Aerodynamics Discuss the physics of flight revolving around the aerodynamics and design of aircraft.

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Old 04-20-2017, 06:13 PM
  #1
vermontbadboy
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Angry engine thust

I have a question. Many years ago I made a model Piper TriPacer from a Sterling kit, about 56" span. Maybe I over powered it ,but the first time I got it to fly,it went into a steep climb ,got up to maybe 100 feet ,stalled ,came down and crashed. I don't think that I held it in the climb mode,maybe I just didn't know how to handle a model[I had flown full scale],so maybe it was just my mistake,. But I have been thinking of getting the model in shape again[I rebuilt it] And see if it will fly now that I have more experience in flying models. My question is would engine down thrust have anything to do with why it went into a steep climb,I other words should it have more down thrust than it had?
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Old 04-20-2017, 07:09 PM
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All Day Dan
 
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I built this 40cc Tripacer with everything set at 0-0-0. The CG is set at 30% of the chord. No trim was necessary. Dan.
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Old 04-21-2017, 10:00 AM
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Welcome to RCU BadBoy. There are much more knowledgeable experts on these forums than I, but I believe there are many things that could have caused your terminal climb. A general inspection by a trusted flyer would help. AD Dan hits on some of the issues, wing/stab incidence and CG. Certainly, a far rearward CG would do it.

Good luck and keep us posted.

Bedford
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Old 04-22-2017, 05:07 AM
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ibuild
 
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I would say so too, that there is many things that individually or perhaps a combination of many things that could cause such behavior, but in my experience - in most cases - a center of gravity that is mis-calculated or perhaps even more often simply not properly checked and being too far aft for the design or configuration would be the cause of an seemingly in-controllable plane and end in a crash.
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Old 04-23-2017, 12:13 PM
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BMatthews
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When the model first lifted off it got enough speed to raise the nose that high but apparently didn't have enough power to maintain speed at that high a climb angle.. And being fairly new to models you didn't recognize that and it stalled and then it was too late.

There are full size examples of this you could easily find I'm sure. New pilot allows the plane to assume too high a climb angle and doesn't watch the airspeed until too late. Stalls and doesn't have enough room to re-establish flying speed and still have the room to pull out.

The idea of checking the trim as mentioned is important. As a full size pilot you know how important it is to consider your loadout. Same for models. So do check that balance point.

The other thing is that after liftoff you should be ready to control the climb as required with some slight elevator stick pressure. Don't let the model assume a high angle of climb on your first few flights. Hold the nose down to a moderate climb and keep the airspeed up while climbing up to the safe altitude. THEN start playing with elevator trim and learning how the model behaves.

If it shows a strong trim change between high power and low power then you can do a few things. My own personal choice is to sneak clost to Dan's 0-0-0 and set the CG to get a nice level flying trim at something short of the full on 0-0-0 settings for a model of something like the Tripacer. My hot zip around aerobatic models I'll go for 0-0-0 and set the CG so they fly like an arrow at any attitude and power setting. But for sailplanes, old timers and if I built scale models I'd go for more like 0, +1 to +1.5, 0. That 1 to 1.5 on the wing gives me a little positive stability pitch up from speed changes and at the same time puts the thrust line at -1.5 to the wing. And it's the wing that is the key player. So from one wing angle I get both downthrust and stabilizer decalage.

Also do not assume that the elevator will end up level with the stabilizer when the model is correctly trimmed to your tastes. The angle between wing and stabilizer was set by one person's opinion. And is likely a compromise based on the presumed buying audience. It's OK if the elevator ends up a few degrees up or down from the stabilizer.

In your case from reading how it flew SOME added downthrust would not be a bad idea. But I'd still want to fly it and just use some forward stick to hold the climb in check. It'll only take a whisper so don't jam a lot in or you'll dork it into the deck.
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Old 04-27-2017, 08:43 AM
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Quote:
Originally Posted by vermontbadboy View Post
. I don't think that I held it in the climb mode,maybe I just didn't know how to handle a model[I had flown full scale],so maybe it was just my mistake,. ?
Hi Vermontbadboy. First let me welcome you to RCU and you will find lots of good information here as you already have in this thread. Please don't be offended but your statement above is I think one of the key points here. It seems that perhaps your full scale experience clouded your judgement of you ability to handle the test flight. I hope this was not your first RC flight at the time. This is quite common that you can believe. I have spent most of my life in full scale flight training and now in my retirement RC training as well as mentoring folks in the hobby.

