FlyASAGuy2005
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Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 4:15 am

Really. Why was/is the ATR such a lumbering a/c? Is it because it was before its time as far as high density turboprop design goes? I'm more talking about the 72 than anything else. I've never flown up front so never had a chance to see the speed tape but I've flown it at Flight Safety and although it is a fun a/c, it is terribly slow. MY climb out of ATL took me on the standard ATL5 departure to get the heck out of the way of jet a/c that would have been behind me if I was flying in a real setting.

Anyone care to shed some light? Also, what is the proposed cruise speed for the 600?


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pilotpip
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 5:18 am

It doesn't need to be fast. 250kts when you're below 10,000 feet (where you spend most of your time in short haul, regional situations) is just as fast in an ATR as it is in a CRJ. The difference is about 1000pph in fuel burn.

It's designed to do what it does and it does it pretty well.
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Starlionblue
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 6:38 am

As pilotpip says the designers traded speed for economy. The segments it is built for are not terribly long, so the lower speed is not a liability.
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Goldenshield
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 9:20 am

Quoting pilotpip (Reply 1):
250kts when you're below 10,000 feet (where you spend most of your time in short haul, regional situations) is just as fast in an ATR as it is in a CRJ.

So where are your turboprops flying? JFK-LGA?

LAX-PSP, and LAX-SAN are 80 miles. Both of these routes spend the vast majority of their time ( >90% ) ABOVE 10,000 feet, and that's with mandatory filed altitudes (11,000.) They can always go even higher if they wanted to (sometimes as high as 17,000, but very rarely given traffic constraints.) To relegate a flight below 10,000 feet just because it's a turboprop is just asinine---with the exception of the Shorts series, as they aren't pressurized, and couldn't really get much higher if they tried.

The ATR, and other cabin-class turboprops, such as the E-120, are still pretty fast, and routinely cruise at FL24-270, and then the E-120 is even certified for FL310 (but with RVSM airspace, it's not going to see it again without spending a lot of money.) Granted, not near-jet speeds like the Q400, but given the time period of when they were certified, they were pretty smokin' aircraft. These aircraft routinely make 3-400 miles trips easily, with the longest that I know of being DEN-DIK.
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tsugambler
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 3:08 pm

The ATR-72 is the only aircraft on which I've ever gotten airsick and actually thrown up. I know it's not really its fault (we had to fly through a storm in Texas rather than over it), but I still don't like it to this day. Give me an ERJ from DFW to SJT any day!
 
pilotpip
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 4:50 pm

Quoting Goldenshield (Reply 3):
So where are your turboprops flying? JFK-LGA?

My RJ regularly goes between LGA and DCA below 10,000. We also go ORD-GRR, CID and other places where time above 10 is maybe 10 minutes. My previous employer did ORD-SPI, SBN, MKE and others and was filed at 9,000.

Despite being "slow" you're going to see very little (if any) difference of block time between an ATR or any other turboprop and a jet on the short routes that you see turboprops on. Definitely not worth the added fuel burn that a turbofan will have.
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Goldenshield
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 5:29 pm

Quoting pilotpip (Reply 5):
My RJ regularly goes between LGA and DCA below 10,000. We also go ORD-GRR, CID and other places where time above 10 is maybe 10 minutes. My previous employer did ORD-SPI, SBN, MKE and others and was filed at 9,000.

You missed my point entirely. The city pairs I listed---and in fact most city pairs in the CA market are short haul---shorter than many of the markets you metioned---yet, they spend the better portion of their time above 10,000. The flights out of Chicago (and Washington) are just restricted because the higher sectors just don't want to deal with the shorter flights.
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doug_or
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RE: Reason Behind The ATR Being So Slow?

Fri Oct 15, 2010 10:06 pm

Quoting Goldenshield (Reply 3):
LAX-PSP, and LAX-SAN are 80 miles. Both of these routes spend the vast majority of their time ( >90% )

That is clearly an exaggeration. Those flights are both in the 20-30 minute range, and it takes around 5 minutes just to get up to 10k,and generally more than that on all the vectors and what not on decent, approach, and landing. It might be a slight majority in ideal conditions, but it is not vast and it is no where near 90%. Please try again.
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Goldenshield
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RE: Reason Behind The ATR Being So Slow?

Sat Oct 16, 2010 1:34 am

Quoting doug_Or (Reply 7):
That is clearly an exaggeration. Those flights are both in the 20-30 minute range, and it takes around 5 minutes just to get up to 10k,and generally more than that on all the vectors and what not on decent, approach, and landing.

