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What is the total EXTRA lift produced using an 80 deg flap? ( 50 deg flap at 30 deg AOA ) and distributed propulsion.

Specs: Ultralight, stall speed 20mph, ROW of 12 x 15" props along entire wing span in DISTRIBUTED PROPULSION, max local AOA estimated at < 9 deg ), electric 25 hp (total), MTOW 500 lbs, low wing in ground effect, max 3 ft height for xSTOL/nVTOL, VG's, full span single slotted flaps with spoilers , Clmax: 3.4.

Hypothesis:

A low wing ultralight accelerates with a short field takeoff, along a runway to 20mph and rotates to 30 deg AoA just prior to takeoff, producing the following:

Total lift = Wing lift (bernoulli/momentum/vortex lift + deflection lift ) + Prop lift + Flap Lift

Example:

  1. Wing lift:

standard wing lift formulas, lift equal to airplane weight of 500 lbs.

  1. Prop lift:

Estimated thrust at 20mph: 5lbs/Hp thrust x 25 HP = 125lbs thrust.

Prop lift = 125 lbs x sin ( radians(30 deg))= 62 lbs

  1. 80 deg Flap Lift

Thrust due to Prop wash on bottom of wing hitting flap = 30%(???) of total thrust = 37 lbs.

  1. Total extra vertical thrust/lift

= 62 + 37 = approx 100 lbs ( 20% extra vertical thrust/lift...!!!! )

  1. Reduced MTOW wing requirements at Vtakeoff to Vflap

orginal MTOW = 500 lbs.

MTOW revised= MTOW original - extra vertical lift = 500lbs - 100 lbs.= 400 lbs.

This is the reason for asking the question:

Could I then resize my wings based on 400 lbs MTOW at Vtakeoff(Vso) to Vf, not 500 lbs?

Is this correct thinking?

See black tuft dwg below showing downwards air flow.

% vertical thrust

Fred
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  • Thrust is generated by the engine. Did you mean lift or drag? – Dean F. Apr 01 '20 at 17:30
  • The edit makes a little more sense. But, are we to assume the engine is directing its slipstream against the underside of the wing. Otherwise, the only contribution to lift is made by the relative wind, not by the engine. Am I missing something? – Dean F. Apr 01 '20 at 23:00
  • Maybe I am thinking about this wrong. Help me with the following scenario: – Dean F. Apr 01 '20 at 23:28
  • You are idling at the AER in a pusher-prop ultralight with the brakes on. Thrust and Lift are negligible. You go full throttle. Thrust increases to X but Lift remains only a small percentage of X. You release the brakes. Lift increases as airspeed increases. Thrust remains X. After reaching TPA, you chop the throttle to idle. Thrust almost immediately becomes negligible. On the other hand, Lift declines in proportion to airspeed. Am I thinking about this wrong? How are you generating Vertical Lift with Horizontal Thrust except through relative motion through the airmass? – Dean F. Apr 01 '20 at 23:42
  • Otherwise, only the lift that is deflected by the flaps from a tractor-prop is converted to Vertical Thrust. The rest remains as slipstream to create Vertical Lift. That begs the question of how many ultralights have flaps positioned behind the prop, in its slipstream? – Dean F. Apr 01 '20 at 23:48
  • Vertical Lift is thrust. A propeller is a little wing going in a circle. A helicopter rotor is a longer wing going in a circle. A regular wing goes straight. It's all the same, except for circle/straight part. Fred seems to be asking what is the lift increase for the main wing as a result of the propeller slipstream - the circular wing's horizontal "downwash" - impinging on the flap. – John K Apr 02 '20 at 00:40
  • @JohnK - Although Thrust from a prop is a form of Horizontal Lift, I don’t think that is what Fred is implying. And, the question has nothing to do with rotorcraft’s Vertical Thrust/Lift. It is, as you said, about the increase in Lift due to prop slipstream. But, Fred goes a step further. Asking how much lift is increased even more when lowering flaps to 80°. The 20 knot relative wind should not be taken into consideration for this question. Only the force of whatever slipstream contacts the flap. And, how many ultralights have flaps positioned behind the prop, in its slipstream? – Dean F. Apr 02 '20 at 00:54
  • The most recent edits make much more sense than the original question. Especially since you have added that you are considering a row of props along the leading edge. Try researching the following aircraft for a lead on your answer: https://www.tecnam.com/us/innovation/p2006t-x-57-maxwell-nasa/ – Dean F. Apr 02 '20 at 03:17

1 Answers1

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A 30° flap is a typical landing setting. Increasing that to 80° is only adding more drag. Your 25 hp thrust will only add a mild breeze and will not help to reach that 30° AoA unstalled. Better use 8° or 10° of flaps (depends on flap chord) and do not rotate to more than 10° AoA.

Peter Kämpf
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