slots in wings

[Woody]

Does anyone have info on closing the slots on the wings?  Does it produce better handling, landing or higher cruising speed?  I understand there are some with the slots covered but why?

[Bill Poynter]

I've requested information on this subject from Neal LaFrance.  He certainly should be knowledgeable on the effects/benefits of the slots.  His Cadet STF doesn't use slots.  Since he had previously owned stock Cadets,  I guess he had his reasons for removing them from the STF.  I really doubt that any benefits would be worth the trouble of getting the mod blessed by the FAA though. 

[pitts2b]

I have ask a swift buddy that has flown with and without the leading edge slots. He says that it adds about 5 or 6 knots but kills the low speed handling of the plane.

[Woody]

My understanding is the Culver wing is stiff so to keep the outer wing flying at low speed you need the slots.  Most wings are flexible which work at lower speed.  The Culver has a spar that is almost as big and as strong as the Stearman so no flexing. I've read that closing the inner slot will increase speed and keeping each slot the same size give a good level wing.

[Brett Lovett]

My understanding is that the prototype Cadet did not have wing slots.  As I recall from Foster Lane's book, the wing slots were added in an attempt to solve a spin recovery issue.  He thought the airplane handled better without them (if I recall correctly he stated that the slots gave the aircraft a slight dutch rolling tendency).  I suspect that the stall characteristics of the elliptical tapered wing without any washout or slots would be rather abrupt (the entire wing stalling simultaneously rather than a gradual root to tip stall like most modern aircraft with washout). 

I'd really be curious to the difference made by the sharp leading edges, which were also on all Cadets but the prototype until removed by Service Memorandum 21.

[Bill Poynter]

Brett,  I spoke with Neal LaFrance about the slots.  His comments were pretty much in line with yours.  I know he did several things to clean up his STF in an attempt to get more speed.  Since he didn't have to satisfy the CAA/FAA with stall/spin performance, I think he figured it was a good trade-off for the speed increase.

Do you know if the sharp leading edges covered the entire leading edge or were they just a fairly short stall strip? 

[Brett Lovett]

Bill, I've only seen a couple of photos where the sharp leading edges were at all visible, and that is the only source I have to suggest where they were located.  Based upon those, I believe the sharp leading edge spanned the leading edge from the rib at the outboard edge of the wing walk, to the rib defining the inboard side of the slots (approximately).  Apparently the idea was to both assure airflow separation for the entire wing inboard of the slots at high angles of attack, and assure attached airflow over the wing tips (through the slots) at the same time.

[Neal LaFrance]

Hi Cadet pilots, Neal is back again to talk about slots on the Cadet airplane. Al Mooney did not designe slots in the wings. During CAA flight test slots were cut in to the wing.  All light aircraft designed in this time period had to have slots. CAA test pilots would not get into an airplane with out slots.
CAR part 23 was the culpert. Read the part on stall spin recovery. Have you seen wing slot in any modern day airplanes. Read FAR part 24. Aerodynamics have come along way in 70 years. The cadet fly’s so much better with out slots. Drag is reduced allowing better air flow over the wing.
Try 15 degree turns with out rudder. Speed increased 4 to 5 knots, airplane approach to stalls very good, Landing speed is decreased, great for short field  landings. You can cover the slots  on the home built Cadet.

[Brett Lovett]

All light aircraft designed in this time period had to have slots. CAA test pilots would not get into an airplane with out slots.

In looking for single engine monoplanes certified in 1940 (same year as the Culver) and 1941, I'm finding only a few were certified with wing slots.  The Howard DGA-15J, Ercoupe 415-C, Porterfield CP-65, Piper J-4F, Piper J-5A, and Aeronca 65-TC were all certified in 1940 without wing slots.  The Rearwin 175 is the only other one I know of with wing slots certified in 1940.  In 1941 the Timm 2SA, Harlow PC-5A, Interstate S-1A, Howard DGA_18W, Piper J-4E, General G1-80 (Skyfarer), Bellanca 14-12-F3, and Taylorcraft DC-65 all without slots, only the Stinson 10-A and Culver LAR-90 with.

[Woody]

Just to be clear ~~~ By covering the wing slots I can gain four to five knots in cruise speed, lower the landing speed and shorten the take off and landing roll out and stop the tendency of the dutch roll??  And does this change the handling of a stall?  There is always trade offs.