You're right. I was exaggerating a little bit (I was on a coffee buzz when I wrote that.) It's closer to 65-70%, but still, it's greater than 0%.
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FlyASAGuy2005
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RE: Reason Behind The ATR Being So Slow?

Sat Oct 16, 2010 11:32 pm

Thanks for the responses. So, the question at hand: they chose economy over speed. Got it. Still lingering in my mind and I guess my original question (As this is tech ops) what in its design inhibits it so much on speed. I have seldom seen one get above 270 or 280kts. 273-275 at best. Engine choice?
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prebennorholm
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RE: Reason Behind The ATR Being So Slow?

Sun Oct 17, 2010 1:35 am

Quoting FlyASAGuy2005 (Reply 9):
...what in its design inhibits it so much on speed. I have seldom seen one get above 270 or 280kts. 273-275 at best. Engine choice?

You are right. The ATR-72 engines are little more than half as powerful as the Q400 engines.

We often assume that ATR-72 and Q400 are head on competitors. They are not. ATR-72 is better seen as a "high capacity Q300".

ATR many years ago evaluated an ATR-82 which would have a new, much stronger wing with faster airfoil, much more powerful engines, stronger landing gears and brakes, and a slightly stretched fuselage compared to the -72. That would have been a Q400 competitor, but they did not move forward with that project.

The ATR-72 is a lightweight, short range, economic plane which was developed as a stretch of the ATR-42 with minimum change.

The Q400 is, compared to its ancestors, mostly a new, much more powerful and faster plane with longer range, and it shares little with its ancestors except the fuselage diameter.

The ATR operators around here use them mostly on 35 - 50 minutes sectors. On sectors, which would be longer than one hour on ATR, they tend to use CRJ-200 instead. People generally dislike turboprop planes.

Last time I was on an ATR-72 (a month a go) for a 45 minutes jump I was seated next to a passenger who complained bitterly that she had to return home on such an "oldfashioned" aircraft. She had been thinking about renting a hotel room and postponing her return flight until next morning on a B737-300. When I told her that the 733 would most likely be twenty years older, then she simply didn't believe me.
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Starlionblue
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RE: Reason Behind The ATR Being So Slow?

Sun Oct 17, 2010 3:36 am

Quoting prebennorholm (Reply 10):
Last time I was on an ATR-72 (a month a go) for a 45 minutes jump I was seated next to a passenger who complained bitterly that she had to return home on such an "oldfashioned" aircraft. She had been thinking about renting a hotel room and postponing her return flight until next morning on a B737-300. When I told her that the 733 would most likely be twenty years older, then she simply didn't believe me.

Classic. And of course this perception is the airlines' own fault. They were the ones who beat the drum about jets being the modern way of going, way back when.
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SchorschNG
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RE: Reason Behind The ATR Being So Slow?

Sun Oct 17, 2010 9:31 am

I just had two trips in ATR42, once a -300 and once a dash -500. I had my GPS with me and was quite interesting to see the crappy climb performance of the ATR42-300 (the airport wasn't either hot or high). The noise and vibration ... unimaginable.
The ATR42-500 is much better. Still slow, but better climb and vibrations & noise reduced by perceived 80%.

So: Why is the ATR so slow?
First, the ATR has a low wing loading resulting in relatively large drag at higher airspeeds. The advantage is better low-speed performance, less need for complicated high-lift devices. But more wing will generate more drag. So the climb airspeed of an ATR (-500) is 170 to 190 KIAS. Compare that to 250 to 300 KIAS for a jet. In consequenze the ATR will climb out at 20-25k with 170 KIAS and a ~220 KTAS. When leveled, the aircraft accelerates to ~270 KTAS.
Note: the ATR always cruises "Maximum Thrust", that is: the engine is set at full power and cruise restrictions (RPM, torque limit, TET). The aircraft will fly as fast as possible.

What is the reason for it being so slow:
> lack of engine power
> more drag

A note about the drag: drag in cruise is usually composed of zero-lift drag and lift-induced drag. The larger my wing, the less is my lift-induced drag but higher is my zero-lift drag. A jet at ATR72 cruise airspeeds (170-200 KIAS) would be close to VS1G. So, it is not a bad design but a design optimized for a certain airspeed.

