alasdair

Finally managed to get the flaps fitted and working late Friday night.
See photos.
I fitted one servo in each wing, each driving two flap sections, using flexible cables, and ball links to cope with the odd angles.

Omitted the cockpit insert and moved the fuel pump and UAT aft a bit to correct nose-heavy CG


view of the top of the right wing and underside of the left wing


One servo drives both flap sections down about 30 degrees. Needed surprisingly little elevator compensation.

Drove to Elvington for the Jet Modelers Meeting on Saturday and it had its first flight that afternoon. My best landing so far.
Though it survived, needed a little tweaking.
I expected to need to good bit of up trim with flap deployment but I had put on too much.
Another two flights on Monday, again tried loop, roll, stall and recovery, and inverted flight and decided that the aft CG shift achieved by moving the pump and UAT aft and leaving out the cockpit detail was still maybe not enough.
Still a little too stable.
Even managed to land with the engine running, but the last landing was horrific. Repeated bouncing with increasing amplitude. Managed to dent the tail end of the tailplane bullet fairing.
No obvious serious damage though, but needs a good check over and maybe a shorter noseleg with a soft spring.

alasdair

SCRATCH BUILDERS, a little advice.

I calculated the CG for my mini Trident using the excellent calculator on Aircraft Center of Gravity Calculator 3 (rcplanes.online)
But be warned. It takes no account of the fuselage (or foreplanes for that matter) It takes account of the wing and tail (H-stab) only, and for most aircraft that works out just fine. For most conventional aircraft that we build you extend the wing shape into the centreline and calculate using gross wing area and it turns out fine.
For my mini Trident I used the page for a 3 panel wing, entered all the dimensions from my CAD drawing, I entered a Static Margin of 10%, the the program spits out an answer
"From wing root to CG = 12.29"
But that's without the fuselage!
Does it matter? I calculated that the fuselage moves the NP, and hence also the CG, forward by a substantial 4.1"
So now the CG should be at 8.2"
The earlier flights (pre-flap fitting) started with the CG further forward, then moved aft a little using a lighter battery.
My latest test flights, after fitting flaps. had the CG at 8.3" and that's after moving pump & UAT and leaving out my cockpit detail (Nigel and the seats and floor and backwall, total 35g)
With that CG the stability and control were great, it handled well and would loop and roll, stall and recover, and fly inverted with a bit of down elevator. There is scope to move the CG back a little more to see if I like it better, maybe with a lighter battery and Nigel back in place. But I think I've found my starting CG for the big one.

JSF-TC

My Feedback: (2)

Alasdair,

Great news.

Can you share how you did that calculation?

Paul

alasdair

Paul, Short answer:-
I used Chapter 22 of "Basic Aeronautics for Modellers" (Second Edition) by Alasdair Sutherland
Can you handle the long answer?

JSF-TC

My Feedback: (2)

As an Aero Engineer I think I can!

The books' author - Alasdair Sutherland - any relation or just coincidence?


Paul

alasdair

Hi Paul,
Yes, it is my book, published in 1995 in UK by Traplet, Second Edition published 2002 with added chapters including Ch22 on CG calcs.
I went to University in Glasgow and completed my degree in Aero Engineering, but then became an airline pilot (with BA) instead.

I made up a term that I called "nose volume ratio" Vn (analagous to tail Volume ratio V-bar) to account for the destabilising effect of long noses
Vn is "excess nose area/wing area x nose arm/MAC" and gets multiplied by an efficiency factor (0.2 in the Trident case as the nose is slim and round)
so nose term in NP equation is 424/441 x 22/10 x 0.2 = 0.42 That times the MAC is how much the NP is moved forward = 4.1" Approximate. You may notice I used 2 values of MAC in there, 10 when calculating Vn and then 9.8 but hey! It's all guesswork anyway.
Do you build your own designs Paul?

For any readers who are not Aero Engineers, it is actually quite simple. I'll go back to basics.
Stability in Pitch is about an aircraft flying along exactly in trim and then meeting an upgust (or a pitch-up) that increases its angle of attack (AoA) by a small amount. The increase in lift (XL) will cause a climb, but what else? If the lift increase makes it pitch up even further then it is UNSTABLE. If that XL makes it pitch nose down, then it is STABLE in pitch, but how stable? Can we measure how stable?
The increase in lift is mostly on the wing and acts through its Aerodynamic Centre (at quarter chord, or 25% MAC for all sections I have checked, give or take a couple of percent).
The Neutral Point is defined as the point where the overall lift increase XL will act. If the CG is exactly AT the NP there is no rotation, neither up nor down, no stability. Neutral Stability.
The aircraft pivots around its CG, so the further the CG is ahead of the XL acting at the NP the more stable it is. Distance CG to NP is the Static Margin, or stability margin. Move the CG forward for more pitch stability, aft nearer the NP for less.
So for a flying wing then the NP position is at 0.25 x MAC back from the LE of MAC.
But the tail also has a lift increase that depends on its effectiveness (how big, how far back) Vbar, times its efficiency factor (downwash, tail height & AR)
[btw I made up my own simple tail efficiency factor Teff = fourth root of wing Aspect Ratio times 0.25]
Now the NP equation becomes NP = (0.25 + Teff x V-bar) x MAC
That works for most models. It is built in to the online calculator.

