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May 10, 2017, 12:50 PM
Registered User

Prop noise when hovering


My Apprentice can't hover (until I replace the electrics) but it can get very close to hovering for a few seconds. However, I've noticed that there is a significant vibration noise that occurs when hovering. Has anyone else noticed it?
I suspect that the stock prop is stalling at zero airspeed due to its relatively high pitch/diameter ratio - this is also predicted by ecalc. I suppose it could also be the cowl or another component resonating but this seems less likely. I'll add some tape to the cowl to change its natural frequency during the next flight. I've also balanced the prop and found the hub to be massively out of balance. This could have been causing vibration.

If it is prop stall then that's ok because it is destined for the rc storage drawers quite soon. I just want to avoid stressing the airframe by exciting it at its natural frequency.

Has anyone experienced this, or have any thoughts on the topic?
Thanks
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May 10, 2017, 05:05 PM
Parkzone junkie
kalmon's Avatar
Quote:
Originally Posted by procat
My Apprentice can't hover (until I replace the electrics) but it can get very close to hovering for a few seconds. However, I've noticed that there is a significant vibration noise that occurs when hovering. Has anyone else noticed it?
I suspect that the stock prop is stalling at zero airspeed due to its relatively high pitch/diameter ratio - this is also predicted by ecalc. I suppose it could also be the cowl or another component resonating but this seems less likely. I'll add some tape to the cowl to change its natural frequency during the next flight. I've also balanced the prop and found the hub to be massively out of balance. This could have been causing vibration.

If it is prop stall then that's ok because it is destined for the rc storage drawers quite soon. I just want to avoid stressing the airframe by exciting it at its natural frequency.

Has anyone experienced this, or have any thoughts on the topic?
Thanks
Almost certainly prop stall. Mine does it to when I hover it. Ignore it, the plane wasn't meant to hover anyways.

The prop only stalls at extremely low airspeed and high rpms.

-Brian
May 13, 2017, 02:53 AM
Registered User
Cheers Brian, I agree. Out of interest, I've taken the leap and ventured out further into rcgroups, posing the question here:
https://www.rcgroups.com/forums/show...-When-Hovering

I have much to learn.....
May 14, 2017, 03:03 PM
A Snickering Hound
Muddie's Avatar
I picked up an Apprentice recently and finally had a chance to fly it today. I'm new to fixed-wing and wanted something I could "grow" with. What a pleasure to fly (okay, it basically flies itself). I believe it would have floated around up there for what seems like forever (it actually took effort to get it to come down). Only thing I changed prior to the first flight was to put some larger wheels on as my field is grass and a little "rough".

Very pleased and looking forward to many hours with this model.
May 14, 2017, 04:01 PM
Registered User

New Wing Profile


I now have a new leading edge wing profile which is closer to a standard semi-symmetric wing. I will also re-profile the trailing edge after the next test flights, which might well be sketchy! The photo shows the initial cutting from a new leading edge datum, before I blended it all in. I'll take another photo once its all smooth. The tricky decision has been to know how aggressive to profile it. It will be a halfway house, although generating a new datum would be pretty tricky to go any further in the future. I might just have to use a template approach.
I'll keep the wing angle stock for the first flight, although I might raise the trailing edge during the test to compare characteristics. After all that I'll do a roof chop to lower the entire wing, which will take it closer to the vertical CofG plane....
May 17, 2017, 01:02 AM
Registered User

Modifying the Apprentice wing


Hi Procat,
Boy it would be nice to have a DHLG wing strapped onto the Apprentice! Flaperons could help slow her down!
I squared the Apprentice tips and put fences on to reduce crosswind effects of lifting a wing and it helps the wing leveling gyros.

On another experiment I put a Radian wing on and she flew sorta like a Radian but with pitch and yaw gyros. You can put an Apprentice RX in a Radian but here are some pics of a Radian Pro wing on an Apprentice. With this you get better glide and slower flapped landings. Not sure of top end speed. I've not flight tested as I'm working on F104 retracts for an Apprentice!
May 17, 2017, 05:25 AM
Registered User
Some fun experiments there Vic!
Do you have any photos of the fences? I'm planning to add them at some point down the line. Also, how did you add the flaps? They will probably be next when I find that the semi-symmetric wing doesn't produce as much lift - sometimes it is nice to float around like a kite! I have a servo and some pushrod parts lying around, are there any pitfalls or suggestions for an optimal design?
May 17, 2017, 05:43 AM
Better SAFE than sorry!
Quote:
Originally Posted by Muddie
I picked up an Apprentice recently and finally had a chance to fly it today. I'm new to fixed-wing and wanted something I could "grow" with. What a pleasure to fly (okay, it basically flies itself). I believe it would have floated around up there for what seems like forever (it actually took effort to get it to come down). Only thing I changed prior to the first flight was to put some larger wheels on as my field is grass and a little "rough".

