Because bureaucracy. I’ve seen it again and again: decisions being made by remote people, then when people on the ground see flaws and and try to report them, they are told ‘it’s above your pay grade’ when they bring them up with supervisors who are scared to fight the decisions made by remote management.
In these environments, people cover their asses by working to exact rule so that when the inevitable failure happens they can’t be blamed.
Over time, this becomes the operating mode - keep your head down, avoid the craziness, make sure you can’t be blamed. That takes hold with project managers and the rest of the staff, and now incredibly stupid mistakes just get carried out by rote instead of anyone pushing back. The space shuttle crashes are an example. Lots of engineers in the program knew of the problems, but just couldn’t get management to listen.
This rot has set in all over the place. Not just governments, but large, old organizations of all sorts. It’s one of the reasons very few corporations survive over many decades. Of the companies that made up the original fortune 500, I think GE is the only one left standing, and it’s a shadow of its former self.
The thing is, when companies become dysfunctional they are eaten by the market and die, and are then replaced by newer, more efficient ones. When governments become dysfunctional they just keep trucking along, becoming more authoritarian if the people don’t like it.
We have an airport in my state that they poured money into for the National Guard. They built a new tower and discovered the light from the beacon was in their eyes. No problem. Stick the beacon on top of the new tower. Nope. Not structurally made to handle it. So they built a tower for a taller beacon. Well you think that would be the end of stupid. Nope. Because of pure politics they moved the Guard unit to another airport which of course got money in the process. But wait, they just spend a ton of money and need to justify the old base. Lets replace the recently refurbished Air National Guard with a ground based unit. If you haven’t guessed it they needed completely different assets to support them. That included more buildings and room for vehicles. So They bought land on the other side of the road that borders the airport. Now we can’t have a guard base with a county road going through it. So a new road is built to go around the new Guard unit along with the appropriate fencing.
Why don’t turboprop planes use bleed-air from the engine to melt ice on the leading edges of the plane?
Nearly all I have seen use the rubber, inflatable boots for de-icing. But on the spectrum of best to worst it seems boots fall in the middle and are less ideal than bleed-air heating from the engine.
What little I have found on this suggested that turboprop engines are not hot enough to provide the heat necessary. But I know that modern jets need to actually cool the air from their engines before sending it to the wings (and whatnot) and that turboprop engines have ITTs of 1200 F or more. Plenty hot.
Additionally, we see those big, fat exhaust pipes on turboprops (one of the things that identifies such an engine). Surely if they need such big, fat exhaust which is very hot they could use it for other purposes. It seems they need to dump a LOT of hot air over the side. But they don’t use it for other purposes.
As always, I am sure the engineers know their jobs well and have good reasons for this. But why?
Your premise is used for other things in airplanes. Small carbureted planes use exhaust heat to prevent carb ice. military jets have used turbine pressure for blown flaps.
I’m not an engineer but commercial turboprops are generally twin engine planes. That means hanging 2 engines on the wings. They’re moving around on their mounts more than you think which means a constantly flexing joint plumbed into a leading edge.
On top of that it would have to plumbed through the plane to accommodate an engine failure. Not something you want leaking into the cabin.
So at the end of the day they have to reinvent the wheel for something already well established. it needs to have attributes that justify the cost of development.
Well, they take bleed air off the compressor side before fuel is added and it becomes toxic. Blown ducts are not unusual in this regard so yes, they do take pressurized air off the engine.
Bleed air is taken from the compressor side of the engine. The fact that the exhaust temps are very hot does not mean that the compressor side air is hot enough for de-icing.
That said, I don’t know specifically why hot air isn’t used. Other suggestions on the internet include there not being enough air on a turboprop to do the job. A de-ice boot doesn’t let any air out (at least it is not supposed to), so it’s probably a lot more efficient in terms of how much air is needed. I can’t vouch for the accuracy of that statement, though it does make some sense. @LSLGuy might have more technical info, though he’s more of a jet guy.
The pneumatic systems have a significant impact on engine performance, there isn’t just an endless supply of unnecessary air for air conditioning / pressurisation, anti-ice, etc.
I don’t really have anything certain to add. But here’s some plausible factors.