However if the aircraft has a foreplane (canard) or a long nose we must take that into account as it will be de-stabilising, it will move the NP forward.
For a foreplane I make up a foreplane volume ratio Vf just like a tail volume ratio, and I include the fuselage area in between (gross area, whereas for the tail I use net area) and as it's in clean air its efficiency is 1, or even 1.1
For a long nose without a canard I make up a nose volume ratio Vn by working out the excess nose area. Normally the nose length on a model is one MAC ahead of the wing Aerocentre and maybe three times as much behind. Thus the fuselage effect on pitch stability is neutral. So Vn = XS nose area/wing area x nose arm/MAC
Then I multiply by an efficiency factor of 0.2 for a slim rounded nose like on my Trident, up to maybe 0.4 for a flattish broad nose like on a lifting section fuselage (example escapes me). That's because the lift curve slope, the lift you get for one degree pitch up, is much less than for a wing (because of the slim rounded shape and very low aspect ratio).

JSF-TC

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Alasdair,

Thanks - and much respect for writing an aero theory book for modellers - not an easy subject. I just ordered a copy from Amazon (plus a replacement for my Martin Simons aero book that I can't find).

I did my Aero degree at Hatfield Poly way back before they changed their name, and have been in the aero industry ever since.

I don't normally build my own designs, but I did jump in at the deep end a couple of years ago and scratch built a Blackburn Buccaneer that I posted a build thread on. That was to teach myself composite building more than anything, but it was a great way to expand my CAD skills as well.

1/7 Scale Blackburn Buccaneer All Composite Scratch Build


I used the full-scale c.g. range plus the on-line calculators to find my initial c.g. After checking the on-line tools against my flying Hawker Hunter, I settled on a 5% static margin which seemed to work out well on the (unfortunately terminal) first flight, at least right up until I lost elevator control. Build of Buccaneer #2 is well underway.

A friend of mine has scratch built a large SU-17/22 and he used 5% static margin also, but it seemed to want to pitch up when slow, characteristic of a too-far aft c.g. After reading your post about the destabilizing effect of the long nose on the Trident it struck me that he may be seeing the same effect. I'll point him to this topic to see if he can apply a similar correction.

Regards,


Paul

alasdair

Hi Paul,
Make sure you get the Second Edition of the book.
It was that build thread of yours that encouraged me to start this one on the mini Trident.
I look forward to seeing a video of your second Buccaneer fly.

alasdair

Another little mod done on the mini Trident.
I mentioned that at the Elvington jet weekend (end of May) the last landing in particular produced some dramatic bounces, ending with the nose so high that I scraped the rear tip of the tail bullet.
I read somewhere that one cause is too firm a noseleg springing.
Also, if you have flaps on an aircraft, but miss out the leading edge HLDs (slats), it flies the approach with an unrealistic nose down attitude. The slats let it come in nose high.
So to remedy both of those I have changed the noseleg. I made up one from thinner wire and used a smaller wheel with a softer tyre. And the leg is an inch shorter, taking away the non-scale nose high stance the model used to have. A nice soft oleo leg would be nice but there just isn't room.

The new leg weighs 20g, the old one 57g, so I could put my cockpit back in and keep the same CG. I might try it with the CG moved slightly aft first.
The wheels are still too big for scale. I'm using 1.75" nose 2" mains whereas scale would be 1.5" nose, 1.75" mains

fumus

Similar to this MD-11 "landing"?

alasdair

Yes, fumus, very like that one.
I didn't get to see mine in slow motion, but having seen that one full screen, frame by frame, several times, it seems to have come in quite nicely, then skipped lightly on the mains, or maybe just flared level an inch or two off the grass, then drifted up a foot or so, then the nose dropped and it landed heavily on the nosewheel, bounced hugely on the nose, pitched well up maybe thirty degrees, climbed to six or eight feet, came down very heavily on the nose, bounced up to seventy degrees and crunched back down.
In my opinion, if the pilot could have held that nose up attitude after the first slight lift it would have come down on the mains and been OK. I believe he may have relaxed on the up elevator when it was 1 foot up, and from then on it was DOOMED.
Maybe that's what I did too. I don't have a video so I can't play it back.
But on my first landing of the weekend it had landed quite nicely and quite tail down as I heard the tail bumper scrape. So that proves it is possible to do a decent landing.
I was very lucky that my horrific looking landing did no real damage at all. My Trident aircraft was re-usable, needed no repairs to fly again, which is said to indicate a good landing (ha ha!). My last landing was also very far away, one problem of using a full size runway; you use it. Next time I'll keep it tight, land close by or go-around, and don't relax on the up elevator in the flare. Maybe I'll get someone to video it too.
Many thanks for that example.