Alec Proudfoot’s DaSH Project update (December 17, 2016)
Hi DaSH Sponsors,
Re: Update on Dec 11 and 17 2016 DaSH flights and attempts
Apologies for this tardy update, we had two flight days in December and then the holidays hit and things kind of got away from me …
On December 11th, we had 4 successful flights at Moffett Field, including yet again the longest DaSH flight ever, this time 1.89 km long (1.17 miles), pretty much the length of the runway minus the takeoff run and some safety margin at the end on landing.
On the fourth flight, we put even longer outer wing sections and wingtip extensions on to put the plane in its “v6-WE’ configuration, with a 40 meter wingspan. (The hope was for slightly lower induced drag with the longer wing.) I flew this test flight — the wind was coming up and was near the limits in maximum speed and gust speed, but it was still flyable. I had a successful flight, but I landed early because it was too bumpy, and at the very end of the flight, the wings started to twist as I was landing, but without consequence. We attributed this to the longer wing with slightly different structure in the new, never before tested longer wing sections.
We had a quick turnaround, and on December 17th, we went back to the configuration we had flown successfully 24 times before, the 36.3 meter wingspan “v5-WE” configuration (v5 with wingtip extensions — the very first flight for DaSH on 12/5/15 was the 33.3 m, v5 configuration, the only time we flew that version of the plane).
We didn’t expect problems with the wing given all the successful flights that had come before, but did want to investigate the wing twist phenomenon to make sure that it wasn’t affecting the lift profile in a negative way, or approaching an excessive amount of twist that could lead to wing failure. So we put indicator sticks at each wing junction, and filmed both sides from the fuselage, so we could make wing twist measurements.
It was cold out, 37 F, and it had been raining the previous day so there was moisture still around. We ended up getting frost on the wing and spent about 45 minutes removing the frost and waiting until after dawn so the sun could help us heat up the wing and remove frost. We got some of the frost removed, but it kept reforming.
When we thought we had an adequate amount of frost removed, we attempted a flight, again with me piloting — I am the heaviest pilot and we wanted the best chance to record any twist (I was running about 4 pounds heavier on this flight than previous ones, too).
Unfortunately, the wing did not attain it’s usual “U” shape on the takeoff attempt, and when we got above 30 kph (18.6 mph), the tips twisted down and then the front wing mount failed from downward load on the mount, causing damage to many of the center wing ribs.
We’ve spent a fair amount of time analyzing the video and looking at the plane parts post-flight attempt. We are reasonably sure that we were moving faster than we had before (typical previous flight speeds were between 20 and 28 kph, depending on pilot weight) and we reached a point where the airfoil pitching moment overcame the torsional strength of the wing at the tips, causing the whole wing to pitch down.
We think several factors played into this — the frost added weight and aerodynamic inefficiency to the whole wing, I was heavier than previous flights and we had a bit more equipment (cameras) than before, and I did not have as much up trim selected as previously.
It was good to discover this while on the ground, and now we need to fix it. Repairing the rib damage will be simple, as will the repair to the front wing mount (the damage is less than the damage from a balky forklift during our load tests in October 2014). We don’t believe the spars themselves are damaged, but we’ll be doing torsion tests of the wing sections and comparing them to unused/undamaged wing sections and spars to make sure. If there is damage to the main spar, we’ll need to fix it, up to and including completely rebuilding a wing section if necessary (we have spare parts for everything).
But after we fix all the superficial damage, we’ll need to make changes to the wing to make sure we don’t have this wing twist problem at any reasonable speed we might fly (it needs to not happen until well above the 30 kph it happened this time, to allow for gusts and to give a good safety margin). We’ll be doing some computer analysis of the wing structure and its aeroelastic properties, and making changes to reduce the twisting moment of the airfoils, probably by putting reflex into the airfoils in the last several meters of the wing.
So, we had a bit of a setback, but that’s what flight testing is all about, and we have a clear idea of how to get to a fix. We hope to get it analyzed and fixed in the next few weeks and be flying again in February.
That’s it for now, more later when we have upcoming flight trials dates set.