Hello All,

Quick question when it came to the design of turbo prop airliners EMB120, ATR, DASH, King Airs....etc when these aircraft were/are designed was the failure of a blade and potential for it to enter the cabin considered from a design perspective when it comes to strengthening to any degree possible those areas of the fuselage were the blades are nearest? Or is that calculated out as such a low probability that more consideration is given to ensure the blades themselves are within tight manufacturing tolerances to mitigate the chance of such a think happening? I know that turboprops have lost blades before with varying degrees of issues but have always wondered if that is something that is considered in the design phase of the aircraft?

Thanks!

TDAero777 wrote:

Hello All,

Quick question when it came to the design of turbo prop airliners EMB120, ATR, DASH, King Airs....etc when these aircraft were/are designed was the failure of a blade and potential for it to enter the cabin considered from a design perspective when it comes to strengthening to any degree possible those areas of the fuselage were the blades are nearest? Or is that calculated out as such a low probability that more consideration is given to ensure the blades themselves are within tight manufacturing tolerances to mitigate the chance of such a think happening? I know that turboprops have lost blades before with varying degrees of issues but have always wondered if that is something that is considered in the design phase of the aircraft?

Thanks!

They normally have a strengthened area around the prop - if you have a look at some ATR and Q400 pics you can clearly see it. It is not a common occurrence though to lose a prop unless combined with other factors (ground strike/crash)

The area of the fuselage in the prop arc has strengthened cabin windows and ice shields. They're only to protect from ice coming off the prop blades. If a blade is slung off the hub towards the fuselage it will penetrate the cabin like a hot knife through butter.

T prop wrote:

The area of the fuselage in the prop arc has strengthened cabin windows and ice shields. They're only to protect from ice coming off the prop blades. If a blade is slung off the hub towards the fuselage it will penetrate the cabin like a hot knife through butter.

Exactly. Those "shields" on the fuselage are to reduce paint damage from ice discarded by the propellers. EXTREMELY RARE are propeller blade separations. Can't quote statistics but maybe one blade separation every 2 or 3 years, worldwide???

Blade failure was a cause of a USMC C130 crash back in 2017. The H-model C130s are grounded now because of cracks in the prop barrel assemblies.

Regulation mandates modern Turbofan engines to have a containment duct which encloses the Fan cowling area of the engine.
Shouldn't the ice shield also designed as a protection device in case of prop blade liberation.?

I suspect this is the primary reason that RJs (while less fuel efficient than turboprops) are on their way out. They've been entirely phased out in the US. Looking out the window on to a massive, and massively powerfully spinning propeller, is disconcerting to much of the flying public.

Shedding a blade? No cowling to contain it. Now: Such an event happening is so unlikely these days but, yeah. A given passenger, looking out the window 2 meters away at (in the public's mind) a blender-speed blade with the potential to rip through the fuselage. Not a great look.

I don't think turboprops have much life left in them, so long as 3-5 rows can visually see the massive torque of the output of these engines.

Happy Halloween!

rjsampson wrote:

I suspect this is the primary reason that RJs (while less fuel efficient than turboprops) are on their way out. They've been entirely phased out in the US. Looking out the window on to a massive, and massively powerfully spinning propeller, is disconcerting to much of the flying public.

Shedding a blade? No cowling to contain it. Now: Such an event happening is so unlikely these days but, yeah. A given passenger, looking out the window 2 meters away at (in the public's mind) a blender-speed blade with the potential to rip through the fuselage. Not a great look.

I don't think turboprops have much life left in them, so long as 3-5 rows can visually see the massive torque of the output of these engines.

Happy Halloween!

Hmm not sure I totally agree - We have 52 prop driven airplanes in the Air NZ domestic fleet and I have never heard of anyone who won't fly them due to seeing the props. Admittedly I never sit right near one myself. In USA they do seem to be on the decline but that's partly due to the greater distances involved, and also the safety incidents of the past.

