Focke Wolf Ta 154 Laser

[Chris Walby]

Thanks for the heads up Eric, mine disintegrated on the starting bench and would have been a bit exciting if it had done it during the 4 point noise test. I intend to put the new FW 3D printed version on the Acrowot for a good many flights until I am totally happy that it won't fail. Not that I want it on a single, but an unnecessary risk with a twin. I am still tempted to make ali back plates just so the prop can't come loose even if the cone fails..

I am running out of things to do with her, just U/C carbon sleeves and the big check sheet. Tempted to go up the flying field and run the engines up as with current restrictions I can't see that it will get her inspected anytime soon (no problem).

Thanks to all who have chipped in throughout the build, much appreciated.

PS - I wonder if it will fit in the car with the wing tips off?

[Rich too]

Sub’d 👍

[Chris Walby]

Good news in goes in the car...after taking everything else out

And engine testing went well. Darn cold and damp thus took a bit of coaxing, but once some heat was the engine they both ran faultlessly.

Please let me know if the link does not work, darn looks like I need a lesson on YouTube 

FW Ta 154 Laser engine test - YouTube

 

Edited By Chris Walby on 28/12/2020 19:04:44

[Tim Flyer]

Looking really great... well done !

[Chris Walby]

Thanks Tim and to add that engine testing went well this morning despite the near zero temperatures both engines started up with help from a finger partly over the carb inlet to act as a choke. As soon as a little heat was in the engine then the glow was removed and normal proceedings prevailed.

Couple of adjustments achieved tick over to WOT with matched rev's within 100 rpm or there about (I took Jon's advice and just Y leaded the throttle servos and made sure there was easy access to the linkages).

In between running the Focke up I started the Acrowot (with the 3D printed spinner) and after a bit of tweaking and a fly of the old girl. The spinner took a bit of a hammering to get it started and I throw everything I could think of o make it part company from the crankshaft. Ok it does not suit the Acrowot, but I have seen more ugly aircraft!

Little bit of work on the main UC and tail ID (left over for a FF 109)

Edited By Chris Walby on 30/12/2020 21:34:18

[J D 8]

Looking and sounding good. Winkle Brown wrote the 154 he flew had good low speed manners. Good luck with first flight.

P. S . That car is a Land Rover good job its a 110, No passengers for me in my 90 when I take my Bleriot to the strip.

[Chris Walby]

Today was the day for the maiden and this is where I am going to need the collect assistance of people that have greater experience and knowledge than myself, so really nearly anyone with a suggestion will do!

 

To recap

• 80 inch wingspan with twin Laser 70's coming in at just over 16 lbs, so porky 
• The wing area is 20% less than the same wingspan DH Mosquito (which weighs less and a thicker wing cord)
• C of G correct empty and 25 mm forward with full fuel load and gear up

So I was expecting something rather fast, protracted take off run and full flap to get it back on the ground, thus I set flap at 10 degrees and 45 degrees and aimed to take off with flaps at 10.

 

Everything run up and tested so all good to go, gentle power up to 80% throttle with a nice straight ground run, rotated on its own at lower speed than I expected and started to climb out.

Now its worth bearing in mind that we have trees on the right turn out so I was aiming for speed then height and then turn, so I gave it some down elevator to gain airspeed

 

Now it seemed quite out of trim as it needed a lot of down to drop the nose, then pitched down so a I decided due to the low air speed I would need full power to gain height.

 

The is when everything started unravel as pitch control became "delayed" and very abrupt so after a coupe very challenging moments I made the decision to fly as far into the crash site as possible (gear up, power to 25% and aim for the ground. I was expecting difficulty in flair and probably a tip stall and this is the odd thing it seemed better at lower speed.

 

Possible theories  

• Pre flight comment by a experienced club member - seems to have a small elevator for size of model
• Possible horizontal stabilizer flex (not fore/aft, but up/down)
• Would a forward C of G  cause a issue like this
• Would the incorrect tail incidence cause a high AOA of the main wing and then manifest its self with the increased speed pitch sensitivity
• Anything else?

The conclusion at the moment is that 10 degree flap is not needed for take off, no idea about the C of G, but I don't thing its too far back and it will fly slowly which is a major surprise.

 

Rather sorry fuselage with some damage behind the TE and a nacelle to sort out, but its mostly in one bit and will fly again.

 

please post if you need any further details, thanks for reading and off to lie down in a dark corner to recover!

 

 

Edited April 30, 2021 by Chris Walby

[Martin Harris - Moderator]

Airflow disturbed over the tailplane at particular angles of attack by the flaps?

[Frank Skilbeck]

Tuck under like you describe could be due to a rearward centre of gravity. 

[Chris Walby]

Thanks chaps, food for thought and Frank I'll get it back on the C of G checker and post the information for discussion. Martin I'll bring the laptop and video up for a club patio analysis session today.

