DA62 Take-Off Roll

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dgger
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DA62 Take-Off Roll

Post by dgger »

It looks like a number of you lucky guys have moved up to a DA62 from a DA42. I am curious, if you could shed some light on the performance - in particular the take off performance as compared to a DA42-VI or NG.

I went through the POH and the payload I get at MOTM in a -VI (approx. 550kg give or take) loaded in a DA62 would give me by and large the about the same two engines operative take-off performance (not accounting for the slightly higher thirst for JET-A). Apparently the extra 24 horses make good for the additional aircraft mass and bulk.

But then, paper is paper. Those of you in a DA62: Have you ever come across a field that you do not feel comfortable going to that would have been fine in a DA42?
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Re: DA62 Take-Off Roll

Post by CFIDave »

I've only been flying my new DA62 in the warmest part of the year so far, but over the past 4 months it routinely uses about 1500 feet of runway for its takeoff roll. Perhaps my previous 2013 DA42-VI (built before the 100 kg weight increase) had a slightly shorter takeoff roll and marginally better climb rate, but it's very close to the DA62.

As you know, takeoff roll on Diamond and other twins is determined by a rotation (Vr) speed that must be comfortably above Vmc in case of single engine failure on takeoff, which means holding the plane on the runway past when it may feel ready to fly. With a no-flaps takeoff, Vr is quite similar across models: 76 knots for the DA42-VI, and 80 knots for the DA42NG and DA62. Vr (and Vmc) is reduced a few knots on all 3 models when using half flaps for takeoff.

Subjectively the DA42-VI and DA62 seem to fly identically, and the shortest paved runway I'd consider for takeoff or landing would be the same for both aircraft: around 2500 feet.
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Re: DA62 Take-Off Roll

Post by dgger »

Thanks for the interesting feedback. I guess, when I eventuelly grow up, I would like to understand just how exactly adding flaps effects a lower Vmc, but it is interesting you point it out, and here is why:

I initially took out an FAA PPL and was training to get my short-field take-offs right. Once in Europe training for my CPL, I found many instructors would readily dismiss them, claiming short-field take-offs in a DA42 would solely exist to fulfill FAA certification requirements, but do little to actually shorten the take-off rolls.

One of the first things I did, once I got my DA42, was to actually try it out. I did short-field take-offs, normal take-offs - I was so desperate to shorten my roll, I even tried to deployed flaps half-way through the take-off roll. I found that it has little impact one my TODR, but I am standing by to admit I am a sub-par pilot ;)

The 2,500ft you feel comfortable with, sounds really good to me. In my NG I would try to avoid anything shorter than 3,000ft at SL/ISA/MTOM and perhaps a little bit of crosswind.
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Re: DA62 Take-Off Roll

Post by CFIDave »

dgger wrote:...I would like to understand just how exactly adding flaps effects a lower Vmc...
Here's the best explanation I found on another website:
When considering the effect of flaps on Vmc, you may be referring to the drag (or lack there of) flaps provide.
Generally, flaps up increases Vmc while flaps down decreases Vmc as a result of drag counteracting thrust as the slipstream quality behind the operating engine is reduced which in turn reduces the engines efficiency and thrust. This will result in a lower Vmc number.

There is also a train of thought that extended flaps have a stabilizing effect much like the keel effect experienced when gear is extended which helps limit the ability to generate adverse yaw.

I also found the below on one of Embry Riddle’s sites concerning muli-engine operations:
“When the flaps are down the wings create more lift than if the flaps were up. However, when lift is created, drag is also created (as lift increase, drag increases).
The side with the operating engine is creating even more lift because of the accelerated air flowing over the wing. When the flaps are extended, the drag caused by the accelerated flow opposes the yaw caused by the inoperative engine allowing the pilot to use less rudder to maintain heading. Having more rudder available to the pilot lowers VMC.
It should be noted more lift on the right wing will cause a roll to the left. If ailerons are used to counteract the rolling of the airplane, the drag from the adverse aileron yaw will actually increase the yaw towards the inoperative engine.”
Takeoff roll in a twin is dependent on Vr, which depends on Vmc (i.e., lower Vmc means lower Vr and shorter takeoff roll). But Vmc is dependent on whether flaps are up or not (i.e., extended flaps lowers Vmc). So use of half flaps would seem to REDUCE takeoff roll.