[Bill Poynter]

I've seen some Globe Swifts with the slots covered, and I found this discussion regarding slots, on the Swift website:

From: Richard G MacInnes <rmacinnes@juno.com>
Is there someone out there who has a real life experience with closing the wing slots only. I've heard so many stories but they are usually "that won"t work" or "it's too difficult to handle" etc. or they did the mod in conjunction with other, more dramatic changed.( Like horsepower, canopies, fillets, small wheels, stall strips.etc.) Just looking for info, not opinions. Dick MacInnes N80923

Dick,
I don't feel there was a whole lot of engineering involved with the wing slots. At Denton, TX in 1979 someone asked Bud Knox why the Swift had wing slots, he answered, "because the Culver Cadet had wing slots." In other words, it was the "in" thing to do for a while. The Stinson and some other WW2 types had wing slots also in that era. I have flown Swifts both ways, and don't see a whole lot of difference. I closed them (temporarily) on my first GC-1A in 1966. It didn't go any faster, or seem to make much difference in the stall either. I flew a friends Swift once that had closed slots, Buckaroo wing tips, and the stall strip removed. That was the only Swift I ever flew that I can say had a mean stall. Gear and flaps down, it would hang on to about 45 mph and in the stall it would break sharply, if it were accelerated at all it would break inverted. Not good! Thankfully, he took my advice and put a stall strip back on. I have flown big engined Swifts with closed slots and think they might gain 2 or 3 mph. I don't see much difference in the ailerons. Maybe these are just opinions! -- Jim

(Swift expert Don Bartholomew also replies...)

Hi All,
Here are my personal experiences with slots. First, I have no experience with closed slots on a stock airplane. The first plane I flew with closed slots was a plane similar to my own. It had a 150 Lyc and half stall strips. What I looked for when I flew it was to check aileron response at stall. I found with both planes, mine open slots, other closed slots, that aileron response was good down into the stall buffet. I didn't see any detectable difference in that respect.

I have never taken a plane and just closed the slots without doing anything else to see if there was any speed change, either in cruise or at stall. I have closed the slots on 210 hp planes with half stall strips and machined stall strips. I had no experience flying these planes prior to slot closure so I can't give any before/after comparison. The planes with the slots closed seemed to fly fine.

It has been said the slots were installed as handles to drag the plane around on the ground. They can be effective for this. If the slots are closed, it is harder for two people to each grab a wing and move the plane.

Dick, this probably doesn't answer your question, and may cause more confusion, but this has been my experience. I am open to other thoughts on the subject. Don and Helo

(Editor's note: Jim mentioned Bud Knox. For those of you new to the World of the Swift, Bud was Chief Engineer for Globe Aircraft. He was still around in the late 70's and early 80's so he was able to answer a lot of questions from Swifters regarding why things were done on the Swift the way they were. Now... not to disagree with what Jim recalls about his conversation with Bud Knox, but... In 1985 I asked Bud why the Swift has slots and he said that they didn't want to take the time, tooling, and effort to put "twist" in the wing to keep the outboard part of the wing flying and ailerons effective while the inboard part was stalling. They felt that the slot was simpler. Bud also said that in his opinion it didn't do any good, drag reduction wise, to cover the slots unless the cruise speeds were gonna be over 170 mph.)

[Woody]

Thanks Bill.  This is good info for me!
Now I'm wondering why the vertical stabilizer is so far forward of the horizontal stabilizer?  Does that have anything to do with adverse power on stalls?

[Bill Poynter]

If you look at the design of the PQ-14 and the Culver V, you'll see that they have the same arrangement of vertical and horizontal stabs.  I'm guessing that it has more to do with achieving structural strength than improving aerodynamics.

[Paul Rule]

One comment on my experiences with the slots...

If you are doing repairs on your slots, work hard to get then the same LH and RH.  I flew an airplane that came out of rebuild that had a measurable difference in the slot size.  If you were not looking for it you would miss it but because of it's handling in flight I got out a tape and started comparing LH & RH.  The slots were 1/4" to 1/2" more open on one side (variable spanwise).  Result?  Real wing heavy on that side!  Didn't fly it enough to install an aileron tab but I am not sure a normal sized tab would have been enough to correct it. 

So, what did I learn from that regarding performance and / or drag??  Learned this -  it does make a difference.  More slot = less lift = higher AOA required = more drag.   Is it measurable in day to day flying or when comparing airplane "A" to airplane "B"?  Probably not unless we take REAL carefull measurments and get kinda scientific about it!!

[10kDA]

Now I'm wondering why the vertical stabilizer is so far forward of the horizontal stabilizer?  Does that have anything to do with adverse power on stalls?

Spin recovery. The horizontal will not blank the rudder when it's that far back; likewise, many other airplanes have the horizontal forward and higher, leaving a fairly large area of rudder un-blanked below the horizontal.