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Jun 08, 2019, 10:49 AM
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Ed Waldrep's Avatar
I had a Kamdax Mig 15 that pitched up bad, I crashed it a couple of times and eventually threw it out. It was a weird setup, MF480 in a bigger duct that was set up for a 90mm fan, thrustline way forward.

My Wattage Mig 15 flew fine though. Maybe it was a CG thing.
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Jun 08, 2019, 01:52 PM
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AntiArf's Avatar
Originally Posted by Ed Waldrep
I had a Kamdax Mig 15 that pitched up bad, I crashed it a couple of times and eventually threw it out. It was a weird setup, MF480 in a bigger duct that was set up for a 90mm fan, thrustline way forward.

My Wattage Mig 15 flew fine though. Maybe it was a CG thing.
Likely. Swept wing CG locations seem up at the top of posted model aircraft mis-information on the internet, and in manuals. Smallish stabs narrow the range also. After building and flying enough of these swept wing jobs, my TLAR derived from past builds will override any posted CG info. Usually notably further than calculated, on swept/tapered jobs. Actually wanted to do just that with my Victor flight yesterday, but the battery was fully forward and I was at a build thread posted CG location that was reported to have been used in flight. Not wanting to add any ballast, it was not flyable at the posted location but was safely landed. Adding a small amount of ballast to get it to about where I initially guessed it was spot on, and only just a few mm forward.
Jun 09, 2019, 10:47 AM
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Thread OP
Hi guys,
Thank you very much for your advices. The Mig is repaired and was through 5 more flights today.

Now, sorry for the length and broken English.

While repairing I reasoned as follows:

1) The "crash" was 7th launch, the previous were OK, perhaps the very first one was rather "untidy". At this first start I learned that the model wants some initial push, and delivered it at the other launches. Then last week the model was launched by my friend who likely wanted the thrust alone to act upon the model.

2) If the model has a very low speed the aerodynamic forces can be "neglected" and the resulting movement of the model is governed mainly by its inertia and thrust. As the CoG is well above the thrust line then the model pitches up very noticeably.

3) Swept wings stall first at tips so when accelerated they generate the lift at roots first, which causes more pitch up and a corresponding tightening of the "loop" (I will call it a loop here, if for a lack of a better term, however it is not a controlled loop, it is rather a flip on back with a reducing radius of curvature).

4) I have seen the similar loop when slowing down the model and applying power. As I was quite confident I can launch and fly the model it seemed to be appropriate to test this "loop" when stalling the plane.

5) I wanted to test (a) moving the CoG forward, and (b) raise ailerons sligthly to give a more washout to the wing.

The result is - neither works 100% .

The calculated CoG (including the fuselage effect that makes for about 10% of the wing chord) has been (almost) spot on. The model is quite difficult to stall, when the elevator is pulled then the Mig starts to "wave" its tail and eventually drop its nose. Nothing dramatic. When some 25 grams of lead is added she almost refuses to stall at all, likely more elevator throws (now about 5-6 mm) would be needed.

When the full power is applied while stalling she immediately loops. If the power is reduced again the Mig enters a dive. If the power is maintained she can enter a spin (not always). The more forwars CoG just makes this spin less likely to happen.

The raised ailerons seem to enable much slower flight (at higher AoA) but the loop is correspondingly "wilder". I did not test the flaperons up at launch.

My conclusion is that I need to be more careful about the throttle stick. Even the recovery from the stall situation is easy if the stick is not just banged forward. Learning every day.

I am happy with the Mig, and thank you for your support.

Jul 20, 2019, 02:31 PM
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Thread OP

Measured performance

Today, I loaded my Mig-15 with the telemetry - Spektrum GPS modul and Altis altimeter and did two flights. The diagrams from the first flight are shown. I tried to glide (with the motor off) at different speeds in hope to get some glide ratio measurement. As expected from previous experiments it is extremely difficult to get reliable data.

Anyway, I selected 5 points from the diagrams and plotted them against the calculated curve. I am quite happy with the fit (when taking 4 out of 5 points shown, also the calculation is with the motor running).

Further, the maximum speed is as predicted in the 160 km/h (100 mph) range, the rate of climb about 30-35 m/s. On the other hand, the Mig cannot be slowed down as much as expected (I suspect the swept wing ).

I will definitely give it more thoughts later, heading for a F5J comp early morning.

Aug 18, 2019, 01:24 PM
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Thread OP

EDF rpm measurement

This may be known already but is new for me. For measuring the EDF speed (rpm) a smart phone with a FFT (Fast Fourier Transform) application can be used. The app in the phone analyses the sound and returns the frequency spectrum. There should be two very distinct peaks in the spectrum: (1) blade passing frequency (BPF), (2) motor frequency.

I used the app called Spectroid for Android. Both measurements were made after flights, i.e. with discharged batteries, and cannot be easier, just place the phone close to the plane, run the motor at max. speed and touch the "freeze" button on the phone.

As shown on the screenshots, the FW F-105 recorded 3101 Hz (herz - cycles per second) and 619 Hz. The EDF in the F-105 has 5 blades, so the BPF should be 5-times higher than the motor speed - which there is. The motor RPM thus was about 619 * 60 = 37 000.
With the Mig it is similar, the motor speed was 673 Hz and the BPF about 6 kHz. As the EDF in the Mig has 9 blades it again matches well. The motor rpm was about 673 * 60 = 40 000.

When comparing both diagrams:
1) The motors are measured to issue about the same noise levels.
2) There is big difference in noise at BPF, the fully ducted EDF in the Mig is quieter the the motor, while the BPF peak for the F-105 is much higher than that of the motor. I think that most of the noise in the Thud comes from interaction of air stream with different speeds with the blades (some air is taken through wing intakes, some from cheater holes.

Thanks for reading, Jan

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