But...the additional drag from deployed flaps during takeoff will also reduce acceleration down the runway, which could INCREASE takeoff roll. So use of flaps might counteract the effect of lowered Vr and Vmc described above.

I think the effect of flaps on takeoff roll then comes down to the available horsepower (actually thrust) to drag ratio. For DA42 TDI models with only 135 hp engines, use of half flaps for takeoff is not recommended in the AFM/POH. But for Austro-powered DA42NG or -VI models with more powerful 168 hp engines, the AFM/POH recommends use of half flaps for short-field takeoffs. (The same is also true for the DA62 with 180 hp engines, where use of half flaps is considered normal for takeoff.)
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Re: DA62 Take-Off Roll

Post by dgger »

CFIDave wrote:
”[...] The side with the operating engine is creating even more lift because of the accelerated air flowing over the wing. When the flaps are extended, the drag caused by the accelerated flow opposes the yaw caused by the inoperative engine allowing the pilot to use less rudder to maintain heading. Having more rudder available to the pilot lowers VMC. [...]”
Fair enough. However, let's look at the back side of the same token. Not only do the flaps incude drag - as said above the side with the operating engine is also creating more lift that will further increase the yaw moment.

Now, we are talking about setting the flaps to APP. My gut feeling would be that the effect of the increased lift should outweigh that of the increased drag. Would you not expect that to imply a higher Vmc?
CFIDave wrote:Takeoff roll in a twin is dependent on Vr, which depends on Vmc (i.e., lower Vmc means lower Vr and shorter takeoff roll). But Vmc is dependent on whether flaps are up or not (i.e., extended flaps lowers Vmc). So use of half flaps would seem to REDUCE takeoff roll.

But...the additional drag from deployed flaps during takeoff will also reduce acceleration down the runway, which could INCREASE takeoff roll. So use of flaps might counteract the effect of lowered Vr and Vmc described above.
Interestingly, the NG POH give you the same take-off roll for both a short-field and a normal take-off, but about 10% improvement on the 50ft take-off distance at SL/ISA/MTOM. In contrast I noticed lowering flaps for take-off in -VI and DA62 gives you a shortened ground roll. Again, according to the POH.
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Re: DA62 Take-Off Roll

Post by nworthin »

I see above comments about a no-flaps takeoff. I thought (but could certainly be mistaken) that the T/O flap setting was a requirement.

While I'm on the topic, any comments about no or reduced flap settings on landing to help with gusty crosswinds or possible shear and/or icing?
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Re: DA62 Take-Off Roll

Post by CFIDave »

Departure with 1/2 (T/O) flaps) is normal on the DA62, but you can optionally take off with no flaps on the DA62 if you wish. The DA42 with Austro engines uses 1/2 (Approach) flaps only for short field takeoffs; otherwise no flap takeoffs are normal on the DA42.

As for landing, reasonable pilots may disagree about the pros and cons of using flaps:

- For gusty crosswinds, there is no consensus. Reduced or no-flaps landings will have you land at a flatter angle with more airspeed to better maintain control during the gusts and/or crosswinds. But landing with full flaps will reduce the amount of time the aircraft may be exposed to a potential wind gust during touch-down, by getting you slowed down sooner. So your choice...
- For landing in icing conditions or with ice build-up DON'T USE FLAPS (not much argument there). This will help you land with more airspeed to counteract loss of lift from ice on the wings, and it will reduce the chance for a tail-plane stall.
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