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# Aeronautical Engineering/model aircraft aeronautics

Question
I need help to settle an argument about aeronautics. Particularly model aircraft.

It has been observed by some that when a model airplane flying with the wind turns back into the wind , some aircraft tend to pitch up and gain altitude indicating an increase in airspeed. Alternately, when flying into the wind, after turning around tend to lose altitude indicating a lower airspeed.

Most model pilots say that this is an “illusion” or “pilot error” and that given no change in throttle or thrust the airspeed remains constant. I disagree! It is my theory that in the first case forward momentum is carried through the turn causing a momentary increase in airspeed and when the model turns to fly with the wind it takes a while for the model, now with a slower ground speed to get up to optimum air speed. Thus airspeed is not constant but cyclical.

I realize that there are many factors involved and that some of my detractors base their belief on their training in full scale aircraft. Model aircraft tend to be smaller, have lower glide ratios and are often based on high performance aircraft designs as opposed to your typical civilian aircraft.  Flying style is also quite different with faster scale speeds and sharper turns for the models.

Am I right? Wrong?
How do Newton's laws of motion relate to this?

Hi Randal,

Sorry for the delay in getting to your question... it's a little out of my area (aerodynamics) as it is more of a flight mechanics/performance issue... certainly airspeed is governed by thrust, drag, and vehicle inertia...  with that said, when an aircraft is flying in one direction, it's momentum is in that direction and I guess first off I question your statement of the momentum being "carried through" the turn...  basically to turn, the lift vector is directed with a component in the new desired direction and that force (F) then accelerates (a) the vehicle (m)  in the new direction as in F=ma... (thus changing its ground velocity and more pointedly its momentum)...  usually its a more gradual process, so the vehicle responds (changes direction) relatively slowly and hence only slight variations in airspeed would result due to the increase in lift (which is required now in order to hold the aircraft level AND provide the force to accelerate it in the opposite direction)... the additional lift brings with it more drag and thus if thrust is not increased, airspeed would decrease...
If you change directions 180 degrees, the change in ground velocity appears to be 2 times the airspeed (regardless of the wind speed)...  The aerodynamics are a function of the airspeed whereas the vehicle momentum is a function of ground speed (an absolute frame of reference)...  since the change in ground speed is the same regardless of wind speed (so is the change in momentum) and hence I would say momentum does not play a role (in that the change in linear momentum is the same regardless of the wind)...

I would have to agree with your detractors in that any change in altitude to likely due to the pilot not (re)adjusting for the increase in lift necessary to make the turn ...  i.e. to turn you roll then pitch up slightly (more wing lift)...  after the turn is made you then would need to return the stick to the position for steady level flight...

Hope this helps a bit... and I didn't disappoint you...

Thanx,
mj

Aeronautical Engineering

Volunteer

#### Mark Janus

##### Expertise

I can answer questions about aerodynamics (high speed and low speed), internal fluid flow, computational methods (CFD, some design, etc.)

##### Experience

25+ Aerospace Engineering, University Professor

Organizations
Mississippi State University AIAA SIAM

Publications
AIAA SIAM

Education/Credentials
PhD Aerospace Engineering