Denver

Hi everyone, first post here.

Looks like the best aviation forum around.

I am fortunate enough to have the oportunity to be chief pilot for a start up part 135 single pilot on demand operation that one of my former students is beginning. We just purchased a C421B 1975. It has a Robertson STOL kit w/ Fowler Flaps.

My question is in regard to takeoff safety in using the approved flaps of 10 degrees for takeoff.

First off the supplament says flaps 10 VMC is 80 KIAS vs. 87 KIAS w/o flaps.

Short field takeoff is rotate at 80 accelerate to 87 at 50' "yikes!"

Normal takeoff from the AFM is rotate at 103 (takeoff safety speed) climb at VYSE 108.

The owner who trained me (on the way from NH to CA) said he always use flaps 10 but higher speeds vr at 90 KIAS and initial climb at 100 KIAS.

The distances are 60% shorter doing it the short field way and doing it the old owners way maybe about 80% shorter than Normal.

I consider it a death sentance if you loose and engine doing this via the short field way. I am good but not nearly as good and current as the test pilot!

The hybrid takeoff seems like the way to go however the disadvantage is that there may be one more item to do when the engine quits after liftoff and that is flaps up. Also as the flaps come up there would be a slight settling as the airplane accelerated to 103 to 108. This may negate the advantage gained from shorter takeoff and 50' distances.

Specifically my question is would I get a better Accelerate go to 50' distance w/ flaps 10 or no flaps? SEROC is probably less w/ flaps but I wonder if angle of climb would be better.

The Robertson STOL suppliment does not give this data. Single engine Climb data is for flaps up only but the supplimented checklist says leave flaps 10 until all obstacles cleared (engine out after takeoff).

I am looking for opinions on the best takeoff procedure taking controllability and performance in to account.

Thank you all

joepilot

I think it would be a good idea to call Robertson and get the straight scoop on things such as how Vmc was reduced. Opinions here are opinions.

Joe

chongololo

I agree, get some info from Robertson on it and do some reading.
It sounds a little fishy to me but I'm sure it's fine.
Does it have the VG's on it? I fly a 414 with the VG kit that increases gross weight but I'm not a fan because there was no increase in the horse power of the engines. So with that extra weight and the same factory engines it's going to be ugly if you lose an engine.
Keep looking for a pilot out there that flies this same plane as you and see what they have to say about the performance numbers.
Good luck.

250 or point 65

Are you talking from a performance perspective or a Vmc perspective? I think the original poster's main concern was rotating so close to Vmc.

chongololo

Just from a performance perspective, I understand the opening post's concern, it is a little slow to rotate but I suppose that's what the makers of these STOL kits advertise.

Denver

That's really it. Those incredible short field numbers sell the kits, and they are impressive. Safe in this aircraft? No I do not think so.

Fortunately The partners in the start-up are fantastic inquisitive people and willing to spend as much as necessary for training and data gathering at altitude. That way we get some real numbers going.

I just do not understand how flaps can lower Vmc, makes no sense to me aerodynamically. BTW the Robertson STOL AFM supplement is all you get. In my limited time I have not been able to find them on the net or any other info.

Those Fowlers are a dream on the landings though.

Looking for info though on cases in certain make and model light twins weather 10 flaps may give you better single engine ROC or AOC performance. Seems this would only be the case in a T-prop or jet where you have much more excess HP on one.

Cheers

joepilot

I notice that you say it is a 421B. This was the one with the big nose and the tip tanks. I thought that the 421C was made in 1975, with the big nose but a straight wing.

In either case, the 421 original split flap did little to increase lift. The Fowler flap, by increasing wing area, may indeed decrease both Vx and Vy. As to decreasing Vmc, I can't understand how.

Joe

xjcaptain

Just think of it in terms of stability. The more stable an object is, the harder it is to displace. With the flaps down, there is more drag. More drag = more stability. With more stability and equal power, the aircraft will react to change more slowly. Thus the lower VMC.

Retracting the flaps prematurely (at too slow a speed) will cause VMC to increase. You could go from having enough rudder authority to running out. Landing gear would have the same effect. Gear down vs. gear up would lower VMC. Problem is that as drag increases climb performance decreases. So it is a balance between being able to maintain directional control, and having the ability to climb.

250 or point 65

I don't buy that.

When we're talking Vmc, we're talking about directional control. It doesn't matter how quickly control is lost, just that it is. What keeps an aircraft stable is opposing forces. We must ask ourselves, does XXX cause a force that opposes or enhances the unstable forces while operating single engine? If there is a prop in front of each flap, isn't the flap more effective at creating lift on the side with thrust? This will cause an increased rolling tendency towards the dead engine, decreasing stability.

I'll bite on the landing gear too. Drag is only stabilizing when its behind the CG. Think about why a dart has a flag. If you throw that dart backwards, the drag caused by the flag will right the dart. This is like the nosegear. Since it is in front of the CG, it is a destabilizing force, unlike the mains.

We can go into the BS behind "keel effect", that one's debatable too.

Denver

That is what I would think. Seems like roll city to me at 87. I'll be up in it doing some data gathering at high altitude with a good CFI in the next 2 weeks.

I will report back with the results.