Can anybody explain why it was designed this way? I am not expert but was it possible to make it a little bit heavier but able to cope with any torque this engine can produce? Now seems it to be a big money maker for manufacturer.
Because to develop sufficient power at high altitudes, they develop significantly more power at sea level – it is just not practical to beef everything up to cope with this.
What I don’t understand, however, why there isn’t a simple limiting system that limits to the day-to-day operational limits
What I don’t understand, however, why there isn’t a simple limiting system that limits to the day-to-day operational limits
The explanation may be tied up with P&W’s refusal to develop a FADEC system for the PT6.
Also, there there a really direct torque measurement on the PT6? How would you implement that? I know how it can be done industrially (there are contactless torque transducers) but I used to be hangared in a TBM/KA maintenance shop for 10 years and never saw any such device.
Because to develop sufficient power at high altitudes, they develop significantly more power at sea level – it is just not practical to beef everything up to cope with this.
But this is true for any combustion engine? Does it mean that Cessna 150 is beefed up? It could be designed that engine brakes the mounts if it’s full throttled on the ground? and full throttle only available let say at 3000FT?
What I don’t understand, however, why there isn’t a simple limiting system that limits to the day-to-day operational limits
I am also very surprised with that! When I did my first takeoff with Turbo Seminole overboost lamps came on. But any car turbo engine has simple device – wastegate that solves this without any user involvement.
Car manufactures could have gone this way also. It’s -20c outside you floor your brand new BMW and all drive shaft is destroyed… But probably were afraid of to much negative feedback. But as airplanes are not so widely used, there is almost no competition such failures are big additional income…
There’s a mention of TBM900 torque limiter in this article .
TBM900 has automatic torque limiting, so it offers “set and forget” power management for takeoff and initial climb. The upgrade is most appreciated by pilots because precise high-power setting of a PT6A can be challenging of ram recovery with speed change and use of the inertia separator.
I have seen one pilot using it. Full throttle on takeoff – no worries. Several other were very careful not to exceed 100%.
Peter wrote:
The explanation may be tied up with P&W’s refusal to develop a FADEC system for the PT6.
They have implemented a FADEC on the PT6C
Turbavykas wrote:
But this is true for any combustion engine? Does it mean that Cessna 150 is beefed up? It could be designed that engine brakes the mounts if it’s full throttled on the ground? and full throttle only available let say at 3000FT?
Normally aspirated piston aircraft are designed to cruise at lower altitudes, you can get 75% power to up to around 8,000ft, so you can design for 100% of the maximum output without much of a weight penalty.
An aircraft flying at FL300 only has 30% of the ambient pressure, so to achieve 75% power at that altitude it needs to be boosted by a factor of 2.5, and to achieve 90% by a factor of 3 – it is just not sensible to “beef everything up” by that factor to accommodate that level of power, which in any case would be excessive.
Cobalt wrote:
Normally aspirated piston aircraft are designed to cruise at lower altitudes, you can get 75% power to up to around 8,000ft, so you can design for 100% of the maximum output without much of a weight penalty.
You also want the take-off performance.
Turbavykas wrote:
I am not expert but was it possible to make it a little bit heavier but able to cope with any torque this engine can produce?
What do you think would happen if you could get full 1800 something horsepower to the prop and applied it willy-nilly? You can encounter this in warbirds, but I don’t think it’s certifiable. Obviously, they could have given you more wiggle room.
One factor is that a certified aircraft needs to have enough rudder/aileron authority at Vs to handle the maximum torque.
Obviously this is one of many reasons why e.g. a Spitfire could never get certified – along with some homebuilts – but it does depend on what Vs you would publish. And there you get another problem: a single engine aircraft must have a Vs of 60kt or lower. Exceptions have been made to raise this to about 65kt where extra crashworthiness was demonstrated (examples are TBM700 C2, PC12).