The engine: turboprop is gas turbine and propeller. The gas turbine delivers relatively constant power (in hp or kW) over entire speed region, the power diminishes with altitude just like any turbofan.
What we actually need is thrust.
The thrust is equal to: THRUST = Shaft Power / True Airspeed
As we see the thrust lapse is proportional to speed increase. Double the (true) airspeed, available thrust will half.
As a turbofan shares some principles of the propeller, it also loses thrust with increasing true airspeed, but the thrust lapse is less significant.

Then there is another thing: propeller efficiency. If anyone considers a turboprop a simple aircraft he is correct as long as he excludes the propeller. A modern propeller is a masterpiece of aerodynamic design. As I wrote in my initial sentence, I had the honor of experiencing progress in propeller design within 3 days (the -500 model has 6-blade propeller). It is like day and night! Propeller efficiency can be assumed at 85-92% for a well designed prop at its design point. It can easily drop to funny levels when going beyond that, especially when the propeller tip becomes too fast (also very noisy). A prop is a balance between the torque (the amount of physical force delivered to the air) and rotational speed. Best for keeping the tip speed low would be minimum torque with high speed (like a turbofan).

So, I wrote quite a bit, what's the conclusion: the main reason why props are slow is the prop itself. The airframe is designed to suite the capabilities of the engine best.

[Edited 2010-10-17 02:54:12]
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FlyASAGuy2005
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RE: Reason Behind The ATR Being So Slow?

Mon Oct 18, 2010 12:53 am

That's what I was looking for. The more technical aspect of why the ATR was such a dog in climb and at cruise. Don't get me wrong. I love the a/c and wished DL/ASA didn't get rid of them but just something I never understood.
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kl671
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RE: Reason Behind The ATR Being So Slow?

Tue Oct 19, 2010 12:53 am

Quoting SchorschNG (Reply 12):
The thrust is equal to: THRUST = Shaft Power / True Airspeed

There is also a significant jet thrust from the engine exhast. In the case of the PW127 used in the ATR72, this amounts to an additional thrust of 325 lbs at MTO over that produced at the prop. (From Transport Canada Type Certificate E-19).
 
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Aesma
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RE: Reason Behind The ATR Being So Slow?

Tue Oct 19, 2010 1:40 am

Do we have numbers like litres/PAX/100Km for an RJ, a Q400 and an ATR72 to compare efficiency ?
New Technology is the name we give to stuff that doesn't work yet. Douglas Adams
 
thegeek
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RE: Reason Behind The ATR Being So Slow?

Tue Oct 19, 2010 3:23 am

Quoting SchorschNG (Reply 12):

The thrust is equal to: THRUST = Shaft Power / True Airspeed

Shouldn't that be: THRUST = Shaft Power / True Airspeed * Propeller efficiency

I think the efficiency would lapse with increasing speed.

For the thrust from the propeller. As kl671 pointed out there is also jet thrust, although I think 325lbs isn't that significant.
 
SchorschNG
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RE: Reason Behind The ATR Being So Slow?

Tue Oct 19, 2010 9:35 pm

You're right.
More exact would be "Thrust Horse Power", which is the Shaft Horse Power corrected with propeller efficiency.
I omitted this to keep it simple.

Quoting Aesma (Reply 15):
Do we have numbers like litres/PAX/100Km for an RJ, a Q400 and an ATR72 to compare efficiency ?

Numbers on the net? No, I haven't found them.
From my own calculations: the ATR rules in short sectors, but jets are very efficient as soon as distance increases. The key is altitude, as turbofans work best at high altitudes. On a 200nm sector the ATR is great, but at a 500nm sector an A320 is already close.
The Prop is hailed for great SFC, but SFC is fuel per hour of thrust. So a smaller SFC doesn't necessarily mean lower fuel consumption per distance.
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kl671
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RE: Reason Behind The ATR Being So Slow?

Wed Oct 20, 2010 12:22 am

Quoting thegeek (Reply 16):
As kl671 pointed out there is also jet thrust, although I think 325lbs isn't that significant.

P&WC's web site states that the contribution from the exhaust thrust is equivalent to an additional 450 shp at the conditions stated. That represents an increase of 16% over the rated mechanical power output of a PW127. 16% of additional power is not insignificant.

http://www.pwc.ca/en/engines/pw127f

Click Features and scroll to the bottom of the page.
 
wn700driver
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RE: Reason Behind The ATR Being So Slow?

Wed Oct 20, 2010 2:12 am

Quoting FlyASAGuy2005 (Reply 9):
what in its design inhibits it so much on speed. I have seldom seen one get above 270 or 280kts. 273-275 at best. Engine choice?