StereoTechque wrote:

Regulation mandates modern Turbofan engines to have a containment duct which encloses the Fan cowling area of the engine.
Shouldn't the ice shield also designed as a protection device in case of prop blade liberation.?

That pretty much means converting prop to the turbofan for a very different set of optimal conditions. Including speed and fuel burn.
If you think about it, turbofans also have only limited protection against fan blade failure. A failure further down in turbine quickly comes to the brink of disaster (QF A380 and a pair of AA 767s come to mind)

StereoTechque wrote:

Regulation mandates modern Turbofan engines to have a containment duct which encloses the Fan cowling area of the engine.
Shouldn't the ice shield also designed as a protection device in case of prop blade liberation.?

Not necessarily, since the "strike zone" of a shedded blade onto the fuselage is forward of the propeller's rotational plane. The forces that act on the blades are forward (thrust) because the blades are pushing air backwards.

Yikes! wrote:

T prop wrote:

The area of the fuselage in the prop arc has strengthened cabin windows and ice shields. They're only to protect from ice coming off the prop blades. If a blade is slung off the hub towards the fuselage it will penetrate the cabin like a hot knife through butter.

Exactly. Those "shields" on the fuselage are to reduce paint damage from ice discarded by the propellers. EXTREMELY RARE are propeller blade separations. Can't quote statistics but maybe one blade separation every 2 or 3 years, worldwide???

Here are a couple of news quotes showing the safety of composite fan/propeller blades:

GE and Safran tout new ‘open rotor’ engine future for sustainable aviation (Seattle Times, 6/14/2021)

There have been repeated instances recently of engine fan blades breaking off during commercial flights and causing serious damage to aircraft. It happened on an older United 777 climbing out of Denver in February. However, these incidents have all involved metal blades, cracked by metal fatigue.

The GE carbon composite blades on its GE-90 and GEnx engines that respectively power Boeing’s 777-300ER and 787 airplanes have never had an incident of a fan blade breaking during their more than 140 million flight hours in service.

Plant tour: Dowty Propellers, Gloucester, U.K. (Composites World, 7/22/2022)

Dowty Propellers (Brockworth, Gloucester, U.K.) supplies large (e.g., 4-meter-diameter) composite-bladed propellers for commercial and military turboprop aircraft, including the Dash 8-400 regional airliner manufactured by De Havilland Canada (Toronto, Ont., Canada) and the C-130J military transport produced by Lockheed Martin (Bethesda, Md., U.S.).
...
The company began supplying commercial aircraft in 1953, became Dowty Rotol in 1960, prototyped a fiberglass propeller blade in 1968 and developed the world’s first all-composite blade in 1984 for the Saab 340 regional airliner. Dowty Propellers has manufactured more than 25,000 blades made using advanced composites and is equally well known for its integrated electronic control systems.
...
“We have never had a composite blade detach in flight,” says Dowty Propellers senior composites engineer, James Trevarthen, “and we want to maintain that high standard of performance and reliability — safety first, quality always. But we also need to prepare for the future.”

CowAnon wrote:

Not necessarily, since the "strike zone" of a shedded blade onto the fuselage is forward of the propeller's rotational plane. The forces that act on the blades are forward (thrust) because the blades are pushing air backwards.

Hi, are you sure about this? Wouldn't the high rpm of the Propeller have a greater effect than the forward force in case of blade separation?

rjsampson wrote:

I suspect this is the primary reason that RJs (while less fuel efficient than turboprops) are on their way out. They've been entirely phased out in the US. Looking out the window on to a massive, and massively powerfully spinning propeller, is disconcerting to much of the flying public.

Shedding a blade? No cowling to contain it. Now: Such an event happening is so unlikely these days but, yeah. A given passenger, looking out the window 2 meters away at (in the public's mind) a blender-speed blade with the potential to rip through the fuselage. Not a great look.

I don't think turboprops have much life left in them, so long as 3-5 rows can visually see the massive torque of the output of these engines.

Happy Halloween!

It probably didn't help that the Brasilia and it's slew of prop system design flaws and poor blade workmanship was popular in the US. Amazing Hamilton is even still in business after those senseless deaths.

