Cowl-flap as speed brake?

[Eric Witherspoon]

I was remembering a belly-board, speed brake seen on a Velocity.

As Wes mentions, on a tractor prop, there's disturbed flow to worry about. That's probably a good reason these are more commonly seen on pusher configs. Also, canard-configured aircraft (generally) don't have flaps, so the need for something to increase drag may drive more designers/builders/operators of those types to want to add the additional hardware. Bob - I'm not sure what you are working on, I'm just saying that there's reasons you saw a control surface on a canard-configured airplane that don't translate to "more traditional" configurations.

[bwilson4web]

This might help explain what I was thinking about:

IF I decide I need a speed brake, my cowl flap will be a little longer and extend beyond 60 degrees.

The whole idea is to increase drag so the plane will descend at a steeper angle. But this is not something I have to do in the first version other than to make sure the linkage supports a larger extension. If the plane needs more drag, a longer replacement and/or a perforated plate (aka., SDB dive bomber) will work.

Bob Wilson

[steveinindy]

Bob, seriously....one has to be very careful with putting speed brakes, etc on aircraft because if you get it wrong you're not going to live long enough to sort it out. A strong enough pitch down (such as might occur in a mis-sized or over-extended speed brake) and you could be rendered unconscious or severely incapacitated as it doesn't take that much negative G to red someone out.


There's a very good reason this sort of thing on commercially built aircraft is extensively wind tunnel tested before they ever try it in flight. I have a hard copy of the wind tunnel test reports for the SBD's dive brakes (you'd be amazed the crap you stumble across from time to time). It's in excess of 100 pages long and is more boring than a French novel....just on the dive brakes. I say this as someone who does not want to hear of another friend (and as pilots, we're all brothers and sisters or at least friends regardless of whether always see eye-to-eye or have ever met) getting hurt or killed.

If you need to get down quickly (such as an ATC request), either use my favorite phrase ("unable") or request a 360 descent to give yourself more "room". A lot of pilots for some reason seem to forget that there's nothing wrong with making a 360 for separation or descent. The idea of "direct to" seems to override even basic piloting skills or non-linear thinking as a problem solving technique in this day and age of GPS and the magenta line of doom. Since we're not trying to avoid enemy fire while putting a bomb onto a Jap flat-top, I can't see any need for a "dive brake" on a light piston engine aircraft. I really don't mean this the way it's going to sound but there's really no polite way to put this: It's far better to learn how to properly fly the airplane as it was design than it is to go seeking engineering solutions to what is most likely a human factors problem. That's not to mention that it's a lot more fun because you get to fly while doing it.

[WLIU]

Here is the interaction between cowl flaps and speed brakes on an airplane that has both - When you want to descend fast you close the cowl flaps to keep the engine from overcooling and you extend the speed brakes to increase your drag. If you have ever seen and engine stop from being too cold (think winter extended glides at idle power) then you will want to keep your engine temps well into the green arcs on descents.

The two controls are used almost always oppositely at the same time.

Best of luck,

Wes

[bwilson4web]

Hi,

This is the aircraft, a Viking Dragonfly:

The center of gravity is ~58" from the prop or roughly between the seat-back and instrument panel.

The new engine configuration:


So the proposed cowl flap / speed brake will hinge just behind the radiator. In normal flight, it will be parallel to the air stream. In climb, slightly down to increase radiator cooling at climb speed, ~80-90 mph.

When over-extended down, there will be two forces:

• drag - significantly increased, it will have a moment arm of about 14"
• radiator drag - maximum airflow will further add to the drag with a shorter moment arm, ~7"
• lift - only from any static pressure increase, it has moment arm of about 35-40" and opposite to the drag force

Without flight measurements, it is too soon to know how these forces will balance but having opposite signs, there is a chance they may be trim neutral. Flight testing and careful metrics will find out.

I'm going with a water cooled engine with a thermostat. During engine warm-up and descents, the thermostat closes to preserve engine block heat. Even with the cowl flap / speed brake maximum open and maximum cooling air through the radiator, the thermostat controls how much, if any, goes to the engine block.

I learned to fly at Hyde Field that has power lines to the North and trees to the South. The nearest airport with hangars is Hartselle AL with trees to the South and the town to the North. Emergency landing fields are often worse with fences, trees, power lines, and all sorts of unexpected things to fly into on the way down. The cowl flap / speed brake provides another mechanism to steepen the approach in addition to a slip.

The operational scenario is:

• downwind - 80 mph, clear traffic, cowl to climb position.
• crosswind - 75 mph, clear traffic, monitor obstacles and keep well above
• final - cowl flap / speed brake to maximum, add power, trim 65-70 mph, clear obstacles
• approach to flair - adjust power to reach numbers at 65 mph
• wheels landing - pull power . . . brakes . . . turn-off . . . close cowl flaps

They are a safety feature to give additional, approach angle control to augment a slip if needed.

Bob Wilson

[steveinindy]

The cowl flap / speed brake provides another mechanism to steepen the approach in addition to a slip.

They are a safety feature to give additional, approach angle control to augment a slip if needed.

Assuming you do it correctly, otherwise it turns what should be an orderly glide to approach into a plummeting spine-splintering slam to the ground. That's assuming you don't pitch the nose over and end up with your engine in your lap.

I'm getting you are set on doing this and I've said all I can as to why there is a better than even chance that it will work out as nicely as you think it will.. If you don't mind my asking, can you PM me your address as I have a release form I would like you to sign before you pursue this any further.

[Frank Giger]

Now that I've seen the plane, I'm wondering why you'd worry about a speed brake for descents - that thing looks ready to slip like nobody's business!

Bring the throttle back, crank it sideways and follow the elevator down!

[bwilson4web]

Assuming you do it correctly, . . .

Not a problem, I am.

. . . If you don't mind my asking, can you PM me your address as I have a release form I would like you to sign before you pursue this any further.

<SIGH>Take this up with the moderators, I have.

Bob Wilson

[Flyfalcons]

Here is the interaction between cowl flaps and speed brakes on an airplane that has both - When you want to descend fast you close the cowl flaps to keep the engine from overcooling and you extend the speed brakes to increase your drag. If you have ever seen and engine stop from being too cold (think winter extended glides at idle power) then you will want to keep your engine temps well into the green arcs on descents.

The two controls are used almost always oppositely at the same time.

Best of luck,

Wes

This is the best way to describe why using cowl flaps as speed brakes is a bad idea.