Very pleased and looking forward to many hours with this model.
Congrats! The Apprentice S is a fantastic plane for learning and building up experience with RC flying. It's still my # 1 choice for beginners as long as you have the proper amount of room to fly it. I'm glad HH is still producing this plane!
May 17, 2017, 11:18 PM
COFPV Addict
Starbuckin's Avatar
Quote:
Originally Posted by VicT
Hi Procat,
Boy it would be nice to have a DHLG wing strapped onto the Apprentice! Flaperons could help slow her down!
I squared the Apprentice tips and put fences on to reduce crosswind effects of lifting a wing and it helps the wing leveling gyros.

On another experiment I put a Radian wing on and she flew sorta like a Radian but with pitch and yaw gyros. You can put an Apprentice RX in a Radian but here are some pics of a Radian Pro wing on an Apprentice. With this you get better glide and slower flapped landings. Not sure of top end speed. I've not flight tested as I'm working on F104 retracts for an Apprentice!
I've seen your post before on this subject... Great work! Might have to check into a Radian wing myself. Combined with the Eagle Tree Vector that combo would probably be awesome for both my Apprentices for longer range flights without having to go beyond a 5,000 mAh battery. That wing should defo have more lift and less drag than the stock Apprentice's wing, meaning more efficient flight. Now that I think of it, I'm standing here looking back and fourth at my Sky Hunter and my "Day Apprentice" and I'll bet that a Sky Hunter wing could be rubber banded to an Apprentice without much modding and make an immediate improvement.
May 18, 2017, 04:49 AM
Registered User
I'm aiming to test fly the 90% completed semi-symmetric apprentice wing this evening. Angle of attack is greater than stock due to re-profiling. This is a byproduct but is necessary to generate the Newtonian lift.
I haven't measured the angle but I can't find any guidance to aim for so it will be a seat of the pants flight!
I will move the CofG forward to make it more forgiving and the wind is minimal, only due to gust to 11mph.
Any advice from anyone regarding the test flight?
Thanks
Procat
May 18, 2017, 06:33 AM
Parkzone junkie
kalmon's Avatar
Quote:
Originally Posted by procat
I'm aiming to test fly the 90% completed semi-symmetric apprentice wing this evening. Angle of attack is greater than stock due to re-profiling. This is a byproduct but is necessary to generate the Newtonian lift.
I haven't measured the angle but I can't find any guidance to aim for so it will be a seat of the pants flight!
I will move the CofG forward to make it more forgiving and the wind is minimal, only due to gust to 11mph.
Any advice from anyone regarding the test flight?
Thanks
Procat
Based on your drawing the increased angle of incidence you've created by re-profiling the leading edge shouldn't hurt you. Newtonian lift shouldn't change much as loss due to lost area where you made the cuts should be close to offset by the increases due to the increased angle of attack. Did you keep the same profile for the entire wing? What did you do with the larger portions in the outboard area in front of the ailerons?

I'd like to see a vid of the flight. looks like a really fun project!

-Brian
May 18, 2017, 08:12 AM
Registered User
Thanks for the words of encouragement Brian, I'll work on a making a video at some point in the next few weeks.

The drawing has developed slightly since that photo was taken. I've maintained a step as per stock wing so the leading edges are all about 10mm further back than stock, other than the short portion that interfaces with the fuselage. The leading edge is quite round for now, slightly larger radius than stock. I've blended the new profile back to 60mm, leaving potential to remove more material later on (and also leaving a flat band for my glassing reinforcement during trials).
That should make any stalls forgiving but it also leaves foam for me to reposition the leading edge slightly higher or lower if I make it sharper.
The trailing edges have gentle tapers as well.
Overall chord length (and so wing area) have decreased by only about 5%. This is in addition to the wing tip chops.
I reckon the AoA has increased by less than 2 degrees.

Unfortunately I don't have a photo of the 90% finished wing here - I'll upload one tonight.
May 19, 2017, 10:48 AM
Registered User

Traumatic Tumbling Testing!


Well that was an eventful 3 batteries worth of flying. The new wing profile had an even more profound effect on flight characteristics than I expected.

First the pros:
- It flies freely and faster, inverted flight is more neutral, the plane doesn't climb so much when more throttle is applied. Basically it is more neutral and needs to be flown more, rather than flying itself like the trainer that it was. This is exactly what I was aiming for.