StereoTechque wrote:

CowAnon wrote:

Not necessarily, since the "strike zone" of a shedded blade onto the fuselage is forward of the propeller's rotational plane. The forces that act on the blades are forward (thrust) because the blades are pushing air backwards.

Hi, are you sure about this? Wouldn't the high rpm of the Propeller have a greater effect than the forward force in case of blade separation?

Maybe it is greater but as the forward force still exists the resultant force still has a forward component and as such will accelerate forward.

Fred


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StereoTechque wrote:

CowAnon wrote:

Not necessarily, since the "strike zone" of a shedded blade onto the fuselage is forward of the propeller's rotational plane. The forces that act on the blades are forward (thrust) because the blades are pushing air backwards.

Hi, are you sure about this? Wouldn't the high rpm of the Propeller have a greater effect than the forward force in case of blade separation?

Yes, although the forward distance isn't as much as I thought. According to a study about a 737-sized aircraft with wing-mounted open rotor engines, a shed blade would impact the fuselage at 11.5 to 12.5 inches forward of the rotor's plane of rotation.

Analysis and Testing of a Composite Fuselage Shield for Open Rotor Engine Blade-Out Protection (Table 4. Test Conditions)

In another finding from that report, the danger area on the fuselage would need 0.560 inches (24 plies) of composite panel thickness to prevent the blade from entering the cabin. For comparison, the minimum fuselage thickness on the DC-8 and DC-9 was 0.050 inches, and on the 727 it is 0.038 inches. (I wonder what the required skin thickness would be if you had an aluminum panel instead of a composite panel.)

https://howthingsfly.si.edu/ask-an-explainer/how-thick-aluminum-covering-wings-commercial-airliner

flipdewaf wrote:

StereoTechque wrote:

CowAnon wrote:

Not necessarily, since the "strike zone" of a shedded blade onto the fuselage is forward of the propeller's rotational plane. The forces that act on the blades are forward (thrust) because the blades are pushing air backwards.

Hi, are you sure about this? Wouldn't the high rpm of the Propeller have a greater effect than the forward force in case of blade separation?

Maybe it is greater but as the forward force still exists the resultant force still has a forward component and as such will accelerate forward.

Fred


Sent from my iPad using Tapatalk

Quoting myself to make a correction, the two major forces existing when operating are forward (thrust) and the centripetal forces of spinning, when there is a separation event there is only 1 force remaining and that’s the thrust as the centripetal force disappears, it is now just a velocity. Ignoring drag of course…

Fred


Sent from my iPhone using Tapatalk

flipdewaf wrote:

when there is a separation event there is only 1 force remaining and that’s the thrust as the centripetal force disappears, it is now just a velocity. Ignoring drag of course..

Centrifugal force is the most prominent though...

77west wrote:

TDAero777 wrote:

Hello All,

Quick question when it came to the design of turbo prop airliners EMB120, ATR, DASH, King Airs....etc when these aircraft were/are designed was the failure of a blade and potential for it to enter the cabin considered from a design perspective when it comes to strengthening to any degree possible those areas of the fuselage were the blades are nearest? Or is that calculated out as such a low probability that more consideration is given to ensure the blades themselves are within tight manufacturing tolerances to mitigate the chance of such a think happening? I know that turboprops have lost blades before with varying degrees of issues but have always wondered if that is something that is considered in the design phase of the aircraft?

Thanks!

They normally have a strengthened area around the prop - if you have a look at some ATR and Q400 pics you can clearly see it. It is not a common occurrence though to lose a prop unless combined with other factors (ground strike/crash)

Those are just ice shields, they wouldn’t do anything to stop a prop blade.

Blade failures are extremely rare and would likely involve some external factor like striking an object or the ground (PenAir’s Saab 2000 at Dutch Harbor comes to mind)

StereoTechque wrote:

flipdewaf wrote:

when there is a separation event there is only 1 force remaining and that’s the thrust as the centripetal force disappears, it is now just a velocity. Ignoring drag of course..