The early turboprops were adaptations of piston aircraft, and used the same de-ice boot and an electric air pump to inflate/deflate them as their piston predecessors did. That probably then became standard practice on new-design turboprops as well.
In jets, the higher speeds mean boots won’t work well; they deform too much under the greater airload. So it’s not that hot bleed air is better, but rather it’s the only option on jets.
All jets are pressurized and (pre-787) they therefore buy into needing a bleed air system, an oversized engine compressor section to deliver the needed excess air, etc. Early turboprops may not have been pressurized. Or used electrical compressors for that purpose. Many current turboprops are unpressurized: Twin Otter, the various PT-6 driven cargo singles (e.g. Cessna Caravan, etc. Adding a bleed system for deicing is buying into adding a large and complex foundation for a pipsqueak need.
In general, the future of airplanes is eliminating hydraulics and bleed air and replacing them both with electricity. Those systems are finicky; as a civilization we’ve gotten pretty good at high powered electrical stuff. Back in the 1940s when so much of complex aircraft systems were born, the opposite was true.
Is there a reason to not take it from the exhaust? I mean, the exhaust is just that and about to be dumped outside the plane. Seems a source of “free” heat (free inasmuch as I can’t see how, at that point, it would rob the engine of any performance unless there is some thrust from that exhaust but I would be surprised if that amounted to much, if anything).
(For this I mean de-icing and not for pressurization of the cabin.)
I would imagine it necessary to plumb it through the body of the plane so both engines can operate the system. lots of valves and plumbing throughout. From a non-mechanic who has worked on his own plane It sounds like a maintenance nightmare compared to heated boots. It would also be toxic so any pipes transiting the body of the plane would have to be monitored for carbon monoxide leaks.
Recovering heat from an engine is an engineer’s dream but the cost and complexity of it has to be the deciding factor. Aircraft design is entirely driven by cost.
I don’t know, I suspect it would be too hot, maybe there’s not enough of it? Once it becomes exhaust it has had as much work as practical extracted by the power turbine to spin the prop.
Ultimately the boot system works fine. I’m not sure there’s a lot of motivation to engineer a different system that also works fine.
I read/saw somewhere (I forget now…sorry) that de-icing boots can have a quirk where the pilot needs to let ice build up before using them. If used too soon they can push the newly forming ice away from the wing but not actually break it up. This lets ice form away from the boots and the boots can do nothing to stop it at this point since they can’t “reach” the ice forming to break it up.
IIRC the article I read said this is what happened on a commuter flight (think ATR style plane) which crashed.
It just seems easier and safer to be able to turn on a heater.
The ATR crash was caused (partly) by deice boots that didn’t cover enough of the wing chord, that is, the boots didn’t extend far enough back along the top of the wing. A design fault that complied with regulations.
The phenomenon of ice bridging is a myth, on modern equipment at least, and the recommended procedure is to turn the deice on when you see ice. Simple enough.
The same article also mentions that small jets use boots because they don’t supply enough bleed air to use a heated leading edge system. Turbo-props presumably have the same issue.
There is also ‘weeping wing’ ice protection, such as that produced by TKS systems. Glycol is forced through thousands of tiny holes and it covers surfaces subject to icing and keeps them clear.
This is mostly used in expensive light singles like Bonanzas and Mooneys and such. It used to be used on business jets, but I think kodern deicing boots have probably replaced it for most uses because it’s heavy and expensive in cleanjng and maintenance.
The exhaust is already spinning the turbine and I would think the maximum work has already been extracted. The exhaust is exhausted! But the bladder system works great off the compressor bleed anyway.
From a re-inventing the wheel side I agree. But if you were theoretically starting from scratch you would gain efficiencies in the compressor section while extracting free power from the exhaust side. A turbocharger doesn’t have to be driven by heated exhaust. It just needs to extract the moving energy in the exhaust. I would imaging a turbine at cruise can contribute a significant amount of exhaust pressure regardless of temperature. There’s no need for efficiency at this point. It also separates contaminated air from pressurized air going to the bladder.
I may be confusing the issue by suggesting a leading edge bladder versus a heated leading edge.