very high altitudes, possibly, due to loss of the damping effect itself, but I need to think about that more. However, I do wonder if the issue has to do with the increase in stall speed as you increase altitude? Still, this is not that significant at any altitude of any airports.
Now, the actual measured "horsepower" of the engine does increase with altitude for a given IAS, but that is more an academic exercise than anything useful, it seems to me.
Foxcow
Well-Known Member
When I pulled an engine on my students, they would automatically put the mixtures, props, and operating full forward while simulating putting the inoperative full forward. I just used the throttle to set the desired zero thrust rpm and adjusted it with changes of airspeed.
Realms09
Well-Known Member
1. In steady flight, lift equals weight.
2. Lift is the product of the lift coefficient and dynamic pressure of the air.
3. Lift coefficient is solely a function of Mach number, Reynolds number, and angle of attack. You can forget about the first two for this application.
4. Indicated airspeed is, omitting a few details, a direct measure of the dynamic pressure the airstream.
5. With the same indicated airspeed, the dynamic pressure will be the same at both altitudes.
6. Assuming the weight is the same at each altitude, the lift coefficient must be the same since dynamic pressure is the same.
7. If the lift coefficient is the same, the angle of attack is the same.
ricecakecm
Well-Known Member
Flysher
Well-Known Member
ahh ok thanks. I got to thinkin about this because yesterday I jumpseated on one of our company lears and it seemed like they held a considerably higher pitch at 41000 versus something in the teens or 20s.
thanks for the explanation
I'm going to guess that you weren't comparing the same indicated air speed at 410 as in the teens or twenties though.
ananoman
New Member
The highest indicated airspeeds are probably encountered in the mid to high twenties. In the 800XP if we are cruising in the mid 20's our IAS is right around 300, but if we are in the high thirties up to FL410 and are cruising at .75, then our IAS is around 220. So yes, you should notice a higher AoA when you are cruising at FL410, but it should not necessarily be 'considerably' higher.
ananoman
New Member
We used 11.5" and 2000 rpm for zero thrust in the Seminole at FSI. The POH listed different settings for various speeds, but I never had a problem with just using the standard setting listed in our training manual.
seagull
Well-Known Member
OK, a little more research, and I'm fairly certain that it is your company that is missing something here. All the equations for V1 that I know of, and I also asked around a bit, and other "techies" were of the same opinion, do not even have altitude as one of the parameters.
This wouldn't be the first time that a company had something messed up. I remember when I was a union tech guy, a certain operator that had just acquired MD-11s was doing some odd stuff. I got a call from their union's tech people in inquire if we were having problems with the fuel system controller moving fuel to the tail tank on the ground -- they said they kept having to defer it to manual. I made some inquiries as to what weights they were operating it, etc., and quickly informed them that the airplane was smarter than they were, because it was moving the fuel to comply with an aircraft structural limitation. I can think of dozens of similar examples from just that one company alone, across all of their fleets. Amazing, sometime, and this is not from a small outfit, by any stretch! Gotten many similar over the years.
seagull
Well-Known Member
Quite true!