Replies to This Discussion

Permalink Reply by Lawrence Bird on July 5, 2016 at 7:54am

<Mine lands best by keeping just a touch of power in and flying it to the ground…  Like to hear from anyone who might have tinkered with this…>

 

Don’t know about the half-n-half scenario, but this is one of the things I’ve always wondered about – if memory serves the very earliest CH701 brochures and info packets (used to come in a 3-ring binder -- still have one somewhere) essentially described dragging her in under slight power – the flat cabin top was described as helping to facilitate this.  I’m reading into this, but back in the UL days is seems that the mission was more to land and takeoff in the boondocks of Ooki-Booki-Stan rather than the chopping the power and standing on the brakes on pavement at Valdez where the whole landing profile changed.

 

Experimental aviation is an interesting place – CH701/750 builders are busy improving their birds by removing the slats and the Cub guys are busy installing them… makes me say; hmmmm…

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Permalink Reply by John Austin on July 5, 2016 at 8:37am

I've got hundreds of hours with Stolspeed VG's and no slats on my 750 (I also tested it with slats, too!). You definitely gain cruise speed minus the slats - I saw about 8 kts increase. In any other aircraft, a 10% increase in cruise for "free" would be extraordinary!  Without slats, the 750 lands much more "conventionally" in that it penetrates better and doesn't suddenly quit flying with low approach speeds. A simple release of back pressure gets you flying again. I really think you'd have to get out a tape measure to see much difference in the landing and take-off rolls.  Chris Heintz even states that he has "no problem" with removing the slats and installing VG's (Slats vs VG's), but from my flight experience I think he overstates the slat's benefits and understates the increased performance with VG's.

On the positive side for slats, it does give you more extreme approach and departure angles - very useful for short, obstructed strips. It also is such a distinctive feature of the plane it does set it apart from all others and that's the only thing I miss! However, one has to consider their normal flight profile - I've got a 2100" strip and I'm flying to other turf strips that long or longer and paved municipal runways, so I really don't need "superSTOL" capability.  However, I do enjoy low and slow and the 750"s unparalleled visibility and off-runway capability definitely enhances safety!

Experimental gives you the freedom to try different things, but I'd say just realistically look at your typical flight and either leave 'em on or take'em off - and it doesn't have to be permanent -  it's not that difficult to take them on and off as desired as long as you've tested and documented both configurations with Phase 1 testing and have the appropriate W&B. Of course, I always add my disclaimer that I'm not advocating that you take them off, just relating my experiences!  ;>)

John

N750A

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Permalink Reply by Lawrence Bird on July 5, 2016 at 10:27am

John; in your experience how does the AOA change for stall (or the speed where the wing can no longer support level flight).  I’m leaning heavily toward the slatless config -- made up the templates and rib form block; just haven’t worked up the courage as I don’t intend to build two wings.  However, my likely airports are exactly what you describe…  Am I correct that the “stall” AOA is the same or less than with Slats?

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Permalink Reply by John Austin on July 5, 2016 at 10:42am

That's a difficult comparison ... with slats, there is no definitive stall - the plane just enters a controllable "mush."  Further increases of AOA just increase the descent rate, so it would be very subjective to identify a specific stall angle.

Without slats, the plane stalls very conventionally - there is a pre-stall rumble and shaking (the plane's trying to tell you you're doing something stupid! Haha!) and a clean break.  Recovery is easy with release of back pressure. My plane had a tendency to drop the left wing which was easy to pick up with a bit of rudder.

Without slats on a departure stall, the AOA is ridiculously steep - you feel like you're nearly vertical!  It would be hard to unintentionally get into this attitude and there is plenty of warning from the buffeting. Without slats and no VG's, departure stalls occurred at 25 kts indicated (of course the airspeed is unreliable at steep AOA) and after adding Stolspeed VG's, the stall dropped to 20 kts indicated.  The VG's appear to have no adverse effect on cruise speed.

John

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Permalink Reply by Lawrence Bird on July 5, 2016 at 1:03pm

Thanks – that’s what I was hoping to hear…  I had a link to Joe Spencer’s slatless results, but lost it and  the last I saw he’s off building his own super-bird of a different flavor these days…

-- Larry

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Permalink Reply by John Austin on July 6, 2016 at 8:01am

Speaking of "different flavors," seems to me that someone out there should come up with a new retractable slat design - that would combine the best of both configurations! I understand there was a Pegastol wing for the 701 that used retractable slats (manually deployed?). However, Just Aircraft's hinged, automatic slats appear they could be easily adapted to the 750, but I don't know if the wing would have to be re-profiled to be optimized for the slats in retracted position? (Now I'm getting waaaay above my pay-grade!) Just thinking out loud!

