Slats vs no slats: Flight test

[tedwaltman1]

Backcountry SQ-2. For grins, I took my slats off yesterday. I’m thinking of making carbon fiber slats (because I can). The possible carbon fiber slat project is another discussion. Yes, I know Randy at Carbon Concepts makes CF slats--I'm exchanging emails with him.

I flew around testing low 50% power cruise, high 76% power cruise and multiple stall configurations, including slight cross control, without slats.

Seems to be about a 3 to 5 mph improvement in cruise speed at 50% power. More like 4 mph.

Maybe 7 to 9 mph improvement (at most) at 75% power. More like 7 mph.

Costs maybe 3 to 5 mph in higher stall speed with higher deck angle (maybe) and definitely a more pronounced, but still minor, stall break.

I definitely thought I’d see a much higher improvement in cruise and similarly more of a loss in stall performance.

Overall, not enough of a cruise improvement to get worked up over. Chief advantage of slats, to me, is flatter deck angle at stall, mush rather than break at stall and much better (safer) cross control stability (not that I plan to approach landings cross controlled).

If anyone has any test #’s without slats vs with slats please let me know.

 

[mvivion]

Interesting report, Ted. I too would have guessed the differences were larger.

MTV

 

[gpepperd]

Ted, your numbers are spot on with what I have found as well in my testing. You didn’t mention rate of climb penalty which seems to be around 200-300 rpm to me. All in all, I can’t imagine flying without slats unless it was for a very long cross country, like Alaska to the lower 48.

 

[Steve Pierce]

I liked the fact that the airplane was more stable when heavy, felt like it was light and flew that way. Huge improvement for maneuvering with a load in bad air.

 

[kestrel]

Chief advantage of slats, to me, is flatter deck angle at stall, ...

This statement caught my eye. My understanding is that what slats do is prevent stall until a much higher angle of attack. The classic "slat landing" photo is of some sort of Cub with the tailwheel about to touch the ground, but the mains still 3 feet in the air?



Example:
https://www.youtube.com/watch?v=RjqewSWG9k4

 

[rotto789]

For me, the insurance policy that comes with the installation of slats far out weigh the speed penalty, at least for my mission. I've watched and read about many incidents and accidents that resulted in damage or loss of the AC and/or life, I know slats would have saved both in some instances. That benefit alone is worth their weight in gold. IMHO

 

[stewartb]

This statement caught my eye. My understanding is that what slats do is prevent stall until a much higher angle of attack. The classic "slat landing" photo is of some sort of Cub with the tailwheel about to touch the ground, but the mains still 3 feet in the air?



Example:
https://www.youtube.com/watch?v=RjqewSWG9k4

At an airspeed you’re familiar with for slow flight with a stock wing, the slat wing will fly flatter. Same as with VGs. That the slats allow you to fly with solid control 15 mph slower than the stock wing stall speed is when you can demonstrate higher AOA, especially with lots of power on. I don’t prefer to land that way. Another factor, the plane in your link has stock Cub flaps. Most of us have longer, deeper chord flaps. Some have double slotted Keller flaps. Gpepperd has the ultimate…. 12’ Kellers. Flaps matter!

 

[vj88]

How about a comparison in turbulence. Ive heard that slats increase movement feeing in turbulence. I have turbulence conscious passengers!

 

[stewartb]

There are other factors at play for most slat airplanes I know, primarily wingspan. Mine is a couple of inches short of 40’, so about 4’ longer than a standard Cub. That changes wing loading and makes the airplane more reactive to the wind.

Back when I had my -12 my wife and I flew home on a very rough day in my Cessna. I sent her home and went right back out in the -12 to see how it responded to the day’s rough air. I think airspeed is the key. The Cessna bumps felt sharper, more abrupt. The -12 just got pushed around but with less jarring. Both planes were lightly loaded and in both cases more weight (wing loading) would have made things more comfortable.

 

[brown bear]

A cleaner / faster plane will probably lose more speed with this type of slats
it’s about % of drag
I have over 2500 hours of slat time and love them ��

 

[skywagon8a]

Maybe 7 to 9 mph improvement (at most) at 75% power. More like 7 mph.

That's about a 10% improvement in cruise speed ... not bad.

Overall, not enough of a cruise improvement to get worked up over.

Consider this..... you are cruising across some remote section of Canada on your way to your favorite Alaska destination. You're low under a cloud deck in a valley and the head wind increases. When a small wind increases in a valley Bernoulli's law says the sides of the valley will cause the wind speed to be greater in the valley than that above the valley. A 5 mph wind increase could translate to 10 or more down low. How's your fuel supply? A slow airplane is more effected by a headwind increase than a faster plane. Take it from someone who found himself in that condition while flying a 185 with over 100 gallons of useable fuel. Believe me, the pucker factor gets high. If I can do anything to make my cruise speed higher by reducing drag, I'll do it.

Since you are talking yourself into making a new set of slats, consider making them retractable or foldable to clean up the wing for cruising.

 

[stewartb]

Ahh, that reminds me of a conversation I had with gpepperd that convinced me to buy my Rev. If forced down in a Cessna you hit the ground at 60mph. If you’re forced down in a slat wing Cub you hit the ground at 20mph. Having that slow speed capability is the best safety tool I could want. It opens up landing areas unavailable to a Cessna. In the worst case, a controlled crash, 20mph is more forgiving than 60mph. I like my slats.

 

[skywagon8a]

If your slats are retractable you will have enough extra speed to make it to the gas pump thereby avoiding the need to be forced down while using the slats. You may have a safe landing, but have a long walk for the gas.

 

[stewartb]

Sat phone. I have friends.