 

Note, I was gentle on the elevator with slow and progressive stick forward with little effect until the snap occurred...it seemed quite responsive in roll, just the nose up and lack of fine response being the issue.

 

Unfortunately the cameraman left his post just prior to the "landing", but essentially its more nose up until close to stall/lot of down elevator.

 

 

[Frank Skilbeck]

Looking at the video, it does exhibit characteristics of a rearward c of g (based on my experience ?), also it looks like the wheels retract backwards, so once retracted there will be a rearward shift in c of g, did you balance it with the wheels up?

[Chris Walby]

Hi Frank,

 

Yes all gear retracts backwards and C of G is correct with gear up and dry. Full fuel pushes this forward by about 25mm.

 

C of G looked well back on the plan IMHO ! So I went to a web calculator and it came  

 

 

 

Plan says 124mm

Cal says 119mm at 5% static margin

Cal says 92mm at 15% static margin

 

So I went for C of G at 120mm  perhaps this is too far back as you suggest and redo it at 90mm. What do people think?

 

The bit I don't get is the snap effect if the C of G is too far back which supports Martin's suggestion around disturbed air over the elevator (as I was flying with 10 degrees flap.

 

 

Edited May 1, 2021 by Chris Walby

[J D 8]

   Test pilot Eric Brown notes in the report of his test flight in a 154 [ short and only low speed as UC could not be retracted ]  that the tail plane was adjustable but does not detail this anymore. In his report he says " Stability was positive around all three axis and harmony of control was impressive, while the effectiveness of all controls was satisfactory at low speeds." In a later interview with Kurt Tank, the designer told Brown this was also the case at higher speeds.

           Hope to see this interesting project back in the air. Cheers John.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[Jon H]

Looking at the video and reading the posts i am not convinced by the c/g being a problem. 

 

I say that as the model generally behaves as you would expect a model to behave when trimmed a bit nose up and generally the model is stable. 

The only instability i can see looks like its due to sudden elevator inputs which Chris describes in his report. 

 

I wonder if there is something wrong with the elevator/servo/linkage and this what was causing the problem. IT could be some slop in a linkage, the servo not being strong enough, or a resonance problem. 

 

In the past i have seen servos do all sorts of amusing things when you hit them with a certain resonant frequency. With two engines humming away you may have suffered some strange harmonic effecting either the servo or part of the linkage. The servo was only able to break through this when full deflection was commanded on the tx. On the full size typhoon in WWII the repeated tail failures were put down to resonance in the elevator mass balance causing it to fail. The resultant flutter in the elevator then tore the tail off. In our models, its more likely this resonance in a linkage would manifest as a bind and this would prevent the surface moving smoothly. Reducing the throttle as you did may have helped by changing the frequency of the engines and could explain why it seemed better at lower speed. It could also be a dry solder joint in a servo causing weirdness at certain frequencies, dodgy extension lead (i had that once on my twin scorpion) or any number of things. 

 

The problem is this is very hard to test as these frequencies will be damped with the model sat on its undercarriage. You could try testing it by having some assistants hold the model off the ground by its wing tips and giving the engines a thrashing while checking elevator functionality. 

 

The only other thing that comes to mind is an incidence problem like on the Hurricane, with engine thrust line perhaps being a contributory factor, but my money is on the elevator itself with that all or nothing response. 

[Martin Harris - Moderator]

Looking at the plan, I would estimate the mean chord around R25 which puts the C of G at somewhere well over 40%.  Assuming your tailplane is somewhere around scale size, the calculated C of G seems more reasonable. 

[Frank Skilbeck]

Years ago I had a small foamy electric Corsair at the recommended c of g it was pretty unflyable, it would nose up and when you pushed the nose down it would tuck under and then be difficult to pull out, moved the c of g forward and dialled down the elevator movement and it became a pussycat to fly.

 

BTW my IS5 canard glider is quite sensitive to c of g, too far forward and it lacks elevator authority, too far back and it is quite unstable (maybe why there was only ever one prototype ?)

[Chris Walby]

Big thanks to all on the forum and at the club for giving advice with the maiden.

 

The plan is to effect the repairs and move the the C of G forward to 90mm which shouldn't be a problem as all the aggravation was trying the get it back to the 120mm mark.

 

I have been given some useful advice and thinking on it the next maiden set up will be, C of G at 90mm, flaps at 10 degrees, elevator on mid rates and gear/flaps away as it climbs out and see how it goes after that....hopefully I can get it trimmed in and see if the flaps interfere with the elevator. Just need to see if I can squeeze a gear down landing in without breaking anything. 

