Mig 25 CG/decalage?

[Roberto Alonso]

Hello all,

 

I have maidened my plan-built MIG 25 today with not-so-great results. I had CG as indicated on the plan, and as with every all-moving-tail model I have, I had both 0/0 decalage and "+2 degrees up" marked on the fuse. We launched  with the stab on the +2 mark, and even then I had to reeeeally pull on the stick, the model felt very nose-heavy during the whole flight, so much that I couldn't really release the stick to operate the trim. Aileron response was ok, speed was good, but it felt very nose-heavy.

 

I would be exceedingly grateful if any builders of this model would care to share their CG point and resulting decalage with the plane already trimmed. I am a bit confused right now.

 

Can't remember exact weight now, but I'm within spec, maybe in the upper third of the range.

 

Any ideas?

 

Thanks! 

[Tim Mackey]

Hi Roberto and welcome aboard.

Not got this exact model, but can say that several models I have maidened recently exhibit the same properties....feeling like they are nose heavy. I put it down to the suggested COG being "too safe" and end up gradually moving it back.

Usually livens up an otherwise "soggy" feeling model.

[Roberto Alonso]

Hi Timbo,

 

thanks for your reply. I am thoroughly familiar with the need to start nose heavy and scoot back to taste, and conservative CG locations on plans, but what I'm seeing here is a different animal - a CG so far forward that the model had a difficult time staying airborne without massive "up" input, or at least these were the apparent symptoms. Somehow I don't think this is Tony's choice for flight behaviour, something must be amiss.

 

Maybe someone who's built and flown one of these will take a minute to measure their actual CG. Decalage info would be brilliant, too.

 

Thank you so much. I am aware I am starting life in this forum with a request for someone to trouble himself for my benefit, but I am confident I will be able to give back the favour some time.

 

Cheers! 

[Tim Mackey]

Hi Roberto - I understand what you are saying, and I am only sorry that I was unable to give you the specific info you need - Tony does visit the forum sometimes, but I will send him a PM to bring this matter to his attention, as he is most likely able to help. Cheers .

[Biggles' Elder Brother - Moderator]

Hi Roberto,

 

I am sure Timbo is correct and Tony will be able to help, but in the meantime...

 

I have no experience of this model, but something you wrote made my ears pick up! When you say you set the stab at "+2 degrees up" I'm a bit confused. Do you mean you set it at +2 degrees incidence? If so that, combined with a very forward CoG could explain the plane's behaviour. You will have effectively built-in a load of down-elevator on a long (because of the forward Cog) tail moment surely? "+2 degrees up" (if incidence) is of course actually 2degrees of down elevator!

 

It might be worth retrying with the stabiliser set a 0degrees before moving the CoG.

 

Sorry if I've misunderstood what you mean by "+2 up"  But if not it might be worth a go.

 

BEB

[Roberto Alonso]

BEB,

 

thanks for your reply. By "+2 degrees up" I meant that, if we take the wing as datum line (zero), the stab is turned 2 degrees in the direction required to command "up", that is, lower leading edge, higher trailing edge. This is a working ballpark for most models, I find.

 

Sorry if I didn't make myself clear - I am based in Spain and English is not my mother tongue. 

[Biggles' Elder Brother - Moderator]

Roberto,

 

please don't apologise - your English is excellent - better than many native speakers I know! The phrase you used is absolutely correct - I just thought I check in case you had set it the other way!

 

Seems the answer doesn't lie in that direction!

 

BEB