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Would converting a Lazair ultralight to 4 x 3 Hp engines ( 12 Hp total ) and using the props along the entire wingspan as distributed propulsion produce a Clmax of 4.5?

I understand the original Lazair had 2 x 5.5 hp engines, for a total hp of 11hp.

I understand the Lazair has no slats or flaps, so the clmax of it's wing would be about 1.5.

I also understand that distributed propulsion adds about a cLmax of 3 to a wing.

So, what would happen if you converted a Lazair to a 4 engine plane with 6' propellers across 100% of it's entire wingspan to generate distributed propulsion?

Would the clmax jump from 1.5 to about 4.5 or is 12 Hp way too low to produce an increase in lift of 3? If 12Hp is too low, how much Hp is required?

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  • $\begingroup$ Even the Lazair 2 with 2 9.5 HP Rotax 185 only manages a modest 200 fpm climb with a 240 lb useful load. $\endgroup$ Feb 14 at 2:29

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It's been done. Thing is, while you may get the theoretical jump to 4.5 Clmax, it's only for the 10 or 15% of the wing that is in the slipstream in the case of the Lazair and it's itty bitty props.

So overall, the effect in total CLmax of two extra engines blowing on the wings is negligible. The 4 engine Lazair's performance was almost all from the doubling of thrust, with maybe a modest drop in minimum flying speed with power on with 4 engines vs 2, perhaps a knot or two.

On airplanes like large commuter turboprops with enormous 14 ft propellers that occupy a significant part of total span, the effect is much stronger. Power settings have a near instantaneous and major effect on sink rate and you are keenly aware of the effect of the slip stream on total Cl.

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  • $\begingroup$ So.. if you spaced out the 4 engines and had 6' props, covering 90% of the wing? Seems like it would jump the clmax from 1.5 to about 4, even with only 12 hp, no? $\endgroup$
    – Fred
    Oct 29, 2019 at 1:25
  • $\begingroup$ Well the velocity of the prop wash from 6 ft blades being driven by 12 hp would be pretty gentle so I think you'd lose a lot of the effect. $\endgroup$
    – John K
    Oct 29, 2019 at 2:54
  • $\begingroup$ @Fred No, not if you leave the wing as it is. You need lots of camber, like slats plus slotted flaps with 40° deflection, plus slipstream. With the slipstream only, not much more than a lift coefficient of 1.5 will be possible. Maybe 1.8 for the full wing. But never 4.5. $\endgroup$ Feb 13 at 11:48
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Please don't run such experiments by yourself!

To change the thrust arrangement of an aircraft is to create a partially new aircraft. To do that, you have to know what you're doing and what, exactly, down to numbers, will happen.

Adding or spreading engines is not an automatic increase of lift coefficient, so the short answer is, no.

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Forget the lift coefficient for a moment. That jump by 3 was for one particular design - this is no magic number that applies everywhere.

Better think that you want to increase lift by a factor of three at a slower speed. Lift is the reaction force from deflecting a stream of air downwards. That stream can be approximated as the flow through a disk with a diameter equal to the wingspan which is 11 m in case of a Lazair. With a mass of 200 kg, standard atmospheric conditions in 500 m elevation (air density = 1.2 kg/m³) and your assumed maximum lift coefficient of 1.5, the flight speed at that lift coefficient is 12.8 m/s. For three times the lift coefficient speed could drop to 7.4 m/s before you stall.

Without flaps all that extra lift has to come from accelerating air downwards over the wing. This would require thrust far in excess of both induced drag and weight of the Lazair. In other words, if you only add propellers, without changing the wing's airfoil such that it would produce far larger deflection angles, the propellers and the wing have to be oriented up in order to make that slow flight speed possible. Thrust has only enough forward component to balance the aerodynamic force on the wing which is now mostly back due to the high angle of attack.

Using four propellers of 2 m diameter each would need 50 HP (= 12.5 HP for each motor) at least, assuming a generous propulsion efficiency of 0.75. If you quadruple engine power and have sufficient control authority for flying an angle of attack between 60° and 70°, go for it! Maybe you should check first, however, whether the beefed-up structure (so the props stay with the motors, and those stay with the wing) and the heavier engines still let you keep total mass at 200 kg.

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Here is some data from RC Groups about a person who built a 100% "scale" electric Lazair about 10 years ago, swinging two 32 inch x 12 props.

Thrust requirement was around 50 lbs. Can't get around that, and loading up with high lift devices will only require much more.

Going back to the lessons we learned about Vbg and lowest drag configuration, particularly with an ultralight, high lift devices simply are not needed for an aircraft that (is very light) and will stop in a few feet anyways. In fact, spoilers might be a better choice for this type (of powered glider).

Oh, if the Fiesler Storch only had retractable slats and flaps!

Miraculous increases in Coefficient of Lift do not come without increases in thrust, which invariably add weight in both engine and fuel. Better to keep an ultralight aerodynamicly clean and focus on engine/prop thrust efficiency and weight savings.

All that being said, mounting the engines on the leading edges of the wings did produce an increase in performance$^1$, so, if you want to go with the original 5.5 hp chainsaw engines, 4 or even 6 of them may produce a fine multiengine ultralight.

It would do Convair proud to have 6 in a pusher configuration $^2$,$^3$, which would help reduce drag as well as improve lift. At least 20 horsepower (15 kW) seems to be optimal for reasonably good performance, especially with floats.

$^1$ Ultraflight Lazair, Development

$^2$ similar to this

$^3$ or perhaps more like this

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