 

[skywagon8a]

Sat phones and friends won't be of any help in some of the remote places I've been.

 

[BC12D-4-85]

Might be able to just tie them down if moveable or at least fill the gap with a plug when cruise is desired

Gary

 

[stewartb]

Pinning mine is simple. I have no desire to fly with them pinned. That’s only a thing for guys that don’t have them.

 

[BC12D-4-85]

By "pinning" does that close the gap? I'm thinking for a long flight where cruise range was primary over STOL. How much does pinning add to cruise?

Gary

 

[stewartb]

I have slat locks that lock mine closed. In cruise that would make about 1/4” difference. I have no idea what the speed difference would be, and don’t care. My Cub with slats is an easy 7-8 mph faster than my old -12, which was faster than my friends Cubs. And with good fuel injection I can run it lean of peak at fuel flows the carb guys can’t use. An airplane is the aggregate of systems. Set them up to suit. Fun stuff.

 

[BC12D-4-85]

Also I guess if they are 4 hour airplanes then it might not make much difference - 25 more miles or so. Make it 6 hour or more plane then I'd think about cleaning it up some.

Gary

 

[stewartb]

If it was important I could stretch my 48 gallons to 7-8 hours. Guys with EFI can do better.

 

[BC12D-4-85]

A MPG vs speed, power, and EGT setting graphs would be useful. There's a couple of SQ's up here. One has a belly tank, the other not. New pilots but seem enthusiastic about the airplane.

Something like these> https://www.youtube.com/watch?v=G5-OyZDytCY

Sometimes endurance is as critical as range.

Gary

 

[rotto789]

By "pinning" does that close the gap? I'm thinking for a long flight where cruise range was primary over STOL. How much does pinning add to cruise?

Gary

yes it does close the gap, I've pinned my (Mackey aluminum) slats down and tested for cruise speed gain, my slats run open about 1/2 to 3/4 inch in cruise, with them pinned down I noticed little to no appreciable gain in cruise speed, at least on my AC it doesn't seem to make much difference. one thing though, if you are going to pin your slats down to see if it makes a difference, make sure to pin all the panels down at the ends and where they they meet, if you only pin them where they meet in the middle you will realize a rather expensive mistake. I know a guy who made this mistake and it wasn't pretty.

 

[BC12D-4-85]

What do folks with slats recommend for best glide speed or configuration? Is it the typical split between Vx and Vy? Have you determined Vx and Vy for your Experimental? I see the potentially low landing impact as an advantage, but just curious about how they glide plus rate of descent with reduced power?

Gary

 

[tedwaltman1]

My Vx --with slats-- seems to be 50 mph as near as my test flights can determine. That's with 1 notch of flaps.

With the throttle at full idle, if I pull full flaps, peg the airspeed at 32 mph, it sure seems like I'm --almost-- coming straight down (in a flat AOA orientation that is). That is with FULL aft stick! So, to flair, I have to add power when at this configuration.

With power, at a high-AOA, with full flaps, as noted by others, I touch down in the low 20's--hard to tell as the airspeed is of course inaccurate at that indicated speed. Plus I can guarantee you I'm not focused on the airspeed indicator if I'm on very short final in this configuration!

 

[BC12D-4-85]

If power quits what's the slat ops equipped procedure for the resulting glide and terrain contact? Just because your relative airspeed is slow your angle and rate of descent still has a significant vertical component.

Here's the BC VIII models printed specs. Looks like Vg might be ~mid-60's. I assume clean configuration and prop full decrease?:

Capacities
Fuel: 59 gallons
Oil: 8 quarts

Performance
Cruise Speed: 115 mph
Stall Speed: 18 mph
Rate of Climb: up to 2000 feet per minute
Endurance: 5 hours (112 mph cruise)
Range: 560 miles with one hour reserve (~9.8gph?)
Fuel Consumption: 8 gallons per hours (minimum LOP?)
Takeoff Distance: 47′
Landing Distance: 28′

Speeds
VNE – 139 mph never exceed speed
VsO – 18 mph stall speed with flaps down
VS – 26 mph stall speed with flaps up
VFE 10 – 60 mph maximum speed 10 degree flaps
Vo – 90 mph maximum operating maneuvering speed
VX – 55 mph best angle of climb speed
VY – 72 mph best rate of climb speed
​
* The above Specifications may vary.

Weights
Empty Weight: 1350 1450 lbs Varies with Bushwh

 

[RaisedByWolves]

How about cht and oil temp increases with the slats? No one that spends 10k on a mod is going to say it’s the worst I’d never do it again.


Sent from my iPhone using Tapatalk

 

[stewartb]

I don’t think slats affect engine temps at all.

When discussing slats there are other factors to consider. Wingspan. Wing chord. Flap length. Flap type. Flap chord. Fuselage length. These all change the way a plane flies. For most of is with slats, our pireps are about more than just slats.

Last I checked a Back Country slat kit was $4K.

 

[BC12D-4-85]

Helio Courier recommends in one of their documents - for a full flap no power landing (see here page 16) a glide speed in the lower-mid 60's to 10-15' AGL, then a quick flare to 3-point. Their flap system (hand crank or electrical) is relatively slow to deploy so I guess they want them out early. With three hands onboard it'd be easier. The slats are automatic as the leading edge airflow separation point moves aft under the wing. Just something to think about if flying a STOL aircraft if the power quits.

Gary

 

[hotrod180]

..... No one that spends 10k on a mod is going to say it’s the worst I’d never do it again. ......

That's the problem with researching mods via pireps,
and that's often the only available info, other than claims by the seller.
I'm not saying it's the case here, but no one ever wants to admit,
even to themselves, that an expensive mod didn't do perform as desired.