I knew that test flying the 90% complete wing was risky but wanted to keep my options open to finish modify things after the test flight. I left the connection between the fuselage and the wing completely stock, which resulted in a 2 degree increased Angle of Incidence (attack).

Now to the cons:
- The increased angle caused the wing to stall and snap roll and death dive in tight loops, something which was dramatic and unexpected. I realised what the problem was and slightly adjusted the wing angle as best I could out in the field. This helped, but not enough. The snap roll was always clockwise.
- The minimum stall speed is considerably higher, meaning the plane doesn't float any more, and landings take considerably more space.
- For some reason, elevator control seemed to be reduced, even when the wing wasn't stalled. This made it tricky to achieve a hover.

The suspected causes, plan and verdict:
- The wing mods moved the CofG aft (40g removed) and I forgot to take tape to strap a weight onto the nose. CofG was already aft, and I had intended to move it way fwd for the test. This is another simple mod, which I believe was one of the major contributors to the snap rolls.
- I will precisely (as much as possible) measure the Angle of Incidence and ensure that it is about 0.5 degrees. I estimate that I increased it by 2 degrees so this was a crass misjudgement on my part! I will trim the fuselage to achieve the correct angle and might pack the edges up during testing to observe the effects.
- I expected the stall speed to increase and plan to fit flaps to regain the low speed flight control when I want it. No problem.
- I hadn't finished cutting or smoothing the ailerons and also the leading edge was quite rough. The cutting is now finished and I'll take some tape to the field to see if a smooth edge helps.
- Although I've cut the wing as precisely as I could, it is possible that I've introduces some asymmetry or twist. I will measure the wing tip weights and look for any differences in shape. This could explain why it always stalled to the right, although it was probably just the motor torque.

So, lots of problems, and possibilities to massively improve the flying characteristics. I will try to adjust one variable at a time and take mods with me to the field. This will obviously help me to identify the true root causes. I've done some reading on the above effects but if anyone has any advice or suggestions, please feel free to chuck it my way.

If it was easy, it would be boring and I'd learn absolutely nothing.

Procat
May 19, 2017, 11:24 AM
Headed down the far side.
David Wile's Avatar
Quote:
Originally Posted by procat

- It flies freely and faster, inverted flight is more neutral, the plane doesn't climb so much when more throttle is applied. Basically it is more neutral and needs to be flown more, rather than flying itself like the trainer that it was. This is exactly what I was aiming for.

Procat
Hey Procat,

If that was what you wanted in the first place, why not just fly it in Experienced Mode? There is no up-elevator throttle mix in Experienced Mode, you have complete control of all control surfaces, and it will not fly itself in Experienced Mode. It seems you worked very hard to make your Apprentice fly better, but all the bashing seems to have made a perfectly stable flying airframe into an unstable one.

Best wishes,
Dave Wile
May 19, 2017, 05:23 PM
Registered User
Quote:
Originally Posted by David Wile
Hey Procat,

If that was what you wanted in the first place, why not just fly it in Experienced Mode? There is no up-elevator throttle mix in Experienced Mode, you have complete control of all control surfaces, and it will not fly itself in Experienced Mode. It seems you worked very hard to make your Apprentice fly better, but all the bashing seems to have made a perfectly stable flying airframe into an unstable one.

Best wishes,
Dave Wile
Hi Dave, thanks for your message. I haven't used anything other than experienced mode for many many dozens of flights. Although the electronics allow more freedom in experienced mode, the basic design of the apprentice is trainer trainer trainer to the core. This includes massive CofG separation which self rights the plane, excess lift, high drag and dihedral, which all amount to a plane that nearly flies itself without any electronic aids. It is not aerobatic in the slightest.
This development is all about understanding the differences between styles of planes. I'm turning a stable docile trainer into an edgy aerobatic plane.

It is a fine balance between encouraging neutrality and agility (neither traits exist in the apprentice in expert mode) and creating a suicidal monster! I was a little rushed in my preparations, hence the lack of tape to secure a nose weight, etc. Anyway, I now understand so much more about angle of incidence and root stalls. As a professional engineer, the dynamics of flight fascinate me, although I currently know very little about it.

Ultimately, it is likely that my developments will result in a total loss of the plane. If that is the case, I'll be happy in the knowledge that I have learned much and enjoyed the journey. I'm not the sort of person who likes to take a product that another engineer has designed within the confines of his flawed brief, and just use it as the manual describes. I prefer to learn the fundamentals and develop from there.

If I total the apprentice, what blank canvas should I purchase next?
Thanks again for your comments.
Last edited by procat; May 19, 2017 at 05:34 PM.


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