 

PS - Dual large elevator servos in the horizontal  stab with very little play, however I suspect that the horizonal stab may have a high degree of flex which I'll sort out. Plus its a small elevator compared to the Mossie. The original plan had a full adjustable tail plane to tweak the incidence where mine is fixed.. perhaps now not the best move.

 

[John Stainforth]

Wouldn't it be safer with no flap at all?

[Chris Walby]

Well I thought the same especially as the take off run was so short, but a well know designer of warbirds has advised me that 10 degree of flap has additional benefits as well as the increased lift in that in a round about way it increases washout which is more beneficial at low airspeed. Better to have more lift, less likelihood of tip stall than reducing drag especially as there appears to be plenty of power on tap.

[Eric Robson]

I am trying the well known warbird designers flap take off next time out with the Ju88 as it has dropped the port wing again on take off. I had too much deflection on the flaps  in the central position to use them for take off and when I tried to alter it I found the screen for setting the flaps has disappeared. The set is less than a year old so I will contact the supplier tomorrow. Meanwhile I have changed over the system to Spektrum and it is ready to go. you may have yours ready again before I get to try it out due to bad weather for the next few days. If not I will let you know the outcome. The rest of the flight was ok but I did not fly again because the wind was getting stronger.

[Martin Harris - Moderator]

I'd normally think along those lines Chris, but when a model behaves so badly at the designer's specified C of g, there's a strong possibility that something else is influencing the model's behaviour.  While my gut feeling is that a more forward C of G will improve the model, you would hope that it has flown acceptably at that position for the designer and if it was my model, I'd try a flapless take-off with a more forward C of G and work from there.

 

It's work in progress but I test flew a Galaxy Mystic yesterday - a well proven design - for a club member who had found it virtually unflyable on a previous visit and had counted himself fortunate to have got the model on the ground without major damage.  From his description of the flight, I reduced the fairly substantial aileron movement and put it in the air.  We had some reasonably nasty turbulence from our neighbouring wood in the NE wind direction but it was an uncomfortable flight with some odd inconsistencies such as a seeming reluctance to turn to the right at times - maybe accentuated by the turbulence.  What was noticeable was a loud buzzing sound so I landed to investigate and found that the left wing's front starburst stripe was lifting along its leading edge for well over half span with up to an inch of its width detached from the main covering.

 

My suspicion is that this was disturbing the airflow over a large part of the port aileron and possibly one side of the elevator.  It will be interesting to fly it again once the trim has been secured and may well illustrate the effect of disturbed airflow.

 

 

[Chris Walby]

Re-maiden it today and if anything it was much worse resulting and in a massive prop hanging moment over the wood followed by getting stuck is a very tall tree. With significant poking it fell its last 30 ft out of the tree.

Autopsy tomorrow, but I think little will be gleamed form the pile of bits. 

[Martin Harris - Moderator]

What changes had you tried?

[Chris Walby]

Moved the C of G from 120mm to 90mm. It was getting further away and not that high over the woods to recover from the stall, but to be honest it only had a broken cowl when it was sitting in the tree. It was last 30 ft out of the tree and the ground that did the damage.

I am coming to the opinion that it wasn't a C of G issue, but don't know enough about aerodynamics or enough of the model to come to a conclusion.

Just put it down to dumb sticks and a difficult model to fly.  

[Martin Harris - Moderator]

It’s often tricky to diagnose these things. The owner of the Mystic referred to earlier had me fly a Maule Rocket for him on Thursday which was pure evil. He’d told me that he’d been advised to try some differential after the previous maiden, which I’d helped him set...what I hadn’t been told was that one of our most experienced pilots had flown it for him and felt lucky to have got it on the ground in one piece. My first flight nearly ended in disaster - on the climb out I turned in turbulence over those same trees and it spun... my first reaction was loss of radio but I regained control in time to recover and landed immediately. Range checks and physical checks were fine although I found both aerials were in the same orientation so we secured them at 90 degrees and flew it again. 
 

Despite the findings I had a slight nagging suspicion that it might have spun although this was dismissed by observers. I climbed out very carefully and once out of the worst of the turbulence found that I needed a lot of rudder to coordinate the turns - during testing at height, I found that uncorrected, the model would rapidly adopt a 45 degree nose up fuselage attitude and would flick from there if provoked slightly even with significant power applied. 
 

The third flight with rudder mixed in with aileron for the owner’s benefit  was a transformation. Incidentally, it was only during this flight that I learnt the whole story!

 

Had it crashed on my first attempt, no doubt the inquest would have blamed the aerial orientation but it illustrates how unexpected aerodynamic issues can confuse diagnostics.  
 

I did a little research on the full size yesterday - and interestingly it seems they have a coupled rudder tab to counter - you’ve guessed it - excessive adverse yaw!

Perhaps we’ll never know in your case - are you aware of any other examples that have been built from the same plan?