Short field take off

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Steve D
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Short field take off

Post by Steve D »

Anyone have any experience using approach flaps to shorten the take off ground roll?
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Re: Short field take off

Post by CFIDave »

Steve D wrote:Anyone have any experience using approach flaps to shorten the take off ground roll?
Yes.

I haven't checked the latest POH/AFM or Diamond's official checklist for the DA42 TDI (Thielert model), but those documents for Austro-powered DA42 NG and -VI aircraft added short-field takeoff with flaps APP numbers about a year ago.

For this to be recommended by Diamond, I guess testing had to first establish a smaller value for Vmc with approach flaps, since Vr must remain at least a few knots higher than Vmc (in case one engine fails right after rotation).

For our DA42-VI, Vmc is reduced from 71 to 68 knots with approach flaps, which permits Vr to be reduced from 76 to 71 knots. Furthermore, Diamond also established a new lower Vx = 77 knots.

What all this means in practice is that you can now be a "show-off" for your friends watching your takeoff from the ground. :) Rotating at 71 knots with flaps to APP and then pitching up to 77 knots causes the DA42-VI to climb away at a really steep pitch angle that has been known to result in comments from other pilots. And last year when I got asked to perform a short-field takeoff by the examiner (DPE) for my multi-engine instructor (MEI) checkride, I got scolded by the examiner (for using too high a pitch angle) until I showed him Diamond's numbers.
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Re: Short field take off

Post by carym »

I can't tell you if flaps on departure helps or not, because I haven't tried it since the POH does not provide an alternative short field takeoff procedure for the DA42-TDI. You can try it for yourself to see if it helps, but remember that it means you have become a test pilot and if anything goes wrong the FAA won't be happy.

Just for reference for anyone else looking, Vmc in the TDI is 68 KIAS, Vr is 72 KIAS (if heavy), and Vy is 79 KIAS if heavy and 77 KIAS if light. These numbers are not very different from those of the -VI. There is no Vx given, but Vyse is 82 KIAS (blue line) and this is where I raise gear. I usually climb out at 90-95 KIAS to give myself some extra margin of safety and a little better visibility over the nose.
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Re: Short field take off

Post by Steve D »

Thanks for the responses guys.

The reason for my question is not to create a higher angle of climb but rather to reduce the actual take off roll. I frequently fly to my game farm and the strip there is dirt. As there is no fuel down there, I have to go with enough fuel for the return trip so I am always reasonably heavy getting out of there. While the strip is in good condition, it worries me having an extended take off roll. Dr Berchtold mentioned that it was acceptable to use approach flaps for a short field take off but, as you have correctly noted, there is no mention of this in the POH.

I am flying down on Thursday and will have 3 pax + fuel so will again be heavy getting out of there. Fortunately my home field runway, while at 4,500ft amsl, is 3,000m long so I will try there when I leave to determine if it makes a big difference.
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Re: Short field take off

Post by CFIDave »

Steve:
To try and answer the real reason for your question -- shorter takeoff roll in a DA42 TDI -- probably requires considering two things:
1. Will the additional drag created by APP flaps be more than offset by the additional lift to permit the plane to lift off the runway sooner, and if so,
2. Will the use of APP flaps lower Vmc, and hence permit rotation (sooner) at a slower speed -- since you still need to maintain a safety margin of a few knots between Vmc and Vr in case an engine fails.

According to Diamond, NG and -VI models both have Vmc lowered by 3 knots with APP flaps, so you might assume the same 3 knot reduction in Vmc would apply to TDI model DA42s. Therefore you might try rotating 3 knots sooner at 69 knots with APP flaps to reduce takeoff roll.
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Re: Short field take off

Post by Steve D »

Thanks Dave.

The C.F.I. at my local flight school tried an "Approach flap" take off this morning and reported that with elevator trim in the normal position, the aircraft flew itself off the runway in a very short distance. He said there was no need to rotate and it was very controlled. He didn't specify at what speed. Also the aircraft was around 100 kgs below M.A.U.W.
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Re: Short field take off

Post by carym »

Steve,
As suggested by Dave what you need to know is the airspeed at "liftoff". If he was up in the air and climbing at a speed that is below Vmc, I would be very worried about the consequences of an engine failure after liftoff. I also wonder how the plane felt with respect to trim. I did this once (by mistake, forgot to raise flaps all the way on a touch and go) and I was pushing on the stick to lower nose in order to get airspeed up. It took a lot of forward pressure. If I lost an engine I would be in a world of hurt.
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Re: Short field take off

Post by Henrik »

carym wrote:I would be very worried about the consequences of an engine failure after liftoff.
Amen.

I know you guys are all completely aware of this one, but just in case there's a few student pilots reading this...

Being low, slow (below blue line), high pitch attitude and full power is an extremely serious configuration in case of engine failure. You'd have to lower the nose *immediately* to avoid stalling the aircraft and slamming back into the ground. (level or inverted) For a piston twin in particular, in this configuration you're completely eliminating any added safety advantages inherent with a redundant powerplant.

I remember training rejected departures in a Seminole, and was amazed how quickly speed dissipates if you keep the same pitch attitude after simulated engine failure. You really have to lower the nose pronto.

If terrain is a non-issue, lift off at +5 kts above Vmc, build speed in a shallow climb & securely depart the airfield at 90 kts...

If terrain *is* an issue, then just don't do it.
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Re: Short field take off

Post by Steve D »

Cary, Henrik et al.

Thanks for the warnings. While I am acutely aware of the issues and am doubly cautious having lost an engine just after take off last year (poor maintenance), it is always good to stress the importance of trim and the perils of flying close to the edge of the performance envelope.

You are of course quite correct re pitch attitude. In discussion with the C.F.I. he stated that the aircraft was completely controllable and did not have a pronounced nose up attitude. He allowed it to fly itself off the deck then held it to accelerate to blue line at which point he cleaned up and flew away as normal. He commented that it was fairly neutral as far as trim is concerned and did not require any unusual inputs. He confirmed that the trim was in the normal take off position. He did not record the airspeed that it got airborne but indicated it was significantly slower than without flaps and used very little runway (the part I am interested in).

Cary - when you experienced a heavy nose up as a result of not cleaning up the flaps, had you reset the trim back to take-off setting before applying power? Certainly in my 42, and I presume most other TDI's, you have a lot of aft trim dialed in on landing. If you forget to reset the trim to the take off setting before you try to take off, you will need a lot of forward pressure to counteract even without approach flaps.

The C.F.I. is flying again this evening and has undertaken to try the take off with and without flaps to ascertain the differences in take off roll and airspeed. I will report back next week when back from the bush.
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Re: Short field take off

Post by carym »

Steve,
I believe you may be correct, I may have not returned the trim to neutral. As you indicated one has to put in a lot of back trim during full flap landing. The incident was frightening enough that I don't want to repeat it again :(

I'll be very interested in hearing the further details from your CFI.


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