What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

Downside of taking off with flaps?

swjohnsey

Well Known Member
I followed a couple of threads about taking off using flaps. I experimented with my RV-4 using flaps aligned with fully deflected aileron (15degrees?). I get noticeable shorter take-off run. Is there a down side?

I normally let it fly off at 65 mph and accelerate in ground effect to 100mph before climbing out at 120mph. With flaps I was coming off at 55 and accelerating to around 80 in ground effect before bring the flaps up and continuing in ground effect 'til 100 then climbing out at 120.

This is a RV-4, IO360 with Hertzel CS.
 
The only down side to taking off with flaps is forgetting to raise them.
One of the reasons I always do it is it becomes part of the routine.
I have an indicator light on my boost pump for the same reason. Once I have established climb and cleared obstacles, the boost pump is off and the flaps are up.
 
Generally I don't bother with take off flaps. A few times it has come in handy to pop off the runway if taking off in heavy or long grass, but suck them up as soon as possible to prevent heating up the engine during the climb out.
 
As Mel says, you need to remember to raise them promptly. RVs accelerate so well that it’s easy to blow right thru max flap speed before they’re retracted - even more so with this ‘accelerate in ground effect’ method.
 
The only down side to taking off with flaps is forgetting to raise them.
One of the reasons I always do it is it becomes part of the routine.
I have an indicator light on my boost pump for the same reason. Once I have established climb and cleared obstacles, the boost pump is off and the flaps are up.

But Mel, did you have a gear over speed horn On the -6 in case you forgot and left the gear down?! ;)
 
Short grass field with obst. at the end might be the perfect Vx use of Flaps 1.

On day one of Phenom initial the instructor said, "Of the five of you in here, three of you will overspeed the flaps on your first sim takeoff." He was right. And the RV speeds up plenty fast too.

99% of the time I'm Flaps 0 on RV takeoffs, but there's an after takeoff checklist to remind me just in case. Yes, I've forgotten <g>

Screen Shot 2020-07-27 at 5.35.09 PM.png
 
Last edited:
This is probably one of those things that for every pro there is a con.

I personally never use them on TO. Part of my reasoning is safety - if engine fails, I do not want the drag. Yes I want them down to land, but remembering to get them down is probably way more likely to happen in extremis than remembering to retract them in extremis.

Additionally I want all of the energy on the airplane I can get during take off, as soon as I can get it. The more energy I have during the early climb out phase In case of an engine failure, the more energy I will have to deal with the options

Where would you rather be 250’ off the ground as the runway goes behind you with a dead engine; with a clean airplane with 100 knots looking at places to go, or retracting the flaps at 85 knots grabbing your arse?.

Again, probably lots of pros and cons, but personally I would rather be through this critical phase of flight as quickly as possible.
 
This is probably one of those things that for every pro there is a con.
I personally never use them on TO. Part of my reasoning is safety - if engine fails, I do not want the drag. Yes I want them down to land, but remembering to get them down is probably way more likely to happen in extremis than remembering to retract them in extremis.
Additionally I want all of the energy on the airplane I can get during take off, as soon as I can get it. The more energy I have during the early climb out phase In case of an engine failure, the more energy I will have to deal with the options
Where would you rather be 250’ off the ground as the runway goes behind you with a dead engine; with a clean airplane with 100 knots looking at places to go, or retracting the flaps at 85 knots grabbing your arse?.
Again, probably lots of pros and cons, but personally I would rather be through this critical phase of flight as quickly as possible.

Flaps are not only for drag. They also produce lift. Why do you think airliners use flaps for take off? On the RV series of aircraft the range of 15° flap is pretty close to best L/D. At this point you are producing more lift than drag.

With proper use of flaps, you will be at a higher altitude when your failure occurs.
 
Last edited:
Don't know about the other nosedraggers, but on the RV-9A on a narrow runway, takeoff with no flaps can make the runway disappear. I always take off with flaps 10 for just that reason, and so that I'm always expecting the same thing.

I also practice instrument go-arounds, and use the same litany on takeoff: full power, 80 knots, flaps up. I leave takeoff power in for a bit to accumulate energy in case the engine goes poop, but if I waited till power reduction to retract the flaps, I bet I'd overspeed them. And with the constant speed prop, the -9A accelerates so quickly that there's little concern for the plane settling with flap retraction.
 
I only use flaps in my RV4 for takeoff when I have a rear-seater.
Makes it easier (& quicker) to transition from the three-point attitude.
 
I've used about 1/3 to 1/2 flaps for almost every takeoff, maybe 2200 of them so far. My flap switch is just to the left of the throttle knob, so I only have to move my thumb over to put them up once I see about 70-75 knots. I like the visibility over the nose better during the initial airborne phase, and the airplane is in trim during the transition from just airborne through 130 knot climb speed, without changing the trim position much. The liftoff speed is a bit slower, and comes a lot sooner with some flaps in.

On a go around, it will be very unlikely that I'll forget to raise the flaps, as it is a normal procedure for me (as others have mentioned).

Disadvantages: takes a bit higher groundspeed to lift the nose wheel, and a flight would need to be aborted if the flaps didn't come up.
 
I use flaps every takeoff, no exceptions and peg the speed at Vy for the first 30 seconds no matter where I am. That, typically, gets me high enough I can return to the runway.