Ok this factor above is what I believe was ninety percent of the problem but it seems you moved on continuing flying RC and since learned a lot. That's great, now that we got through that there is another factor here that I would like you to understand about specifically the Sterling Tri Pacer model and no other tri pacer model that I have ever seen. Take a look at this video and take note of the horizontal stab/elevator:

https://www.youtube.com/watch?v=P5sCp7s33y

The stab/elevator on the Sterling Tripacer if built according to plan is simply humongous, almost 50% greater in size to what would be considered scale. It almost turns the model into a tandem wing aircraft. Most never notice this and I once was asked to test a new ship for a gentleman who has just completed his. Sure enough it had the giant tail feathers and when I pointed this out he did not believe me. So since I still kept a hanger over at the airport we popped over there and pulled his model out next to a full scale tripacer. Wow what a contrast ! That particular model designed in the fifties for old escapement rudder only RC was looking for the most pitch stability possible of course.

Now of course I tested this model in this form and it was OK perhaps not great and remember this model was intended for engines starting at about .19ci displacement and these days folks are hanging .40 and up on them. Later on after I pointed out the giant stab/elev to him he rebuilt it with a much more scale stab outline and I also suggested some down thrust. This he did and I also tested it in this form and all I can remember is it was a better flyer.

So my best suggestion is most definitely add four or five degrees of down trust. Yes 0-0-0 is an ideal but the Stirling will greatly benefit from down thrust. This is especially so if the original decaledge and stab/elev outline is used.

Actually it would be fun flying one these days with the giant stab and watch the guys reaction when you point out that fact to them. Please do restore your ship and enjoy

John
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Old 04-27-2017, 08:55 AM
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Old 04-29-2017, 02:27 PM
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I took a look at plans for the pacer and the wings have a flat bottom with a positive incidence, both conducive to a high climb rate.
This configuration is made to climb with a increase in air speed (normal) I believe it was designed as a free flighter also, but over powered and not closely monitored you will get what you got.
down thrust will help but maintaining a proper angle of attack is the way to go.

ALL day dan
your wing appears to be of the symetrical type and shouldn't exhibit the high climb tendencies of the sterling kit.

Last edited by donnyman; 04-29-2017 at 02:30 PM.
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Old 04-29-2017, 03:28 PM
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Donny, you have a sharp eye. Yes it is symmetrical. It's the NACA 0013.5 set at my usual 0 degrees. It flew straight and level with a little a click or two of the trims. I gave it to a guy when I ran out of room for the new planes. He gave it to another guy and he gave it to another guy. They all loved it as much as I did watching them fly it. I had one of the large Sterling kits on single channel. Never did get any decent flights on it. Dan.
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Old 04-30-2017, 07:17 AM
  #10
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I don't have all the answers, that's a fact! but if one carefully listens to many that complain or bemoan a plane for the most part do not undestand the aerodynamics of what they are trying to fly, for one reason or another.

Threads such as this is and should be a place to learn, but instead it becomes a depository of opinions of things that has long be established, those that don't know and wont admit it, speak as if what they say is fact.
have you noticed how many tend to over power their plane as opposed to flying on the wing? a lot can be said for a minimally powered bird not to mention the amount of fun derived from trying to keep it airborne. and by doing so truly learn how to fly a plane that isn't a miss guided missle, thus the un avoidable stall which many don't recognize be it low or high speed.

Many would learn a lot if there ego where not in the way. now just for the record...... I am not excluding myself from what I said.
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Old 05-15-2017, 01:40 PM
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You might also get some good feed back over in the "kit building" section of the forums. There are people there who mess older kits.
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