How much negative until bom strike

[Crumpet]

Hiya guys,

I always find myself on my Trex 700 pro dfc HV being gentle with my negative pitch, e.g. from an inverted hover i'll VERY smoothly add negative over a few seconds, with the fear of a booms trike.

I run 710mm evolution blades 1950 headspeed and +-12 degrees.

Could I literally just bang it to -12 and watch it fire up (i.e am i overthinking it) or should I be gentle?

I dont know how to find the limit without (literally) hitting it

Martyn

[Gate88]

You'll be fine.

Even if you're flying hard 3D you'll stuggle to hit the boom with -12.

ive only ever seen somone do it once in the air, and he was ragging the arse off it so hard that something had to give!

[jasoncook]

I think 700mm are the largest they recommend for the 700E you could do with checking that , I have 690 sab,s on mine , I have never worried about a boom strike, I just keep the blades tight and the dampers in good condition and i run 2050, I think full back elevator and full neg may be a different story,,

[Crumpet]

I think 700mm are the largest they recommend for the 700E you could do with checking that

Thanks for picking that out.

any idea how i could find out if they are okay?

[jasoncook]

You could call Midlands they should know, I was flying with alex hawtin who was sponsored by spin blades the info came from him on the blade length , If i remember right he had to use spin blades 710,s but as they were 710 mm and he had to fit the nitro boom which was longer than the electric version,

[trillian]

Thanks for picking that out.

any idea how i could find out if they are okay?

Since I have tried all sorts of blade combinations on different helis I just physically check if I'm ever in doubt. The most logical way is to turn the main blades by hand until you find a spot where the blades are parallel with the boom and the tail blades are pointing as near as possible in an imaginary line up to the rotorhead. Then run a tape measure from the underside of the nearest main blade grip towards the tail blades and see what the distance is. Keep in mind also that the blades will never be able to form a straight line towards the tail, they will always be bending in an arc, so if you have any clearance in a straight line you should be fine (assuming the boom doesn't bend upward! )

Regarding a boom strike I doubt that 710s vs 700s would make enough difference in length alone, it would be much more about how rigid the blade is, weight distribution, dampers etc. and obviously how aggressively they're flown.

The other variable is how far the blades are from the boom. I think of my helis the Kasama Faifa is the closest of the 700s and it's about 150mm.

[coolice]

Hey all.

Length is not the main problem with boom strikes, too much length would give a main to tail strike. Increased length will allow the blade tips easier access to the boom, but other factors are the main contributers.
The problem with the DFC's is the reduced height between the rotor disk and tail boom, making a boom strike possible if conditions are right. By conditions I am referring to rotor rpm and blade tightness.
The boom strike Alex had was caused by running his blades too loose in the grips, with DFC it is important to run the blades tight in the grips to avoid any lead/lag of the blades during loading and unloading. They had been changed and not tightened enough unfortunately. There was a video on HF of a similar boom strike to Alex's mid rainbow and it happened at the same time during the manoeuvre, right at the top of the arc as collective pitch was reduced, following the initial pitch pump upwards and rpm changed, causing the blades to lead/lag which then allows them to get closer to the boom.

At 1950 you have a much higher chance of a boom strike during aggressive manoeuvres, increased rotor rpm puts the blades under much greater centrifugal loads, so they stay straighter in flight. On my own 700e I run around 2000 in IU1 for less aggressive 3D and have IU2 set to 2150 for any flick stuff, such as walls, tic-tocs etc stuff that has aggressive pitch changes where the disk is loaded and unloaded quickly. I also run my blades tight, blade stop tight and this stops them lead/lagging.

Your +-12 degrees won't be a problem in a straight vertical ascent/decent, but if your blades are loose and you were to unload the disk following a pitch pump and input back elevator for example, a boom strike could be a risk.
.

[Rachel]

I have been trying for years to boom strike in mid air, not managed it yet!

[Varelco]

You are just as likely to boom strike with the heli upright as you are inverted.

Coolice is spot on, it's a common misconception that the blades flex causing boom strike which isn't the case at all.

[trillian]

You are just as likely to boom strike with the heli upright as you are inverted.

Coolice is spot on, it's a common misconception that the blades flex causing boom strike which isn't the case at all.

At high headspeeds no but at low headspeeds yes the blades do indeed flex. Extreme lagging at low headspeeds is the easiest way to boom strike though, like if you do an auto and sap all the headspeed then land a bit hard you get all the bad things happening together.

But anyone worrying about 'coning' of the rotor disc, I think that is extremely minimal at any decent headspeed.

---basically, to boomstrike in the air you either have to fly extremely aggressively, have very short clearance over the boom and too soft dampers or bog heavily with very loose blades at high positive pitch so that they lag massively. But as to whether blades flex, yes they definitely do! But it's very unlikely they would contact the boom through flex alone.

I had a boomstrike in the air once. It was a Century Predator gasser and what happened was that in an aileron roll the lower Jesus bolt sheered at the point where the heli was inverted allowing the entire main shaft to shove itself about an inch into the frames, driving the blades closer to the boom and the combination of inputs allowed the tip of one blade to chop into the boom (and it nearly chopped it off completely).

[Varelco]

The blades lag in the grip more at low headspeeds due to less centrifugal force pulling them out straight, the blade doesn't flex/bend to the point where it would hit the boom. I'm not sure if you just said the same thing just differently lol.

[Morus]

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I can create a boomstrike in 10 sec on any of your heli Rachel

[Rachel]

Hahahaha Tamas, no thank you ��������

[KevDavies]

I have been trying for years to boom strike in mid air, not managed it yet!

Amateur!! I smashed a Knight 3d up on its maiden on my very first piro flip attempt back in the day LOL. Vids on my youtube of the aftermath