My home strip is grass & gravel, short, on top of a plateau and surrounded by trees, exactly the place you do not want an engine failure. The faster I can get higher, the better the chance I have of being able to clear the trees and make it to a clearing or nearby road.
 
Thanks for the food for thought Mel.

Although the airliner example is certainly true, it is a bit apples to oranges to our case. Our fat short Hersey bar wings perform very differently than an airliner wing optimized for high altitude cruise. A no flap takeoff in a 767 would take a LOT of runway.

AFAIK L/d max is achieved flaps up. Never flew any best glide profiles with the flaps partially down. Yep you may produce more lift at the same speed with partial flaps, but you will also produce more induced drag.

Simple test might be timing successive take offs, 0 to 80 knots, 80 knot climb to 300’. There might be some combination of partial flaps that get you there quicker, but I suspect it will be within a few seconds either way.
 
Last edited:
Mark, Thanks for that link. Very interesting data, thanks Nigel.

Interesting that with that data, Nigel and a couple of other sage flyers stated a choice to do normal take offs from paved runways no flaps. I also suspect that most people use a higher airspeed than 1.2 Vs during climb out on TO. The nose attitude at 1.2 Vs would not be a fun place to be with a sudden engine failure after TO.

Again, many pros and cons to weigh here.
 
Have read this thread a couple of times.

I rarely use flaps except on grass. I have forgotten to retract flaps taking off on pavement once. The amount of trim needed is how I knew flaps were down.

Starting to think about using flaps more often to become more like Mel developing a reflex action check to make sure they are up.
 
Flaps are not only for drag. They also produce lift. Why do you think airliners use flaps for take off? On the RV series of aircraft the range of 15° flap is pretty close to best L/D. At this point you are producing more lift than drag.

With proper use of flaps, you will be at a higher altitude when your failure occurs.

Another good post where I would have used a LIKE button here if it were available instead of posting to say I AGREE.
 
Flaps

I quickly transitioned to using flaps during takeoffs in my -8, ...
it seems to reduce/eliminate the "skittering around" just prior to liftoff.
 
Disadvantages: takes a bit higher groundspeed to lift the nose wheel, and a flight would need to be aborted if the flaps didn't come up.

I'd move it to the ADVANTAGES. Wouldn't it be better to know your flaps aren't working as you depart your home base instead of finding out when you use them to land at another airport and then find out they don't retract?
 
Having forgotten to raise flaps on a number of occasions in a C-172 and Cherokee, I went looking for some help. I found that the GIFT acronym check immediately after takeoff helped a lot. G-Gas (fuel pump off), I-Indicator (correct heading), F-Flaps (up), T-Trim (climb speed).
 
Here's a little bush plane technique that works in the RV too: while on the ground, move your stick full deflection to one side. Look out the window at the down aileron. Now move the flaps down to match the aileron. This is the maximum lift position of the flaps. Any more flaps deflection than this and you'll just get more drag, but very little extra lift.
 
Why do you think airliners use flaps for take off?

Well, not sure that it applies to RVs... yes, flaps are partly extended on most airliners, but only as much as it takes to nicely cover the 2nd segment. The primary use to extend the flaps here is to extend the slats. These are superior lifting devices, and are usually extended together with the flaps, being actuated by the same lever. Even looking at the flaps only, one look at the double slotted fowler flaps of an airliner reveals that the analogy might as well stop here.

The flaps on the -3/-4/-6/-8 are just plain flaps, meaning they are more drag producing than anything else. Nigel Speedy has provided some very good data (again: https://vansairforce.com/community/showthread.php?t=134158) and, having just flown similar tests in my -6, can only echo his measurements and observations.

Conclusion, on RVs using plain flaps (no slot between the flap LE and the wing TE), the use of partial flaps on take-off offers a slightly reduced take-off roll thanks to the increased ground effect. This can be used on bad surfaced fields when the aircraft can levitate off the ground at speeds as low as 35-40kts, then let to accelerate in ground effect to Vx, before rotation/flaps retraction/climb speed schedule occurs.
 
After reading through all the posts I've decided to make a 15 degree flap setting my standard take off tail low. My RV-4 flies off at around 60 mph, I keep it in ground effect and bring the flaps up when I hit 80 and keep it in ground effect until I hit 100 then let it climb out at 120. All this happens very quickly with IO-360 with CS prop. I have 500' AGL by the time I hit the end of the runway and 1,000 AGL turning downwind.
 
Down Side???

.... The biggest and only down side I can come up with is very simple! If people
see you doing this they will all start doing it, and then you won't be able to out
perform them as easily. Thanks, Allan..:rolleyes::rolleyes:
 
From my perspective the answer is it depends!
160hp fixed pitch from grass, one notch/15* of flaps.
160 hp fixed pitch from less than (about) 2500' tarmac runway, one notch
160 hp fixed pitch from longer tarmac runway, no flaps
180hp/ 200hp c/s from any surface > 1200', no flaps
Soft runway, one notch
More than 15kts of wind, probably no flaps
180 hp fixed pitch or 160hp c/s, depends on runway length, surface, approaches and wind.
 
Flaps

I read thru this post kinda quick so maybe I missed it but my perspective has alway been use of flaps on take-off using the technique Gash described setting by down aileron. Why? Because I buy my tires and brakes. Figure slower lift off saves wear. Just another thing to consider.
 
Back
Top