Centrifugal force is the most prominent though...

There is no centrifugal force though, as soon as the blade breaks it’s just velocity in the direction it was going when it broke, no acceleration in that direction so no force.

Fred


Sent from my iPhone using Tapatalk

On a military transport I know relatively well there is a Propeler Break zone identified which have various requirement to ensure a propeler break doesn't have desastrous consequences.
Shields or armouring are not a appropriate way to protect the different system in there as a detached propeler is considered having infinite energy... For the design/certification you need to avoid placing critical stuff in this zone, and for the non avoidable like cables and pipes the risk is mitigated by duplication and separation of the critical ones.
The cargo hold loading plan is also considering the Propeler Break zone.

Any airplane with propellers out on the wing that I've worked on does have an extra layer added on to the fuselage in the vicinity of the props so any ice, gravel, etc. that gets thrown by the prop will damage that extra replaceable layer instead of the actual fuselage. If a blade let go it might slow it down a bit but it almost guaranteed won't stop it, luckily this is an extremely rare occurrence.

I'm sure it's possible to design the airplane so the prop can't come through, but it would add a substantial amount of weight to the design and also wouldn't do much to help the fact that once the blade lets go it's likely that the rest of the prop, gearbox and engine are probably going to come apart as well due to the massive unbalance.

Final report of what happens when a blade lets go in flight on an ATR: http://www.tsb.gc.ca/eng/rapports-reports/aviation/1994/a94q0037/a94q0037.pdf.

rjsampson wrote:

I suspect this is the primary reason that RJs (while less fuel efficient than turboprops) are on their way out. They've been entirely phased out in the US. Looking out the window on to a massive, and massively powerfully spinning propeller, is disconcerting to much of the flying public.

Shedding a blade? No cowling to contain it. Now: Such an event happening is so unlikely these days but, yeah. A given passenger, looking out the window 2 meters away at (in the public's mind) a blender-speed blade with the potential to rip through the fuselage. Not a great look.

I don't think turboprops have much life left in them, so long as 3-5 rows can visually see the massive torque of the output of these engines.

Happy Halloween!

No.

The replies above that reference "ice shields" are correct, they are in fact that, designed to protect the fuselage from ice shedding off the prop blades. They do routinely get removed to inspect the fuselage underneath them for dents. I've worked on turboprop airplanes for almost 20 years and have never heard of a blade being thrown in flight, or even on the ground unless it hit something first and then broke off.

The other thing to remember is that if a blade separated from the prop assembly in flight, where that blade went would not be your only concern, I'm thinking the imbalance this caused would be much worse than the damage caused by the liberated blade.

I've sat next to the prop many times and I find it cool to watch the pitch changes and deflection of the blades at take-off power. As I said, I work on prop planes so I know how they are put together and how stringent the inspections on the blades are, makes me confident to sit anywhere on the plane and let the prop drone put me to sleep.

Here you go... Back home a Jetstream 41 hit a big assed bird on final approach and shed a prop into the cabin...

http://avherald.com/h?article=4f2a35e6&opt=256

From that report it seems that the airplane was on roll-out after touching down with the propeller in reverse pitch. Probably still at a fairly high RPM all the same. The crew reported significant vibration which I would expect to be the case in this instance.

KingOrGod wrote:

Here you go... Back home a Jetstream 41 hit a big assed bird on final approach and shed a prop into the cabin...

http://avherald.com/h?article=4f2a35e6&opt=256

Wow, I've never heard of a Kori Bustard before. From https://nationalzoo.si.edu/animals/kori-bustard:

The kori bustard is one of the world's heaviest flying birds, however, it lives on the ground and is reluctant to fly unless in serious danger.

Male kori bustards range in weight from 24-42 pounds (11-19 kilograms), and females are roughly half the size of the males. They stand about 5 feet (1.5 meters) tall.

I thought birds that heavy that were capable of flying would've been hunted to extinction already. Perhaps a change to the existing bird strike regulations is in order?