6
$\begingroup$

The Q400 has a cruise speed of 360kts, and on flightradar24 most Q400s that I looked at reached similar speeds during cruise. The ATR 72 has similar specifications yet only flies at 280kts. I'm currently designing a regional turboprop as part of a university assignment and I can't find a drag polar that gives 360kts as anywhere near the optimum speed for L/D ratio, 244kts was the cruise speed for my initial configuration. Is the Q400 likely flying at a much higher, less efficient speed? Why would this be done? Or has better technology allowed it to fly efficiently at this speed?

Edit: According to a pilot on this flight sim forum they used to cruise at around 200kts. This video shows Flybe (budget airliner) cruising at 240kts, FL240. Maybe the flights I've seen on flight radar are operating under different parameters at the moment, with fewer passengers and lower oil prices maybe 360kts is the best speed.

Improve this question
$\endgroup$
9
  • 2
    $\begingroup$ Are you sure the drag polar is at cruise altitude? 244kts IAS at FL300 may very well be 360kts TAS. $\endgroup$
    – Efe Ballı
    Nov 22, 2020 at 16:45
  • 2
    $\begingroup$ @EfeBallı It doesn't really change your point, but service ceiling for the Q400 ist FL270. Most cruise below FL250 because they don't have oxygen masks (for passengers). $\endgroup$
    – Bianfable
    Nov 22, 2020 at 17:25
  • 2
    $\begingroup$ The designers of Q400 made the plane light enough to be fast. The major weight savings come from main ldg gear, which is so light it collapses approximately on every other landing. Sorry... I'll get mi coat... $\endgroup$
    – Jpe61
    Nov 22, 2020 at 17:27
  • 1
    $\begingroup$ Oh dear god don't, you'll fail the course 🤣 $\endgroup$
    – Jpe61
    Nov 22, 2020 at 17:29
  • 3
    $\begingroup$ I'm sure I read somewhere that the Q400 cruises at 400kts TAS, it's a major selling point of the Q400 as it cruises almost as fast as a jetliner. I don't have any data as proof, but I don't believe 400 kts is too far off its best range speed. @Jpe61, not sure about weight but it surely saves a fair bit of drag to retract the LG into the nacelle and eliminate fuselage protrusions for LG housing. 100% agree on LG fragility $\endgroup$
    – Efe Ballı
    Nov 22, 2020 at 17:45

1 Answer 1

Reset to default
11
$\begingroup$

Here are the cruise performance tables for the Q400 at the heaviest and lightest weight (click to view larger):

Q400 Performance Tables
Source: from the Dash-8 Q400 performance manual, which came included with a simulator product I purchased. They probably got their data from a QRH (Quick Reference Handbook), which usually includes such tables in the In-Flight Performance section.

At ISA conditions at FL250, the long range cruise is flown between 209 and 188 KIAS (depending on weight) and the high speed cruise is flown between 230 and 243 KIAS. The corresponding TAS at FL250 (assuming ISA conditions) are:

  • 188 KIAS: 281 KTAS
  • 209 KIAS: 312 KTAS
  • 230 KIAS: 343 KTAS
  • 243 KIAS: 363 KTAS

That last value (for high speed cruise at light weight) seems to agree quite well with the max. cruise speed of 360 KTAS, which one usually find for the Q400. But best range speed at that weight would be around 188 KIAS (281 KTAS), quite a bit slower. At higher weight, the speed differences between best range and high speed cruise are smaller though.

At typical weights the Dash-8 will still be faster than the ATR 72-600. I don't have exact performance tables for that aircraft, but Wikipedia says cruise speed is 280 KTAS. So yes, the Dash-8 is built for higher speed. The biggest difference is the choice of engine: PW150A with 3782 kW for the Dash, but PW127M with only 1846 kW for the ATR, more than a factor two difference! While this difference probably causes a higher best range speed for the Dash compared to the ATR, in absolute fuel usage the ATR is still more economical.

Improve this answer
$\endgroup$
3
  • $\begingroup$ Thanks. Do you mind sharing where you got these tables? $\endgroup$
    – Rory McDonald
    Nov 22, 2020 at 20:20
  • 5
    $\begingroup$ @RoryMcDonald I always include a link to the source if it is publicly available. In this case, I couldn't find a public source. I got the tables from the Dash-8 Q400 performance manual, which came included with a simulator product I purchased. They probably got their data from a QRH (Quick Reference Handbook), which usually includes such tables in the In-Flight Performance section. Maybe you can find a QRH on Google somewhere... $\endgroup$
    – Bianfable
    Nov 22, 2020 at 20:33
  • 1
    $\begingroup$ I embedded the source info into the answer, in case a comment clean up ever accidentally sweeps that away. $\endgroup$
    – FreeMan
    Nov 24, 2020 at 19:23

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .