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Watching helicopters come in to land, on occasion, I have noticed that they approach with a forward speed of a few knots, never land straight down. If there is sufficient landing space, would it be possible in some cases to replace the helicopter with a small fixed wing aircraft with low stalling speed?

The Helio Courier is an aircraft which is capable of flying slowly:

With a minimum-control speed of around 28 mph, the Courier is perfectly suited for confined off-airport operations. The first one was certified in July 1954 and powered by the 260 hp Lycoming GO-435-C2B2. - Wikipedia

I want to design (on paper at least), a tricycle retractable aircraft which is capable of the 20-200 knot speed range. Why has no one developed such an aircraft, and can the helicopter be replaced in certain situations by such an aircraft? Will it be technically feasible but commercially infeasible? (Edit: can I do this with increasing wing area and power only?)

EDIT: What I am looking for is an aircraft that can meet these helipad specifications, see page 25 in the document : https://www.faa.gov/airports/resources/advisory_circulars/index.cfm/go/document.current/documentNumber/150_5390-2 and the image titled "Figure 2–7. VFR Heliport Approach/Departure and Transitional Surfaces: General Aviation". 4000 ft approach and 500 ft obstacles outside that. I can fire up my favorite flight sim and try the approach. Stopping within the helipad could be a problem.

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    $\begingroup$ I think perhaps you are not watching those helicopters closely enough. They may APPROACH with a forward speed of a few kts, but (from my observation, at least) they always reduce that forward speed to zero before they actually contact the ground. Indeed, many have skids rather than wheels, so a landing with appreciable forward speed would make sparks :-) $\endgroup$ – jamesqf Jun 21 at 16:22
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    $\begingroup$ Checkout the stall speed of the Antonov AN-2. $\endgroup$ – Brian Drummond Jun 21 at 22:46
  • $\begingroup$ Good point about the forwards speed - this means the landing roll is zero for helicopters, and they can land on a helipad. Any forward speed needs to be stopped. The An-2 has a stall speed of 26 knots but can only go as fast as 100 knots. $\endgroup$ – stackex555 Jun 22 at 14:44
  • $\begingroup$ Wikipedia puts the An-2 max speed at 139 knots; retractable wheels could be expected to increase that; as a starting point it might be close to your goal. $\endgroup$ – Brian Drummond Jun 22 at 15:28
  • $\begingroup$ @jamesqf - I was a passenger on a CH-46 that made a rolling landing at NAS Cubi Point back in 1982 and then taxied to the tie-down location. I didn't get the feeling that it was anything extraordinary. $\endgroup$ – Bob Jarvis Jun 22 at 17:35

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At least one airframe close to these specs already exists, its a highly modified Wilga by the name "Draco". It can get off the ground in 120 feet and has a stall speed of ~37mph and cruises at ~180mph. With its epic 4000 ft/min climb rate it can get out of tight spaces fast as well. The only thing it lacks compared to your specs is the retractable gear as it was designed as a big gear bush plane.

enter image description here

(source)


The same guy (Mike Patey) also built a pretty wild Turbolance experimental that has some epic short field and climb performance as well and its a retract.

His overall secret sauce is effectively putting a PT-6 on the front end of a small airframe to add a huge amount of power to a light airframe.

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  • $\begingroup$ Draco is not a homebuilt, it's a (heavilly) modified wilga. Apparently registered in the "experimental exhibition" category. $\endgroup$ – Peter Green Jun 21 at 16:32
  • $\begingroup$ @PeterGreen thanks for the note! I have updated to reflect. $\endgroup$ – Dave Jun 21 at 16:50
  • $\begingroup$ While "Draco" is indeed impressive, It looks like the actual quote is that a new set of wings reduced its stall speed by 20mph, not to 20mph. The takeoff and landing speeds seen in videos look rather faster than 20mph, though they involve exceptionally short ground runs. $\endgroup$ – Chromatix Jun 21 at 18:14
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    $\begingroup$ Specifically: "He designed a completely new airfoil that dropped the stall speed about 20 mph to about 37 mph." This is in a similar class to WW2-era STOL aircraft. $\endgroup$ – Chromatix Jun 21 at 18:15
  • $\begingroup$ A retractable would be fine, but some sleek looks would be what I am looking for. Can it land on a helipad? $\endgroup$ – stackex555 Jun 22 at 14:45
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Technically possible? Yes. In practice it's so difficult to do that we use helicopters, tilt rotors and VSTOL technology instead.

The byproduct of lift is drag (called induced drag), so if you have a wing with a shape and size that delivers high lift at low speed it will generate a great deal of drag at higher speeds, limiting the effective top speed of the airplane. A wing that is optimized for higher speed produces much less lift at low speed.

If you want the best of both you have to change the wing, which is what flaps and slats do: they increase the lift surface area and change the shape of the wing. If you look a the wing of a commercial jet you see just how complex it is to do that, and understand the limitations. To have an airplane land at 20 knots yet cruise efficiently at 200 you'd need to change the wing shape radically in flight in a way that doesn't interfere with the flight characteristics or present control difficulties. That is a huge engineering challenge, and would likely result in a product that has so much complexity and cost that it isn't better than the alternatives.

You can get close-ish, there are airplanes with a 40kt landing speed and a cruising speed of 160 or so, this is done by strapping a really powerful engine onto a STOL airplane, and powering past the induced drag. 40kt landing speed isn't 20kt landing speed though, that extra 20kt is going to be a toughie.

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    $\begingroup$ In essence, you'd need your Helio Courier (or equivalent) to turn into a Bonanza once clear of obstacles, then turn back on final approach. Tall order indeed... $\endgroup$ – Zeiss Ikon Jun 21 at 11:14
  • $\begingroup$ A very good way to put it @ZeissIkon! $\endgroup$ – GdD Jun 21 at 12:05
  • $\begingroup$ Actually I was thinking of increasing the wing area, keeping the payload constant and increasing the engine power to compensate. Some sort of delta wing (Delta Dyke) may be more compact and suitable. $\endgroup$ – stackex555 Jun 22 at 14:47
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    $\begingroup$ @stackex555 Increasing wing area doesn't often pay off. Absent other changes, you have to quadruple the wing area to cut stall speed in half, and that only if you can keep weight constant. Since bigger wing is more weight, this race is hard. $\endgroup$ – BowlOfRed Jun 22 at 17:16
  • $\begingroup$ Interesting that the Dyke "delta" is actually a very low aspect trapezoid, and the "Flapjack" a very low aspect semielliptical. May be a better low speed wing. The Dyke has good top speed, but stalls at 70. But power would help both. $\endgroup$ – Robert DiGiovanni Jun 22 at 18:25
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The planned Vought XF5U "Flying Flapjack" fighter had what you're looking for, but with a top speed of 460mph and a 20mph landing speed. A turboprop version may have taken it to 500mph and nearly zero landing speed.

The prototype V-173 was built and flown and proved the concept sound. It had much smaller engines so its performance was significantly worse, only 140mph top speed with a 40mph landing speed. At one point it came out of an emergency landing on a beach unscathed.

The concept was abandoned because jets appeared. The military was interested in the top speed of the design, which was higher than that of conventional piston planes. They were happy to deal with jets' much higher landing speeds in favor of significantly better top-end performance.

Flying Flapjack prototype V-173

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The Dornier Do-29 seems like a really good contender.

Weird German plane

It's a tilt-rotor type, which could swivel its engines downwards to improve STOL performance down to a mere 15kt stall speed. Its maximum speed of 157kt is a little short of what you want, but no doubt this could be improved with modernised engines and a bit of aerodynamic work.

Only two prototypes were built, in the late 1950s. Flight tests were successful, but the concept was not further developed in that form. One survives in a museum.

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  • $\begingroup$ Absolutely. Tilt those rotors up, make props bigger. (someone may have thought of that). $\endgroup$ – Robert DiGiovanni Jun 21 at 18:38
  • $\begingroup$ I think the props are already the maximum size for ground clearance, given that this is both a taildragger and a pusher. The engines are of course returned to horizontal for cruise flight; other photos imply they're linked to the flaps. $\endgroup$ – Chromatix Jun 21 at 18:44
  • $\begingroup$ They were probably into increasing lift by "curving" the air flow into a normally turbulent area at high AOA. The "Flapjack/V-22 Osprey approach is what I had in mind. Larger props are more efficient. Wanted to try tilt rotors on the Catalina too. $\endgroup$ – Robert DiGiovanni Jun 21 at 19:12
  • $\begingroup$ Making the landing gear retractable should bring it to 180 kts already. $\endgroup$ – Peter Kämpf Jun 22 at 10:31
  • $\begingroup$ The Flying Flapjack needs a high landing gear and high angle of attack. Recent RC scale models have demonstrated the concept. A variable incidence wing would help. Tilt rotors are an attractive idea, especially lightweight electric tilt rotors. Another successful tilt rotor that comes to mind is the Canadair CL 84. youtube.com/watch?v=q6SxyIoSvMM&vl=en 3000 hp needed, though. $\endgroup$ – stackex555 Jun 22 at 14:58
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Have you looked at Autogyros for your usecase? Or does it have to be "fixed wing" truly?

They tend to have extremely low stall speeds, need very short runways and are capable of reaching speeds near what you ask for with 180 kts. https://en.wikipedia.org/wiki/Carter_PAV

This thing is still in testing but allegedly was able to do 174 kts in trials. Another advantage of autogyros is that they're fairly cheap compared to traditional planes.

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  • $\begingroup$ Autogyros are great, if the rotors could be enclosed within the wings that would be great. $\endgroup$ – stackex555 Jun 22 at 15:00
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A huge challenge indeed, but then again so are cell phones. For low speed landings a low aspect ratio with slats and flaps is what you want. Making them fully retractable is certainly within the range of today's technology.

Next up would be a light and powerful source of thrust. Here the old PT6A turboprop fits the bill. The more power the better, and blowing the propwash over the wings and control surfaces is helpful too. Power would make your plane stand apart from old biplane designs, and make your top speed goal readily attainable, in addition to improving climb performance.

So your project would not be without cost, but you would wind up with a highly versatile aircraft resembling a miniature airliner. (You don't have to sweep the wings as much as they do). I would pursue it!

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  • $\begingroup$ Hi lift, yes and as I think of it, what about FBW and relaxed stability? That worked for the Mirage 2000 and reduced its approach speed by 50% if I am correct, to 135 knots. $\endgroup$ – stackex555 Jun 22 at 15:03
  • $\begingroup$ Deltas are a different kettle of fish for STOL as they can take advantage of vortex lift at high AOA. But they are inferior to straight wings in lifting capacity. Experience with the Delta Ray foamie helped launch me into a 2 year study of aircraft design. Reducing wing loading with deltas only adds to low speed woes in wind. Slats and flaps on a low aspect straight wing are what you want, retractable for higher speeds, like airliners. $\endgroup$ – Robert DiGiovanni Jun 22 at 17:23
  • $\begingroup$ But "relaxing stability" of forward set CG back towards nuetral will help take some load off the tail and downforce off the total lift the plane needs (good for STOL). But relaxing it to fighter plane level, no, not for this application. $\endgroup$ – Robert DiGiovanni Jun 22 at 18:15
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I originally asked the question:

If there is sufficient landing space, would it be possible in some cases to replace the helicopter with a small fixed wing aircraft with low stalling speed?

Looks like DARPA beat me to it: https://news.usni.org/2014/03/19/darpa-awards-contracts-search-460-mph-helicopter

Many great answers here: I can summarize the solutions available as follows:

  1. Add high lift devices to a 200 knot light aircraft, such as leading edge slats, double slotted Fowler flaps, blown flaps, canards and flaps such as the Robertson conversions, in-wing powered rotors or auto-rotating gyrocopter type rotors. (The last one is an idea I have not seen anywhere in print)

  2. VTOL type aircraft such as the Canadair CL-84 fits the bill very well: 15 passengers and 200 knots or more. A smaller aircraft with less power would work, perhaps with modern electronic autostabilization.

  3. Aircraft with low wing loading or variable incidence delta wings or low aspect ration wings such as the Flapjack. Variable incidence will solve visibility problems.

One option not mentioned are lighter than air hybrids, but I will not go into that right now.

Aircraft can match the high descent rates required for obstacle clearance (500 ft obstacle at 4000 ft from landing area), but land with too high a forward speed to stop within the helipad. It all depends then on the combination of how slow you can land the plane, and the landing roll required.

Taking off in a short space will require high power as well as a low take off speed, but low take off speed does not seem so critical.

The VTOL concept has been proven to be able to meet the requirements. It would be very interesting to see a mini V-22 or CL-87 and some of the newer electric VTOL proposals I think have this.

Without the 20 knot landing speed, the goal of landing in small spaces in an aircraft has been nicely demonstrated by this video I came across recently. Really amazing:

The 20 knot landing speed appears not be necessary, if the aircraft can land and stop within a short distance: this short distance is a variable and I would attempt several combinations of landing distances that will be useful for the aircraft operator. Basically I am looking for instances where the helicopter can be replaced by a STOL airplane. A piston single with high lift devices and a lot of power would be suitable with antilock brakes or reverse thrust.

Using a flight simulator has been useful for this purpose so far: I hope to do some testing and report. Here is one such trial:

Edit:

There are two ways to approach this: take the fastest two seat kit plans and add flaps and retractable slats, increase wing area or reduce fuel, or make the plane larger.

The Helio Courier is being developed as a turbine powered version: A re- arrangements of parts to make a sleeker aircraft will more or less result in what I am looking for:

http://www.helioaircraft.com/ourplanes_stallion.htm

The Stallion features a 750 shaft horsepower Pratt & Whitney PT6A turbo-prop engine. The plane is designed to be fully maneuverable and controllable at 37 knots and has a cruise speed of 175 knots.

Edit: It occurred to me that Light Sport Aircraft, with their stall speed limited to 35 knots, might fit the bill if they are fast:

This one with a further STOL kit would do nicely: Stall Speed with Flaps 53 km/h / 29 kt Max Cruise Speed 305 km/h TAS / 165 kt Landing Groundroll and Takeoff Roll 150 m / 490 ft

https://www.jmbaircraft.com/aircraft

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I didn't see anyone mention the rule of thumb that very few aircraft designs have a stall speed slower than 1/4 the cruise speed. You are asking for a design with a stall speed that is 1/10 the cruising speed. That's asking a lot.

So, let's back up a couple steps. Keep in mind that lift offsets weight in level flight. Angle of Attack and airspeed combine to create lift. Change the airspeed a lot, and you have to change the Angle of Attack a lot to maintain the same lift. Increase the Angle of Attack too much, and the wing stalls. Also keep in mind that when you change the Angle of Attack of the wing, you are also changing the Angle of Attack of the fuselage. One angle will have the least drag, like an arrow pointing into the wind. All other angles are shoving either the roof or the floor into the wind, creating a huge amount of drag.

Leading edge slats and exotic flaps can increase lift (and drag) at lower air speeds, and dropping the control surfaces to below the level of the wing gives you better attitude control at those lower speeds, but all of these things create extra drag at higher speeds, requiring a LOT of horsepower to pull the airframe through the air at those higher speeds, and the frame needs to be strong to take that kind of stress, and that implies more weight which again implies more power, or exotic materials and related construction techniques to keep the strength high but the weight low...

The point here is that such an aircraft would be very complex, very powerful, probably involve exotic materials (to bring the weight down while keeping it strong), and so would be extremely expensive. So, then, the question becomes:

"What is the use case that justifies the expense of such a design?"

Likely, the answer would be, "Some rich guy really wants a cool airplane that will do this." Otherwise, people will come up with far simpler, less expensive aircraft to do whatever it is they want to do with an airplane.

So, if your use case is, "I want to take off and land on a helipad," it would be simpler and cheaper to buy a helicopter and learn to fly it.

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Take a look at the Rutan Grizzly. If I remember correctly they called out 25 Kt. minimum fully controllable flight speed. I don't remember the cruise speed, but it was less than 200 knots.

The way I heard it the Helio Courier with its' geared engine etc. became sufficiently expensive to operate that helicopters became a lower cost commercial solution and the Courier went out of production.

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I can see one reasonable possibility here: fan lift.

You don't want to build a Helio Courier update and try to make it go fast in the air; you want to build something like a Bonanza or Skymaster and make it land really, really slow.

Lift fans in the wings, covered with panels or louvers when not in use, might permit this. For takeoff, you'd power up the fans and execute a "normal" takeoff, except you only need fifty feet of roll to build up the 20 mph for the wings to add enough to fan lift for takeoff; when landing, you'd power up the fans around the time you go to full flaps, slow down like making a STOL landing, and drop into the extended helipad space. Additional smaller fans in stabilizer and fin could add full three-axis control below what would otherwise be minimum control speed.

Whether this is practical, well, probably not -- but you're not trying to make a full VTOL like a navy F35; you're trying to augment lift enough to use a fifty foot runway in and airplane that can cruise like a fast GA design.

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  • $\begingroup$ Actually, a helicopter might be better than a fan, as it's high aspect slower rotor will be much more efficient and top speed will be not bad. But a fan is possible. $\endgroup$ – Robert DiGiovanni Jun 21 at 15:29
  • $\begingroup$ @RobertDiGiovanni I won't argue that a helicopter is more practical. The OP is asking for alternatives. $\endgroup$ – Zeiss Ikon Jun 21 at 15:45
  • $\begingroup$ The fan would be good if there is lots of power (just like jets, no?), so the F35 does apply. But for 20-200 mph it is 3rd choice. A large tilt rotor could work too! $\endgroup$ – Robert DiGiovanni Jun 21 at 18:34
  • $\begingroup$ Fans can be run off a battery similar to the one in a hybrid or electric car, and recharge from an added alternator on the engine while in cruise or on the ground. As with hybrids, you get big-engine power for a while, with small-engine fuel economy. $\endgroup$ – Zeiss Ikon Jun 21 at 19:03
  • $\begingroup$ Excellent idea. Could have an electric fan for a short, high powered burst of thrust for STOL. I was into putting an electric on the nose of the JU-52 for the same reason. Definitely a possibility. $\endgroup$ – Robert DiGiovanni Jun 21 at 19:17
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The Westland Lysander was a WW2-era aircraft with about your requested top speed and with a reputation for excellent STOL performance, though its stall speed is actually about 60kt. It was used for inserting and recovering spies in occupied territory, in which the ability to take off in less than 1000ft from some random field or beach was extremely valuable.

Lysander, showing extended flaps & slats

Another WW2-era aircraft in this performance range was the Gloster Gladiator, the RAF's last biplane fighter. It had a stall speed of 46kt with flaps extended, and a maximum speed of 220kt, making it capable of cruising at around 180kt.

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One possible way to approach this is to take an example of an actual larger aircraft (4 or 6 seater) that meets the 200 knot requirement and fit a smaller 2 seat cabin and reduce the weight.

If we replace the large cabin with a smaller one and use the same wings, tail and engine and systems we could end up with a lighter, smaller plane that will do 200 knots, and will take off and land slower.

But can this aircraft land at 20 knots? The aircraft in question is this one:

The Spectrum SA-550.

https://en.wikipedia.org/wiki/Spectrum_SA-550

Maximum speed: 250 mph (402 km/h; 217 kn) at 15,000 ft (4575 m), Cruise speed: 250 mph (402 km/h; 217 kn),Stall speed: 66 mph (106 km/h; 57 kn) flaps down, power off

So we have the 217 knot cruise. In order to lower the stall speed, the wing loading has to be reduced, since the stall speed is proportional to the wing loading, we calculate the stall speed at empty weight + 2 passengers and fuel as 500 lbs.

Stall Speed (new) =

Stall Speed at Old Weight x SQRT(New Weight / Old Weight)

This gives 45 knots stall speed. To get the magic 23 knots we would have to reduce weight to 800 lb, while the weight limit for LSA aircraft is 1320 lb. With retractable leading edge slats and full span flaps, this would bring the stall speed down further. At the LSA weight limit, the aircraft in question would give 29 knots as a stall speed.

Best look for an LSA that can be fitted with a turboprop, which is lighter, but uses more fuel. A sleek fuselage, mid to high wing and retracts and we are done. In theory.

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