 

John

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Permalink Reply by Lawrence Van Egmond on July 6, 2016 at 10:42pm

I built my 750 without slats and used VG's and am pleased with the results. I do not get a break at stall, it just just starts to sink wings level. With full power and the stick all the way back it just slowly climbs at what seem like about 20 miles an hours. 

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Permalink Reply by Samuel McNair on July 10, 2016 at 10:57pm

For what it is worth I have an 801. I Put STOLSpeed VGs on, removed the slats and picked up about 12+ knots of cruise. With careful measurement I found that my stall speed increased by 1.5 to 2 knots. And the addition to TO & L roll is around 50-75 feet, a small enough change that I had to run a lot of tests to really get a repeatable measurable change. Rate of climb improved. And stall is more gentle and predictable. Mine has a cargo pod and still retains enough weird factor without the slats to draw a crowd most places I stop. VGs on the elevator helped keep the nose up. If you have round strut tubes, the fairings make an incredible difference in speed (~8 knots) and give about 100 fpm increase in ROC due to decreased parasitic drag.

When I got my 801 with slats and round struts, it struggled to make 78-80 knots TAS. With no slats, strut fairings, the pod on (which streamlines the gear even though it add 3.5 ft2 of frontal area), a nose strut tube fairing it now cruises at 101 KTAS at 75% with the doors on and with doors off and all of that turbulent air in the cabin it cruises at 85 KTAS at 65%. The drag is so reduced that when you chop power it no longer throws you forward against the seat belt and shoulder harness.

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Permalink Reply by Lonny M. Kirk on July 11, 2016 at 7:20am

Your info supports what came as a surprise to me. My plane came with strut fairings. my first flight I was doing 90 MPH at 4,800 RPM (5300 redline). I installed VG's on the elevator and with a bit of throttle have no issue with keeping the nose off.

I have not had the opportunity to do more flight testing but my bird was much faster than what I took from everything I read.

I have a short strip over 40' obstacles and that may be the one thing the slats do for you, but I will see how it goes. I am leaning toward slat removal if I get comfortable in and out of my strip.

thanks for the reply.

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Permalink Reply by Walt Snyder on August 23, 2016 at 12:04pm

Interesting that Carbon Cub is trying out a half slat as mentioned. Re fixed vs retracting, the serious STOL guys seem now to go with fixed, though there were a few with floating trailing edge. The retracting is more complicated and heavier, and weight is everything to some in serious STOL, so perhaps that's why. 

Walt Snyder

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Permalink Reply by Lonny M. Kirk on August 23, 2016 at 2:18pm

This looks like what I had envisioned. I thought to keep the slats inboard and remove them outboard where the cleaner flow is.

I did not know about wanting a wing root to stall first. I assume this is a factory setup and if so what do they know that apparently we don't?

Retractable slats have got to be the better option but I wondered if this might not be a compromise that kept some of the step angle of approach while picking up some in cruise speed. 

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Permalink Reply by Samuel McNair on August 23, 2016 at 12:30pm

I had an 801 and did extensive tsets with slats on and off. The results totally agree wirh the results that John got: landing distance, stall speed (1.5 to 2 knot difference), cruiSr improvement was closer to 9 knots, stalls much gentler.

Observation, if you look at the videos on the Savanna Web site (701 clone with slats ir no slats) ir clearly shows the benefits of the "beanie mod" smoothing out the airflow and getting lift off of the cabin roof. It us next on my mid list after airfoil lift strut covers which have HUGE cruise and ROC impact.
Next observation. Unless that Cub had huge washout in those wings, he is real likely to get killed. Everyone in the STOL business and conventional AC knows that you want the root to stall first and the tips last. Otherwise you have a stalled wing and no roll control. If you don't belive me that the slats belong outboard look at the integral ones on the wings of a Stinson or the retractable ones on a ME 109. I may not be an aerodynamicist, but those guys were.
Anyone ever seen design details for retractable slat mechanism?

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