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I am interested in better understanding the performance differences between different planes that seem to be similar in many important ways but that differ greatly in performance.

For example, take single engine turboprops A, B, and C that all have a regulation stall speed minimum of 61 knots, and that all have similar power to weight ratios at takeoff. Turboprop A has a takeoff field length of 1,200', B is 1,600', C is 2,400'. If they all have the same stall speed (and a similar lift-to-weight ratio, I would think) and similar power to weight ratios, shouldn't they have similar takeoff field length requirements?

Lancair Evolution

  • 550hp at takeoff (750hp in flight)
  • 4,550 lbs MTOW
  • max speed ~295 knots
  • 1,200' field length
  • ~10:1 glide ratio

Epic E1000

  • 960hp at takeoff (1200hp in flight)
  • 7,500 lbs MTOW
  • ~330 knots max speed
  • 1,600' field length
  • ~17:1 glide ratio

Daher TBM 900*

If these planes have the same stall speed (and therefore, my intuition would think, similar ratios of lift to weight and drag), and have similar ratios of power-to-weight, how can they have such different field length requirements and glide ratios?

EDIT: UPDATED INFO

Stall speed is as defined by the FAA for Part 23 aircraft certification. Here's a link to how the FAA requires the stall speed to be validated.

Wikipedia defines this Vs0 speed as:

Stall speed or minimum flight speed in landing configuration.

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    $\begingroup$ Stall speed is not a very accurate way of judging lift characteristics of a wing. A wing can stall at any speed technically, the "stall speed" is given for a particular configuration. Are your quoted stall speeds in the landing configuration? Or clean? $\endgroup$
    – Ron Beyer
    Sep 4, 2016 at 23:45
  • $\begingroup$ The stall speed as defined by FAA regulations for single engine airplanes and multi-engine airplanes below 6,000 lbs MTOW that don't meet a certain climb rate minimum. I believe that they allow the high lift devices to be extended when they calculate it. $\endgroup$
    – Charles847
    Sep 4, 2016 at 23:51
  • $\begingroup$ Which planes are these? I'm sensing C is a TBM 850? $\endgroup$
    – Pugz
    Sep 4, 2016 at 23:57
  • $\begingroup$ Lancair Evolution, Epic E1000, TBM900. I think the TBM850 only has 700hp for takeoff. $\endgroup$
    – Charles847
    Sep 4, 2016 at 23:59
  • $\begingroup$ Biggest difference is probably the wing design, including leading and trailing edge devices. Takeoff and landing performance can also be greatly affected by how well the brakes work, which have nothing to do with aerodynamics but certainly changes the ground rolls (in the event of an abort on takeoff). $\endgroup$
    – Lnafziger
    Sep 5, 2016 at 4:13

3 Answers 3

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Are we comparing apples to oranges again? The differences in take-off length are far too big with such similar performance numbers, and I agree that they should be closer together - if we are really comparing the same thing.

Take-off means that the aircraft has to gain an energy difference with a potential and a kinetic component. FAR 23.53 demands a height of 50ft, and FAR 23.51 a speed of 1.2$\cdot\text v_{S}$, but adds more conditions that could be significant here. If the speed for continued safe flight is found to be higher, that speed must be used as the basis of the take-off performance.

The 61 kts stall speed limit only applies to aircraft below 6000 lbs MTOW, so two of the three are not constrained by this limit. Interestingly, the heavier aircraft have longer take-off lengths.

If the airplane weighs more than 6000 lbs and is certified in the commuter category, FAR 23.59 comes into force. It relaxes the height requirement (35 ft instead of 50) but adds a factor of 1.15 to the demonstrated length.

If all take-off distances would had been determined with the same rules and if the stall speed in take-off configuration would be equal between all three types, the take-off distance would be very similar. But I am sure both conditions are not met.

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    $\begingroup$ If you read closely I am pretty sure that the 61 kts stall speed requirement applies to all single engine planes and "multi-engine planes 6,000 lbs MTOW and under that do not meet a certain single engine climb performance". So I believe that the 2 larger planes were both subject to 61 kts requirement and the Lancair being a kit plane was voluntarily self-subjected to the requirement, they prominently tout that they have designed and tested the plane at an FAA-defined 61 kts stall speed. $\endgroup$
    – Charles847
    Sep 5, 2016 at 14:00
  • $\begingroup$ So are you saying that the TBM 900 might have a higher "safe flight speed" than its FAA specified V<sub>s0</sub>? That would be interesting and exactly the sort of thing I'm trying to understand. What would be the physical difference between an airplane that can fly safely at its stall speed versus one that can't? $\endgroup$
    – Charles847
    Sep 5, 2016 at 14:04
  • $\begingroup$ @Charles847 In this pilot report rotating speed of the TBM 900 is just under 90 kts. This smells like a stall speed much in excess of 61 kts. To fly fast, stall speed must be picked as high as possible in design. The higher the wing loading, the higher the top speed. $\endgroup$
    – Peter Kämpf
    Sep 5, 2016 at 15:18
  • $\begingroup$ you are correct. According to the below article the TBM has a special exemption for a 65 kts stall speed. Does that account for the differences? flyingmag.com/pilot-reports/turboprops/tbm-850-even-faster $\endgroup$
    – Charles847
    Sep 5, 2016 at 16:09
  • $\begingroup$ @Charles847: Take-off distance should scale with the square of stall speed (roughly), so the "corrected" value should be 12% shorter. This reduces the 2,400 ft to 2,100 ft - still not enough. But I think the stall speed given in the article is for the landing configuration, so we can safely add another 5 kts for the take-off configuration. This would bring the 2,400 ft down to 1,800 ft. $\endgroup$
    – Peter Kämpf
    Sep 5, 2016 at 19:00
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You need to dig a little deeper here, we need area, aspect ratio and airfoil type of wing AND propeller.

For example, a low aspect wing will stall at a lower airspeed, but not glide as efficiently as a high aspect wing. The Lancair and Epic bear comparison here.

Also, check into slat and flap configurations available for low speed flight on all types. The Daher may be able to "hang everything out" for a low speed airline-like landing, but may have a smaller, lower aspect wing than the Epic, there for, poorer takeoff performance.

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This is likely a difference in optimization, contrasting speed and efficiency at cruise with other performance parameters like acceleration on the runway or stall characteristics.

Total drag will make some difference, design optimization of the prime mover will likely be a big factor, and as others have said stall speeds may be stated for different high lift device configurations, brakes, spoilers.

As for the prime mover, a turbo-jet is best at high speeds and less efficiently creates thrust at taxi speeds, a high bypass turbo fan is best at medium speeds and is reasonable at taxi speed.

A propeller can be designed/selected for a target performance, with a climb prop(optimized for low speeds) you will get much more thrust at low speeds for quicker 0-60 times and steeper climbs but you will suffer reduced cruise speed and efficiency while operating at higher rpm. Adjustable pitch props help provide good low speed and cruise performance but the still have an optimum sweet spot.

At the extremes of fixed pitch you have the STOL performance of bush planes that will nearly hover with a slight headwind and a top speed of 80, and Reno air racers that may actually get out and push to get the plane moving because at low-taxi speed the prop is almost completely stalled and needs some relative airflow to get a useful angle of attack but they have a top speed of 3-400.

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