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I would be interested in comments from flyers having more experience than me. That would be most of you. My Mulvihill ship flies just fine until it gets into lift and then it stalls. Big stalls. U shaped stalls. At this years NATS the plane was in good lift and was writing huge W’s in the sky. When it fell out of lift the stalls dampened out before it reached the ground and that caused me to think the trim was not way out of line. But still I’ve seen other ships, in lift, riding the upwelling air to greater altitudes with out all of the stalling.
So what do you think? Is there something amiss with my trim set up or is this stalling to be expected in lift? BTW…it doesn’t take much lift to up set the balance.
Earl
Do the stalls force the plane in the opposite direction of its intended glide turn?
I mean, does it normally glide right and do the stalls give it a left-vector?
Do you have any wash-in on the wing?
I have some advice based upon answers to above questions.In addition to Dan’s questions, where is the CG? Sounds just on the verge of being at or very slightly beyond the neutral point.
I was just reading an excellent explanation of this exact phenomenon in the 1999 NFFS Symposium in an article by Peter King. The section titled “Paradox of dynamic longitudinal stability” may have the answer you’re looking for. You may get by with a simple re-trim if the model is a proven design or at most a new stab of different design.
Cheers!Thank you one and all.
Lets start with Dan. The plane is trimmed to fly right/left. In lift, the circle tightens up and to the best of my knowledge there are no warps, intended or otherwise.
Dean I set my CG following a formula by Rene’ Jossien published in 1999 NFFS Symposium. I can give you the details if you wish. The formula yields a CG of 101%.
Pete I have the document you mentioned and I have read it, but obviously it needs to be studied it’s pretty dense. I agree with you that the problem could be some discordance between wing and tail plane.
Please keep the suggestions coming.Earl
I’m betting on the CG. Unending stalls after an upset. Move it forward 10-15% and give it a try.
101% works on Civy Boys but I have my doubts on a Rubber Model.
Dean, I’m studying the Peter King article right now. Do you have access to it? I’m learning a lot.
Earl
My first idea doesn’t apply, so i won’t post it and confuse the issue.
I would have to believe there is a problem with tail volume and CG in some combination. Is this a new design or something already proven?Dan, it;s a new design. Working out the kinks.
Yes, I have the article, but I was talking off the top of my head. It has been my experience that formulas are only a starting point, and highly suspect at the extreme ends of things. 101% is really extreme. Look at similar models that are successful. Locked-down Mulvihill types would be best. If you are much different from them, be suspicious.
An extremely aft CG will produce a slow, floating glide, but will be easily upset, and reluctant to recover. I tried this with some small HLG’s back in my callow youth. They had cambered plates for stabs. Gentle hand glides were real floaters, but any gust and it was over.
Put down the books, move the CG 10-15% forward, jack up the stab TE maybe 1/64″, and try again.
Seventy to eighty percent CG seems about average for most locked up Mulvihills I’ve seen with 25-30% stabs. smaller stab might go farther forward.
Civy Boys had 50% stabs, 1 to 1 1/2 deg decalage and the CG at 100%.
They had a reputation of having a floating glide and also being sifficult to trim and wildly inconsistent.Well, it appears that you guys are correct. I went back to the formula and found that there are several variables where I could substitute different values. For instance, rough weather vs calm weather and even a choice between experienced vs inexperience. By changing these variables I was able to bring the CG forward to 85%, just what you guys suspected. I had the ship set up for best glide and calm weather.
Thanks for the help – I may call on you again.
Earl
Lol. Usually your weather observation would be spot on, but not this morning. I went to the field about 8:00 AM. The sky was clear blue and the wind was out of the southeast at 1 –3 mph. Temp in the mid 60’s. Wonderful.
Without changing any of the afore mentioned trim I wound the beast for three flights; 1st 35/in/oz, 2nd 50 in/oz and the last one to 60 in/oz. I’m using a half-length motor to keep it on the field. All flights went well and I came home with an intact model. However, I could see a tendency to stall with the slightest longitudinal up set. To me, this confirms the observations made by you and Pete and Dean and Jossien’s formula.
Now I have to figure out how to move the CG and tail plane settings to match different conditions.Earl
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