No doubt engine choice & propeller design has a lot to do with it. But I would think that a 0 degree wingsweep probably limits speed with regard to induced drag. Must be great for short field lift though.

I remember my days working on the Dash 8-300/100s. Looking back, the field performance was pretty excellent given the available power and relative heavy weight of the airplane. I know it's not an ATR, but they're pretty similar where all that counts (my understanding...)

Quoting SchorschNG (Reply 12):
If anyone considers a turboprop a simple aircraft he is correct as long as he excludes the propeller.

LMAO!... Because it's true! Just working with props and then transiting over to the CF6s, CF56s & VAE2500s I work on now is testimony to the relative levels of complexity involved.
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pilotpip
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RE: Reason Behind The ATR Being So Slow?

Wed Oct 20, 2010 2:30 am

Quoting Goldenshield (Reply 6):

You missed my point entirely. The city pairs I listed---and in fact most city pairs in the CA market are short haul---shorter than many of the markets you metioned---yet, they spend the better portion of their time above 10,000. The flights out of Chicago (and Washington) are just restricted because the higher sectors just don't want to deal with the shorter flights.

No, I didn't. Often you will see very little difference getting above 10,000 on a short sector like that and it definitely doesn't make sense to burn a significantly large amount of extra fuel to climb longer to go faster for a short time than it does to get back to cruise power.

A great example was STL-SPI. I did this a few times while at TSA. Had two turns there one day and did one at 11,000 and 12,000 and the other at 9,000 and 8,000 respectively. Going above 10,000 saved me about 3 minutes block.
DMI
 
Okie
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RE: Reason Behind The ATR Being So Slow?

Wed Oct 20, 2010 2:50 am

Quoting kl671 (Reply 18):

P&WC's web site states that the contribution from the exhaust thrust is equivalent to an additional 450 shp at the conditions stated


It also states estimated. K1671's number is the one that is certificated for the application.
In anycase we would have to agree that the exhaust needs to point the correct direction.

Quoting pilotpip (Reply 20):
A great example was STL-SPI. I did this a few times while at TSA


I did about a once a fortnight OKC-STL-IND and back from about 89 till 06. The afternoon STL-IND sometimes was a TSA ATR. Did you fly that route? Usually opted for the morning flight on the Barbie Jet.

Okie
 
SchorschNG
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RE: Reason Behind The ATR Being So Slow?

Wed Oct 20, 2010 12:38 pm

Quoting kl671 (Reply 18):
P&WC's web site states that the contribution from the exhaust thrust is equivalent to an additional 450 shp at the conditions stated. That represents an increase of 16% over the rated mechanical power output of a PW127. 16% of additional power is not insignificant.

The additional thrust is most relevant at low speeds and altitude. At cruise conditions it is effectively zero.

Quoting wn700driver:
No doubt engine choice & propeller design has a lot to do with it. But I would think that a 0 degree wingsweep probably limits speed with regard to induced drag. Must be great for short field lift though.

Actually, if flying below M0.7 the wing sweep is of limited importance. Modern wing profiles are much better than stuff used in the 1950ies. The ATR is slow due to its engine.

Quoting pilotpip:
No, I didn't. Often you will see very little difference getting above 10,000 on a short sector like that and it definitely doesn't make sense to burn a significantly large amount of extra fuel to climb longer to go faster for a short time than it does to get back to cruise power.

Usually the fuel to climb is well invested. The TurboProp saves less when going high, so it might be of no importance for short sectors.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]
 
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Aesma
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RE: Reason Behind The ATR Being So Slow?

Sun Oct 24, 2010 9:26 pm

Quoting SchorschNG (Reply 17):
The key is altitude, as turbofans work best at high altitudes. On a 200nm sector the ATR is great, but at a 500nm sector an A320 is already close.

But the A320 is quite bigger, I was thinking about CRJs, ERJs and E-jets. In the ATR capacity they're not reputed to have a great fuel economy.
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SchorschNG
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RE: Reason Behind The ATR Being So Slow?

Tue Oct 26, 2010 8:34 am

Quoting Aesma (Reply 23):
But the A320 is quite bigger, I was thinking about CRJs, ERJs and E-jets. In the ATR capacity they're not reputed to have a great fuel economy.

For short legs (below 200nm), certainly not.
When going above 500nm, both aircraft will probably come out even.
From a structural standpoint, passengers are the worst possible payload. [Michael Chun-Yung Niu]

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